13 MAINLY CHEMISTRY CONTENTS of this section Page 13.1 Accepted practice when handling chemicals 1301 13.1.1 Hygiene and organisation 1302 13.1.2 Before the activity starts 1302 13.1.3 During the activity 1304 13.1.4 After the activity 1305 13.2 Chemical reactions 1305 13.2.1 Fossil fuel experiments 1305 13.2.2 Gas preparation 1310 13.2.3 Gas reactions 1316 13.2.4 Heating substances 1320 13.2.5 Metals 1324 13.2.6 Acids, bases and salts 1329 13.3 Gas cylinders 1334 13.4 Gas syringes 1334 13.4.1 Gas syringe experiments 1334 13.5 Molecular models 1336 13.5.1 Types of molecular model 1337 13.5.2 Commercial models 1338 13.5.3 D-i-y models 1340 13.6 Plastics 1341 13.6.1 Preparations 1341 13.6.2 Sources of plastics 1342 13.6.3 Burning plastics 1342 13.6.4 Heating plastics in the absence of air 1343 13.7 Techniques in advanced chemistry 1343 13.7.1 Quickfit apparatus 1343 13.7.2 Refluxing 1345 13.7.3 Distillation 1347 13.7.4 The use of tap funnels 1348 13.7.5 Removing water from organic solvents 1349 13.7.6 Recrystallisation 1349 13.7.7 Vacuum filtration 1350 13.7.8 Melting point determination 1351 13.7.9 Nitration 1352 13.7.10 Electrophoresis 1352 13.8 Titration 1352 13.8.1 Volumetric flasks 1353 13.8.2 Pipettes 1353 13.8.3 Burettes 1353 13.8.4 Standard solutions 1353 13.8.5 Indicators 1354 13.8.6 Carrying out a titration 1356 13.9 Water 1356 13.9.1 Water purity 1356 13.9.2 Measuring purity 1357 13.9.3 Water purification 1358 13.9.4 Water storage 1361 13.10 Ground-glass jointed apparatus 1361

13.1 Accepted practice when handling chemicals Serious accidents in school laboratories are rare because of the high standard of instruction ! and vigilance by science teachers, helped by technicians, during the handling of chemicals and equipment by pupils and staff. However, technicians and teachers must not be complacent. The example set by teachers and technicians will influence the attitude of pupils towards chemicals; the adoption of a casual approach will cause the same in pupils. Out of school, it is

2006 Mainly chemistry 1302 © CLEAPSS 2004

hoped that pupils will apply their chemical knowledge, including an awareness of hazards obtained from school lessons, when handling other materials, eg, mixing concrete, applying paint stripper, spreading garden chemicals, using bleach etc. If these materials are handled in an irresponsible way, accidents will occur which could cause severe harm to the user as well as to innocent bystanders. It is hoped that there will be fewer accidents at home and in industry if the basic rules of storing, pouring, mixing, heating and dispensing chemicals are learnt at school. During a lesson, steps must be taken to ensure that pupils follow instructions carefully and treat health & safety advice seriously. Unfortunately, there are pupils who behave in an irresponsible way and need to be dealt with firmly in accordance with departmental policy. Consideration and patience also has to be given to those pupils who are scared of using chemicals and Bunsen burners and need to gain more confidence. Pupils are less familiar with fire than they were in a previous generation. This section is to remind you of the accepted practice that forms the basis for health & safety in the laboratory when handling chemicals.

13.1.1 Hygiene and organisation Pupils, teachers and technicians should observe all of the following. ! a) They should not consume food (including sweets) or drink in the laboratory (see section 3.5); a chemical-free area should be found. Many schools allow or encourage pupils to carry bottled water. However, pupils should not drink this in laboratories. b) They should ensure that benches are wiped at the end of any practical work involving chemicals (see section 13.1.4). c) At the end of any activity involving chemicals, they should wash their hands with soap and water and dry them hygienically; paper towels are most practicable. d) They should ensure that coats, bags and non-essential books are stored away from the bench so that they do not become contaminated nor present a trip hazard. It will be necessary to clean eye protection equipment periodically as discussed in sections 3.2.4 (Eye protection, Maintenance) and 15.12.3 (Chemical disinfection).

13.1.2 Before the activity starts Before any practical work begins involving hazardous substances, a teacher must consult ! and follow any risk assessments to minimise the chance of an accident occurring with the chemicals and procedures adopted, including preparation and disposal. This is required by both the Management of Health and Safety Regulations and the COSHH Regulations. The method by which this assessment is carried out should be outlined in the science department health & safety policy. More advice is given in section 2.2.2 and in Guide L196, Managing Risk Assessment in Science. Be especially careful of text books which often contain incomplete instructions or recommend methods now known to be hazardous. Check first with Hazcards or this Handbook. If in further doubt, contact the CLEAPSS Ηελπλινε.

While experience is no guarantee of safety, if you are inexperienced,

! do not carry out a hazardous chemical reaction without supervision from a more-experienced colleague. Technicians are very busy during the school day and so departmental policy should stipulate adequate notice for requests. Thursday lunchtime during the week before is often a recommended deadline. Adherence to such a system will not only help to avoid possible © CLEAPSS 2005 1303 Mainly chemistry clashes in the demand for equipment but allow sufficient time for the preparation of special apparatus, such as glass delivery tubes, and permit room changes to be planned, as necessary, well in advance. If materials from local shops are required, time can be arranged to purchase them. Accidents occur when people are in a hurry, have less time to think or are interrupted, preventing them from concentrating on the task at hand. Any new procedures should be trialed by the teachers or technicians before they are carried out by students. Making sure you Required items should be written clearly in a diary or on special request sheets, with are using the particular attention paid to chemical names and/or formulae. For instance, the right chemicals substances sodium nitrate (NaNO3) (OXIDISING AND HARMFUL) and sodium nitrite (NaNO2) (OXIDISING AND TOXIC), appear very similar if written carelessly but they have very different properties. There have been changes in chemical nomenclature which can be confusing to technicians who are inexperienced in dealing with chemicals or who have only just returned to work after a number of years away from a laboratory. Some suppliers use older or common names but education has adopted the IUPAC convention; eg, VWR International1 sells acetone (HIGHLY FLAMMABLE and HARMFUL) but schools use propanone (it is the same chemical!). Section 1.3 contains a list of chemical names. Technicians will need to know the volumes and concentrations of a solution. It is not enough to say “dilute”; how dilute? Making up New and/or inexperienced technicians will also require help from teachers in working solutions out quantities to make up solutions. The CLEAPSS Recipe Cards not only give instructions for preparing solutions suitable for GCSE-level and A-level but include health & safety information as well, acting as risk assessments for the operation of making up the solutions. Instructions to Pupils need to know what is required of them during practical work and written pupils instructions (including health & safety advice from risk assessments) must be clear and well presented. For instance, the amounts of materials to be used by pupils should be given in terms of height in a test tube, so many grams or so many millilitres. Vague instructions referring to ‘spatula amounts’ or ‘heaped spatulas’, will lead to spills and variable experimental results depending on an individual’s interpretation of the phrase and the type of spatula used; see Spatulas in section 9.11.4. We have found poor instructions in modern text books which, if followed, could give rise to poor results or worse: for example, toxic fumes might be produced in the room. Teachers and technicians should ensure that published instructions are comprehensive and encourage safe practices. If experiments are to be designed by pupils, teachers should scrutinise their plans for unsafe procedures before any practical work is attempted, ie, plans should have a health & safety section. Pupils should politely inform laboratory technicians of any materials or apparatus they require, well in advance and in writing. Moving materials Bottles of chemicals and equipment should be moved from prep room to laboratory and vice versa in trays or stacking boxes, preferably on trolleys. Hazardous materials must be moved when it is safe to do so. Moving such items at the ends of lessons, when corridors are full of pupils, is rarely likely to be safe. For carrying bottles of chemicals, see section 7.4.1 (Carrying and dispensing corrosive liquids). Chemicals should be placed in a secure position in the laboratory before they are used. Technicians may need to enter a laboratory before the previous lesson ends and it would be unwise to leave these materials in such a position that the pupils present could interfere with them. Dispensing the It is good practice for technicians to place the relevant Hazcards with the chemicals that chemicals are handed out. This means that, if an accident should occur in a lesson, remedial advice is very quickly obtained and time is not wasted. They can be printed off the Science Publications CD-ROM. If activities involve many different solids, it is best not to have the reagent bottles on display; the contents can easily become contaminated with other chemicals, eg, by pupils mixing up the spatulas or putting the wrong tops back on the bottles. Solids can be placed

1 Formerly Merck Ltd and BDH Ltd. Mainly chemistry 1304 © CLEAPSS 2004

in dry 100 ml, labelled beakers which are placed on trays or on oil-tempered hardboard mats to protect the bench from spills. Specimen tubes could also be used; see Specimen (sample) tubes or vials in section 9.11.4. Solutions are best dispensed from convenient-sized bottles (with hazard warnings), placed on trays to catch any spills. Pupils should not dispense chemicals from a Winchester bottle. Solutions or liquids with a TOXIC or CORROSIVE vapour should be dispensed from bottles on trays in a working fume cupboard. Clothes For laboratory work, clothes (eg, ties, scarves and sleeves) must not hang freely. They can soak up chemicals from spills and some loose-fitting synthetic fabrics are flammable. Long hair should be tied back. Technicians and teachers should consider wearing substantial footwear which is easy to remove if a liquid should spill over them; buckles are particularly difficult to undo quickly. Open-toed footwear is not advisable. Pupils carry around heavy bags containing books, sports gear, etc. As these can lead to clutter in the laboratory, it is essential to find space for them so that only the minimum number of books is on the bench and no bags block gangways or present a trip hazard.

13.1.3 During the activity Pupils should stand, rather than sit, during practical sessions with chemicals and/or when ! heating so that they can move quickly away from spills etc. There must be no interference with another person’s work, such as altering chemicals or labels. Appropriate eye protection should be worn whenever there is a foreseeable risk to the eyes, as discussed in section 3.2.2 (Eye protection, Safety considerations). This should be enforced from the time that the first pupil leaves the bench to collect some chemicals until the last pupil has finished and washed up. Departmental policy should stress that the importance of wearing eye protection is enhanced if the teacher wears goggles/spectacles during a demonstration or while the pupils are carrying out a practical. Visitors to a laboratory must also wear eye protection if practical work is in session. If safety screens are specified for any procedure, then they should be clean and large enough to protect all pupils and the demonstrator. Bench protection is important when using Bunsen burners, chemicals which stain the surface, eg, iodine solution or silver nitrate and reactions which eject materials, eg, the screaming jelly-baby demonstration. Transferring and Spatulas (one per substance) should be used for transferring solids to and from mixing chemicals containers. Beakers, test tubes etc should never be more than a quarter full after all the reagents have been added. To mix chemicals in a test tube, either use a stirrer or hold the test tube at the top, between thumb and forefinger, and waggle it from side to side, pointing it away from the body and other people. (Never shake the tube violently with a thumb over the end; gases can be evolved which force liquids to spit out and contaminated fingers may not be washed.)

Warn pupils not to touch chemicals with their fingers. m Liquids should be dispensed from a bottle by pouring away from the label. This prevents chemicals damaging the label. The stopper should not be placed directly on the bench but held in the hand. Section 7.4.1 (Carrying and dispensing corrosive liquids) illustrates a useful technique for pouring from a beaker. With some pupils, teat pipettes (droppers) lead to misbehaviour and accidents and, in these circumstances, are best avoided. The top of the bulb of a plastic pipette can be cut off and, by placing the finger over the cut-off bulb, small volumes of liquid dispensed safely. If teat pipettes are used, pupils should also be provided with beakers to fill with water for rinsing the pipette. With adequate notice, Mainly chemistry 1302 © CLEAPSS 2004

technicians can prepare test tubes and other small containers such as vials, with small volumes of solutions which pupils can then mix directly with other chemicals.

Moving beakers of hot liquid from a tripod to a heat-proof mat can lead to scalds. Before any transferring, the Bunsen burner should be removed or switched off. Special beaker tongs are expensive. Another device, silicone-rubber ‘hot hands or fingers’, are available for small containers, though these are also expensive. Holding a beaker with ordinary tongs from the top is safe only if the tips of the tongs are correctly aligned. A cloth or dry paper towel can be used with care. It is best to allow beakers to cool in their own time on the tripod and gauze.

Heating When a substance is heated in a test tube, this should be held with a test-tube holder (not ! chemicals tongs) and pointed along the bench, away from all faces. Highly flammable liquids should never be heated directly in a flame; a beaker of hot water (from an electric kettle) should be used instead. Electric heating mantles should be considered when higher temperatures are required for flammable liquids. Unless there is a specific reason, do not heat liquids or solutions to dryness. See also section 13.2.4 (Heating substances) and section 7.4.2 (Handling flammable liquids) for further details.

Observing To make an observation, pupils must not look directly down on to a reaction. Instead they ! reactions should raise the apparatus level with the eyes or leave the apparatus on the bench and bend their knees.

Smelling The technique is tricky. Pupils should learn it using safe substances, eg, carbon dioxide, ! chemicals ethyl ethanoate, dilute ammonia solution. Only when teachers are sure that pupils have mastered the technique should they be allowed to use anything more hazardous. For some pupils, this will never happen. Test tubes should be held about 30 cm from the face, level with the nose but pointing away. The vapours can then be gently wafted towards the nose with the other hand, the test tube being brought closer if necessary. Vapours must never be inhaled deeply. Alternatively, a deep breath can be taken and then attempts made to smell the gas. Only a little gas is inhaled but the smell can be detected.

Pupils with known breathing difficulties, eg, asthmatics, should

m not be asked to smell any chemical.

Spills and Teachers should tell pupils to report accidents involving spills and breakages im- breakages mediately. Spills should be mopped up quickly as suggested on Hazcards or in section 7.7 (Chemical spills). After a glass breakage, a person should be checked for any cuts or glass in the eye. Pieces of glass should be carefully swept from the floor or bench into a dust pan. Larger pieces of broken glass should be wrapped up in newspaper, so that no jagged edges protrude through, and placed in a special bucket or bin, solely reserved for broken glass and cracked glassware. This procedure protects the cleaners and refuse collectors from cuts. (A lump of Blutac is very useful for picking up tiny pieces of glass, if it is then disposed of safely.) Mainly chemistry 1304B © CLEAPSS 2004

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© CLEAPSS 2004 1305 Mainly chemistry

Laboratory work The school should have a policy about out-of-hours working, eg, any pupils working in a at other times laboratory during lunch time or after school must be under the supervision of a teacher and the head of department must be aware of these arrangements.

If teachers or technicians are working in the laboratory or prep room during the holidays, they should inform others (eg, secretary, caretaker) that they are present. It is unwise to carry out hazardous activities (eg, diluting concentrated sulfuric acid, climbing a ladder) with no other people around to provide aid if there is an accident. A risk assessment should be carried out; this should consider whether other staff nearby would know how to provide appropriate help.

13.1.4 After the activity Science department policy should stipulate that teachers and pupils make sure that materials left for disposal will not cause harm to technicians who, when clearing up on their own later on, might be unaware of any hazard. All glassware should be rinsed out immediately after use by those using the equipment and placed in a convenient position for technicians to collect it. All bottles should have the correct stopper replaced. Sinks should be cleaned, so that no solids remain (especially iron filings). Benches should be wiped over with a damp cloth or paper towel to clean up small spills and to remove any water lying on the surface. Pupils often attempt to rinse pipettes by holding them at right angles to a stream of water m from a tap. It is no wonder that other pupils may then be squirted in the eye with water, or worse. The only safe procedure is to rinse a dropping pipette in a beaker of water. Equipment lists for practical work should specify beakers for water whenever dropping pipettes are requested.

On leaving the Teachers or technicians should be the last to leave a laboratory, observing any policy laboratory such as locking the door. If there are any specific problems, technicians should be informed, as well as the teacher of the class about to enter the room.

Do not forget the cleaners, who need to be warned about any possible problems in a laboratory, eg, the possibility of broken glass in an area where there was a breakage or any special apparatus that must not to be touched.

13.2 Chemical reactions Before any chemical reaction is carried out, an assessment must be made of any risks to ! health the chemicals might cause and how these risks could be minimised. Once a substance has been identified as hazardous to health, perhaps from a catalogue symbol, the Hazcards1 are the starting point of any risk assessment. Warnings, restrictions and precautions should be added to the teachers’ and technicians’ notes, and to the pupils’ worksheets. The experiments discussed below have given rise to particular problems in schools and the details given should help with measures to reduce the risks to acceptable proportions.

13.2.1 Fossil fuel experiments Fossil fuels include coal, oil and natural gas. Coal tar and oil are known to contain polycyclic hydrocarbons containing four to six rings of carbon atoms in their molecular

1 The Hazcard provides a general risk assessment. Schools need to confirm that the procedure used conforms to that on the back of the card; if not, a separate assessment must be made. For post-GCSE work, the book by SSERC, Hazardous Chemicals, a manual for Schools and Colleges, Longman, ISBN 0050032046, may be needed. Mainly chemistry 1306 © CLEAPSS 1992

structure. Similar molecules are found in tobacco smoke, cutting oils, lubricants, soot, etc. If these substances are absorbed into the body, either through the skin or by inhalation on a regular basis (ie daily), then there is a possibility that cancers could form. Risk to the teacher or technician is minimal because these experiments are only done a few times each year and gloves can be worn to prevent skin absorption. Crude oil is known to contain more than 0.1% benzene and, as this substance is now not allowed to be used in schools1, a substitute should be used (see below). Another problem is the risk of fire. With the quantities specified below, the risk is extremely small but vigilance is important throughout these experiments. Advice on dealing with natural gas fires is given in section 4.1.3 (Gas fires).

The distillation of coal The diagram shows the standard arrangement for the destructive distillation of coal. It is ! important to burn off the gases from this reaction as they are potentially harmful.

5 g of coal Boiling tube

Coal gas Heat burning

Water

The reaction can be carried out in a well-ventilated laboratory by responsible pupils in years 10, 11 and above. Watch carefully those children with known breathing difficulties. The smell from the experiment can be cut down considerably by limiting the amount of coal to a maximum of 5 g per group and using a water trap2 to dissolve the sulfur3 gases and ammonia (TOXIC). Some technicians wash the coal beforehand to remove surface dust.

It is good practice to allow time for the reaction to get well established before attempting to light the coal gas which comes out of the side-arm test tube. When the heating is stopped, the bung in the water trap should be disconnected and the trap removed immediately to avoid suck-back. The water often becomes cloudy because colloidal sulfur is formed from the reaction between hydrogen sulfide (TOXIC AND HIGHLY FLAMMABLE) and sulfur dioxide (TOXIC). The presence of ammonia in the water can be demonstrated by adding an equal volume of 1 M sodium hydroxide solution (CORROSIVE), an anti- bumping granule, warming the mixture and detecting the ammonia gas with damp red litmus paper. This is best shown by the teacher using several samples collected from the pupils.

1 COSHH (Amendment) Regulations 1991, SI 1991 No 2431, states that benzene may only be used in ‘industrial processes and for purposes of research, development and analysis’. 2 The addition of a few drops of universal indicator solution adds learning to the experiment. 3 Throughout this section of the Handbook, the new (1992) British Standard spelling of sulphur (and its compounds) has been adopted. Suppliers’ catalogues and textbooks are expected to adopt it progressively. © CLEAPSS 1992 1307 Mainly chemistry

The distillation of crude oil In view of the toxicity of benzene and tars in crude oil, many teachers do not perform this ! important experiment. However, it is possible to use a substitute for crude oil and carry out fractional distillation.

Crude oil The original CLEAPSS substitute for crude oil consists of 50 g of paraffin wax, 50 g of substitutes engine grease, 125 ml of new engine oil and 125 ml of white spirit. This mixture is warmed on a water bath until the wax dissolves. After cooling, 100 ml each of two petroleum ether fractions are added1. If this recipe is used, it is important to use grease free from certain additives, ie sulfurised isobutylenes and molybdenum disulfide; Castrol LM grease is suitable. Otherwise this mixture will give rise to harmful and obnoxious smelling vapours. Vaseline should not be used. A safer alternative (without the most volatile component) consists of the following mixture: liquid paraffin (55% v/v), paraffin (20% v/v), white spirit (11% v/v), petroleum ether (100-120 °C) (4% v/v), petroleum ether (80-100 °C) (4% v/v) and petroleum ether (60-80 °C) (6% v/v). The addition of powdered charcoal produces a black colour. It is safe to heat small samples of this mixture with a Bunsen burner from the beginning of the experiment. The apparatus A suitable version of the apparatus is shown in the diagram; an alternative is to use a boiling tube in which is inserted a two-holed bung through which a delivery tube and thermometer are inserted. It is important that the thermometer bulb is level with the side-arm outlet so that its reading truly reflects the composition of the condensing mixture.

o 0 - 360 C thermometer

Side-arm boiling tube Small test tubes to Hot water collect each fraction Mineral wool + 5 ml of distillation mixture

Mineral wool is used to absorb the liquid and to provide a surface for smooth distillation. An added bonus is that the residue sticks to the wool which can then be hooked out after cooling and placed in a polythene bag for disposal. If petroleum ether (40-60 °C) is present in the sample, then the initial distillation should be carried out using a water bath. The resulting fraction is collected in a small test tube and immediately corked to avoid evaporation and fire hazard2. Fractions can be collected in small test tubes at approximately 50 °C intervals up

1 40-60 °C, 60-80 °C (both HARMFUL AND HIGHLY FLAMMABLE), 80-100 °C and 100-120 °C (both HIGHLY FLAMMABLE) are all available. 2 The boiling tube may be heated with a Bunsen burner only if the sample is small and the low boiling range fraction is omitted. Mainly chemistry 1308 © CLEAPSS 2006

to 300 °C. The height of the fraction in the small tubes indicates the volume of the various fractions. If possible, the equipment should be dismantled immediately after the distillation is over. Because the residues are potentially hazardous, the side-arm boiling tubes are best not cleaned but kept for the next time the experiment is carried out. Although the distillation can be carried out in a well-ventilated laboratory by responsible pupils in years 10 and 11, it is perhaps better done as a demonstration, since it requires a certain amount of experience to choose the moment to change collecting tubes.

‘Cracking’ experiments A typical arrangement for this practical is shown in the diagram. The reactant can be either decane or liquid paraffin. The advantage of the latter is that, with the boiling point being high, it will not vaporise too easily and then condense in the delivery tube.

Procedure The catalyst (usually aluminium oxide) must be heated strongly before any attempt is made to warm the paraffin. Carbon forms on the catalyst turning it black. The gas comes off very quickly when the experiment is working well, so collect enough test tubes of ‘cracked’ gas for further experiments, placing each one in a beaker of water. ‘Suck-back’ of cold water into the hot apparatus is a common problem especially if the heat is removed from the apparatus during the reaction. It may also mean that all the alkane has been used up and the reaction is finished. Avoiding ‘suck- To stop ‘suck-back’, the apparatus should be raised out of the water immediately and back’ heated again. This will expel the water from the delivery tube. The apparatus can then be allowed to cool or placed back into the water to continue the heating. The apparatus should also be raised immediately the reaction has finished. The use of a Bunsen valve (see diagram) on the end of the delivery tube sometimes stops ‘suck-back’ occurring.

Testing the Experimental worksheets often ask for the ‘cracked’ gas to be tested with bromine for product unsaturation, ie double bonds. As only small amounts of alkenes are formed in the experiment, only 1 ml of very dilute bromine water (a very pale orange solution) should be used; otherwise gas is displaced from the test tube and not enough gas is present to Mainly chemistry 1310 © CLEAPSS 1992

react with the bromine. A stopper should be attached before shaking the test tube. Bromine in 1,1,1-trichloroethane1 (HARMFUL) is not recommended.

13.2.2 Gas preparation Gases can give rise to particular hazards ranging from unpleasant smells to explosions; breathing difficulties and faintness can be caused by their inhalation. Consequently, whenever gases are prepared in school laboratories, steps must be taken to control them. This diagram shows the several stages which may be required to prepare a sample of a gas for a chemical investigation.

Collection of the gas over water Reaction to prepare the gas Collection of the Drying gas by upward or downward delivery

Making a solution of the gas in water

The reaction Apparatus The diagram shows the traditional apparatus for preparing gases.

The tube must end below the level of the liquid

Table 13.1 Amounts of reagents to make 1 litre of various gases

Gas Hazard Reagents Minimum quantities to produce 1 litre classification of gas

1 See the HAZCARD for bromine. © CLEAPSS 1992 1311 Mainly chemistry

Ammonia Toxic 1) 0.880 ammonia (IRRITANT) is heated 2.1 ml of 0.880 ammonia. alone.

2) Ammonium chloride (HARMFUL) and 2.3 g of ammonium chloride. excess calcium hydroxide (with a little water) are heated together. Carbon Highly flammable Concentrated sulfuric acid (CORROSIVE) is 2.9 g of sodium methanoate and at least monoxide and toxic slowly added to sodium methanoate. 2.5 ml of concentrated sulfuric acid. Carbon - 2 M hydrochloric acid is slowly added to an 42 ml of 2 M hydrochloric acid. dioxide excess of calcium carbonate (marble chips). Chlorine Toxic 5 M hydrochloric acid (IRRITANT) is slowly 51 ml of 14 % sodium chlorate(I) solution added to sodium chlorate(I) solution and at least 17 ml of 5 M hydrochloric (CORROSIVE). acid. Dinitrogen - 20% hydroxyammonium chloride solution 5.8 g of hydroxyammonium chloride in monoxide (HARMFUL) is added to a warmed 50% solution 30 ml of water and 81 g of iron(II) of iron(II) ammonium sulfate in water. ammonium sulfate in 160 ml of water. Hydrogen Highly flammable 3 M hydrochloric acid is added to excess zinc 28 ml of 3 M hydrochloric acid. and 1 g of hydrated copper sulfate. Hydrogen Toxic Concentrated sulfuric acid (CORROSIVE) is 4.1 g (2.1 ml ) of sulfuric acid. chloride slowly added to excess sodium chloride.

Hydrogen Highly flammable 2 M hydrochloric acid is added to excess 42 ml of 2 M hydrochloric acid. sulfide and toxic iron(II) sulfide (HARMFUL). Nitrogen - A mixture of sodium nitrite (TOXIC AND 2.9 g of sodium nitrite. OXIDISING) is warmed gently with excess ammonium chloride (HARMFUL). Nitrogen Toxic Concentrated nitric acid (CORROSIVE) (70%) is 8 ml of 70% nitric acid. dioxide added to an excess of copper turnings. Nitrogen Toxic 1) 35% nitric acid (CORROSIVE) Is added to 30 ml of 35 % nitric acid. monoxide an excess of copper turnings. 2) 50% sodium nitrite solution (TOXIC AND 2.9 g of sodium nitrite in 6 ml of water OXIDISING) is added to iron(II) sulfate with 12 g of hydrated iron(II) sulfate in (HARMFUL) dissolved in 5 M hydrochloric enough acid to cover the crystals. acid (IRRITANT). Oxygen - 20 vol hydrogen peroxide is slowly added to 50 ml of 20 vol hydrogen peroxide. manganese(IV) oxide powder (HARMFUL). Sulfur dioxide Toxic 2 M sulfuric acid (IRRITANT) is added to 21 ml of 2 M sulfuric acid. excess sodium sulfite and warmed.

The thistle funnel tube must be lower than the surface of any liquid in the flask, otherwise the gas will escape out of the thistle funnel. If the reaction accelerates rapidly, the gas pressure in the flask can force liquid up the thistle funnel, resulting in possible splashes of corrosive liquids from the top of the thistle funnel.

The apparatus in the following diagram produces a steady stream of gas1 because control of the reaction is more sensitive. Dead space is reduced by using a Pyrex Büchner flask. Plastic bottles are also suitable but they will become top-heavy and will need fixing securely to a retort stand.

1 H G Andrew, Safety II, School Science Review, 61 (No 215), 1979, p292. Mainly chemistry 1312 © CLEAPSS 1992

Andrew’s gas generator

Separating funnel Rubber tubing

A liquid reagent

Buchner flask The other reagent

Estimating It is important to estimate the volume of gas that is required, otherwise an experiment quantities can get out of hand (and so will the class !). For instance, the preparation of too much nitrogen dioxide has led to laboratories being evacuated. Table 13.1 can be used to find how much reagent is required to produce 1 litre of gas at room temperature and pressure. For example, if five 250 ml gas jars of chlorine are to be filled (in a fume cupboard, of course), Table 13.1 shows that 64 ml of 14% sodium chlorate(I) solution (CORROSIVE) are required, plus another 10 to 15 ml of solution to allow for dead space and small leakages. Always consult the Hazcard for a particular gas to obtain further information about the reagents used and a general risk assessment.

Drying gases Apparatus There is no point in drying a gas if it is going to be collected over water in the next stage ! The following diagram shows two arrangements for drying a gas, depending on the physical nature of the absorbent. It is imperative that any filler in the U-tube is loosely packed, otherwise there will be a pressure increase in the apparatus containing the reaction mixture resulting in a possible explosion ! With liquids, the delivery tubes must be connected the right way round: otherwise the drying agent will be forced into the gas collecting apparatus.

Wet gas Dry gas in out

Wet gas Dry gas in out

Apparatus for drying Apparatus for a gas using a drying a gas loosely-packed solid using a liquid

When drying gases, concentrated sulfuric acid should be used with

! extreme caution, if at all. Suggested drying An effective, safe drying agent suitable for most purposes is silica gel, a solid which will © CLEAPSS 1992 1313 Mainly chemistry

agents need to be heated in an oven at 120 °C before use. If the self-indicating variety is used, the colour change (blue when dry, pink when wet) will show when regeneration of the dying agent is necessary. The gases for which it should not be used for are listed in Table 13.2 below.

Table 13.2 Gases for which silica gel should NOT be used

Gas Suggested dying reagent Ammonia (TOXIC) Soda lime (IRRITANT) or calcium oxide (CORROSIVE). Chlorine (TOXIC) Calcium chloride (solid or saturated solution) (IRRITANT). Hydrogen sulfide (TOXIC AND HIGHLY FLAMMABLE) Anhydrous calcium chloride (IRRITANT).

Hazards with Table 13.3 illustrates hazards that are associated with some traditional drying agents ! drying agents which should consequently be avoided if at all possible. Table 13.3 Problems with drying agents

Drying agent Hazards Concentrated sulfuric acid Many accidents have occurred when concentrated sulfuric acid (CORROSIVE) has been used (CORROSIVE) to dry chlorine (TOXIC) generated from hydrochloric acid, either by the acids being inadvertently confused (see Hazcard for chlorine) or sulfuric acid being sucked back into the reaction flask. Where sulfuric acid would be used and a liquid is necessary, it is considerably safer to use saturated calcium chloride solution (IRRITANT) which is efficient for most purposes. Excess anhydrous calcium chloride may be added to maintain its effectiveness. Calcium oxide (CORROSIVE) Lumps should be broken in the fume cupboard to prevent inhalation of the dust. Magnesium chlorate(VII) Combination of strong dehydrating action and oxidising power gives a serious fire risk if used together with combustible matter. Explosive with some organic liquids. DO NOT USE. Potassium carbonate (anhydrous) Irritant. Phosphorus(V) oxide (pentoxide) Violent reaction with water. (CORROSIVE)

Dissolving gases in water Slightly soluble Carbon dioxide, chlorine (TOXIC) or hydrogen sulfide (TOXIC) come in this category. The gases gas should be passed through water using the same apparatus as described for drying a gas with a liquid drying agent, shown in the previous diagram. The gas needs to be supplied at a steady rate of about one or two bubbles per second, in a fume cupboard if a toxic gas is used. It is often difficult to ascertain whether the right concentration of gas in water is reached without performing a time-consuming volumetric analysis on the solution. It is best to check that the proposed solution behaves as required before it is given to the pupils. The concentration of any of these gases in water will decrease on standing, so the solution should be prepared, as required, immediately before use.

This apparatus should not be used for the very soluble gases, see below. Very soluble Ammonia (TOXIC), hydrogen chloride (TOXIC), nitrogen dioxide (TOXIC), and sulfur gases dioxide (TOXIC) are very soluble in water. Suck-back is the major problem when dissolving these gases in water but the arrangement shown in the diagram avoids the problem. As suck-back starts to occur, the water level in the funnel rises until the level in the beaker becomes lower than the rim of the funnel, at which point the solution in the funnel falls back into the beaker under its own weight. Mainly chemistry 1314 © CLEAPSS 1992

Gas in Rubber tubing

Inverted filter funnel

Beaker of water

Because these gases are all toxic, this operation should always be carried out in a working fume cupboard.

Again, the concentration of any of these gases in water will decrease on standing, so the solution should be prepared, as required, immediately before use. Sulfur dioxide solution will also gradually oxidise to sulfuric acid on standing. Substitutes Commercial soda water or carbonated water from Sodastream are convenient sources of aqueous carbon dioxide solution. CLEAPSS Recipe Cards (1991 Edition) and Hazcards have alternatives for chlorine water and sulfur dioxide solution.

The fountain experiment The old, but popular, demonstration, known as the ‘fountain experiment’, illustrates the ! solubility of either ammonia, hydrogen chloride or sulfur dioxide in water in a spectacular fashion but it needs to be practised before being presented in front of a class. As there is a reduction of gas pressure in the reaction flask, a safety screen must be used along with safety spectacles for all.

The method The traditional, round-bottom flask must be dried (in an oven at 120 °C for 30 minutes) and must then filled with dry gas, either prepared chemically or from a cylinder. Once filled, the bung with the long delivery tube is attached to the flask and is sealed with a clip at position A. The flask is placed in hot water and the clip opened to allow a small volume of the gas to expand out. After a few minutes the apparatus is completed as shown in the traditional diagram below.

Iced-water is poured on to the round-bottom flask which reduces the pressure in the flask and water rises into it. The sudden reduction in pressure caused by the gas dissolving in water allows air pressure to push more water into the upper flask producing the fountain effect. Universal indicator, with a little 1 M hydrochloric acid or 1 M sodium hydroxide solution added1, to alter the colour of the water brings a dramatic colour change to the demonstration.

1 For ammonia, use a little 1 M hydrochloric acid. For hydrogen chloride or sulfur dioxide, use a little 1 M sodium hydroxide solution. © CLEAPSS 1992 1315 Mainly chemistry

Traditional apparatus for A narrow jet A round-bottom the fountain produces a flask containing experiment better the gas fountan effect

Position of the clamp

Position A

Add an indicator The volume of water in to give a colour the conical flask must be change as well! greater than the volume of the round-bottom flask.

A simpler version This version1 consists of three steps as shown in the diagram and the instructions for use are given below.

Gas in

Step 1 Step 2 Step 3

a) Fill the Büchner flask with the gas in the fume cupboard.

b) Attach rubber tubing with a clip on the delivery tube and a squeezed teat on to the Büchner flask.

c) Invert the flask into a trough of water (with universal indicator) and remove the clip from the delivery funnel. This allows water to be drawn into the flask. The fountain should then follow!

1 Adapted from SSERC, Standard Grade Chemistry: Practical Guides, Volume 2, SSERC, 1989, Topic 8/9, p3. Mainly chemistry 1316 © CLEAPSS 1992

Gas collection The three basic methods of gas collection are shown in the diagram.

Bee-hive shelf

Downward delivery Upward delivery Over water (upward displacement (downward of air) displacement of air)

a) Downward delivery (or upward displacement of air) can be used to collect all gases except those that are lighter than air, ie hydrogen, methane, (both HIGHLY FLAMMABLE), and ammonia (TOXIC). The gas should be dried before it is collected. It is important that the delivery tube reaches as far down the gas jar as possible.

b) Upward delivery (or the downward displacement of air) can be used to collect hydrogen, ammonia and methane. The gas should be dried before it is collected. It is important that the delivery tube reaches as far up the gas jar as possible and the gas jar is held securely by a clamp.

c) Collection over water can be carried out with all insoluble or slightly soluble gases ie not ammonia, hydrogen chloride, nitrogen dioxide or sulfur dioxide. Tests to see if Small pieces of test paper are placed at the mouth of the gas jar or flask. Test papers can the gas jar is full be bought commercially (eg, litmus paper) or prepared by soaking filter paper in the reagent. It is recommended that tweezers are used to hold the papers.

Table 13.4 Tests to ascertain whether a gas jar is full

Gas Test Ammonia Place damp red litmus paper at the mouth of the gas jar. When full, the litmus paper turns blue. Chlorine Place damp blue litmus paper at the mouth of the gas jar. When full, the litmus paper turns white. Hydrogen Time the collection of the gas in a boiling tube (50 ml). Add a bung and take the boiling tube to a flame at least 1 metre away from where the gas is collected. If the hydrogen is more than 75% pure, there will be no pop but it will burn with a blue flame. Allow a 250 ml gas jar five times as long to fill as the boiling tube took. Hydrogen chloride Place damp blue litmus paper at the mouth of the gas jar. When full, the litmus paper turns red. Hydrogen sulfide Place damp lead nitrate paper at the mouth of the gas jar or flask. When full, the paper turns black. Sulfur dioxide Use the same test as for hydrogen chloride or place damp potassium dichromate(VI) paper at the mouth of the gas jar. When full, the paper turns blue-green.

13.2.3 Gas reactions Hydrogen Whenever hydrogen is used in experiments, there is danger of explosion from hydrogen-air ! mixtures. The mixture is explosive over a wide range, (4 to 74% hydrogen) and the ignition © CLEAPSS 2005 1317 Mainly chemistry

temperature is about 500 °C. However, the presence of transition metals and/or their oxides lowers the ignition temperature and makes ignition more likely.

Need for good Hydrogen is the lightest gas of all and the molecules move extremely quickly so that, if connections there are any leaks, the hydrogen will escape. This will cause a drop in pressure within the apparatus and allow air to be sucked in, resulting in a mixture which might be explosive. Therefore, it is essential to make sure that all connections are secure before the experiment starts. Hydrogen source Hydrogen is best obtained from a cylinder because a good flow rate can easily be obtained, leading to better purging, and there is no need to dry the gas. See section 9.9 (Gas cylinders) for general advice on handling and storing cylinders. If it is prepared chemically, then follow the methods of the previous section (13.2.2); do not use concentrated sulfuric acid to dry the gas as any accident could result in the acid being sprayed over the audience ! The best drying agent is a U-tube of calcium chloride granules. If the use of a liquid to indicate gas flow is felt to be essential, then glycerol or saturated calcium chloride solution may be used in a wash-bottle of minimum volume instead of the U-tube. In any case, the flame height will indicate the flow rate.

Experimental precautions when burning hydrogen Sensible precautions should be taken to minimise the risks to pupils and teacher as detailed ! below. a) The apparatus volume should be kept to a minimum, so reducing the force of any explosion. b) Connections must be firm. c) Safety screens should be used. d) Eye-protection must be worn by all pupils and the teacher, even when safety screens are in place. e) The audience must be at least two metres away from the apparatus. f) The hydrogen should be obtained from a cylinder if at all possible. Experimental precautions when reducing with hydrogen The precautions listed above for burning hydrogen should also be followed when carrying out ! reductions with hydrogen. Because the apparatus has additional volume, it is even more necessary to ensure that the apparatus is completely purged of air before a Bunsen is brought near to light the hydrogen jet or heat the oxide. (Hot oxide even below red-heat can trigger a hydrogen-air explosion.)

Excess Copper(II) hydrogen Hydrogen from oxide burning a cylinder

Calcium chloride drying agent Heat

Absence of oxygen must be demonstrated positively by the demonstrator collecting a sample of the emerging hydrogen in a test-tube by upward delivery, taking the tube to a Bunsen flame at least one metre away and igniting it. If the hydrogen ‘pops’, the apparatus is not yet purged. When there is no ‘pop’ and/or the hydrogen burns with a pale blue flame1, then the hydrogen

1 This can be extremely difficult to see. Mainly chemistry 1318 © CLEAPSS 2005

can be lit. (It may even be possible for the tube of burning hydrogen to be carried back to the apparatus and used to ignite the hydrogen jet!)

Substitutes for hydrogen when reducing metal oxides If the object of the practical work is to reduce metal oxides or to investigate the formula of an ! oxide, then it would be safer to use an alternative to pure hydrogen.

Methane The explosive mixture range for methane/air mixtures is from 5 to 15% and the ignition temperature is 750 °C, much higher than that for hydrogen. Methane can be used directly as a reducing agent in place of hydrogen but, as it requires a higher temperature to complete the reduction, the process will be slower.

The combustion tube is connected directly to the natural gas supply; the gas tap needs to be only half open. Once the air has been purged out of the system, the gas may be lit and the size of flame reduced to a convenient level. If too much copper oxide is used, the material under the surface is not reduced, so use a minimum amount of oxide and spread it over as wide an area as possible within the combustion tube. The combustion tube will need to be heated to a very high temperature and alternatives to a single Bunsen flame may be required: section 9.10.3 (Special types of burner) makes some suggestions. Methane/ Methane is used to flush the air out of the apparatus and then act as a carrier gas at the hydrogen end of the experiment when the hydrogen production slows down.

Methane (natural gas supply)

Buchner flask Dry the gas with anhydrous calcium chloride Granulated zinc and 3 M hydrochloric acid

Metaldehyde/ A small tablet of metaldehyde is placed in the combustion tube and heated with the metal methane oxide. This has the advantage of increasing the reaction rate for a given oxide temperature but the possible disadvantage of complication. Teachers will decide whether to explain in detail the action of the metaldehyde or simply to describe it as ‘helping the methane to reduce the oxide more quickly’.

Hydrogen reacting with chlorine Explosions of mixtures of hydrogen and chlorine have been initiated by UV light (eg, sunlight, ! fluorescent lighting) or traces of metal oxide. Several explosions with no identifiable cause are recorded. The DES1 and Topics in Safety2 recommend that these mixtures should not be

1 DES Safety Series No. 2, Safety in Science Laboratories, Third Edition 1978, HMSO, ISBN 0112704735, p39. 2 Association for Science Education, Topics in Safety, ASE, 1988, ISBN 0863571042, p47. © CLEAPSS 1992 1319 Mainly chemistry

made in schools but that the reaction should be demonstrated by burning hydrogen in chlorine.

Burning hydrogen Burning hydrogen in chlorine is safe if carried out in a fume cupboard and full in chlorine precautions are taken before lighting the hydrogen jet in air prior to introducing it into a gas jar of chlorine. This should not be attempted unless the hydrogen can be obtained from a cylinder as it is difficult to manage a safe and reliable flame from a generator. Oxygen Reactions in oxygen will occur more vigorously than they would in air. The oxygen should be ! prepared as in section 13.2.2 (Gas preparation), dried by passing it through a U-tube of anhydrous calcium chloride and collected by downward delivery. Oxygen mixture, ie potassium chlorate(V) and manganese(IV) oxide, should not be used to prepare oxygen as it is unstable.

Table 13.4 Reactions of elements in oxygen

Substance Amount and technique Aluminium Use a 2 cm square of foil or a small turning held in tongs. Calcium (HIGHLY A teacher-only demonstration. Choose a turning about 0.5 cm long and scrape it to remove some oxide and FLAMMABLE) make it easier to ignite. Hold it in folded ceramic paper with tongs, ignite in a Bunsen flame and plunge it into the oxygen. Eyes should be shielded from the flash (see details for magnesium below). Carbon Use a lump of charcoal in a deflagrating spoon heated until it glows before insertion into the gas jar. Iron Use steel wool held with tongs in a flame until it glows before insertion into the gas jar. Magnesium Always use ribbon, about 3 cm in length and tied around the deflagrating spoon. The light from the reaction is (HIGHLY very bright. Pupils must cover their eyes before the burning ribbon is placed in the oxygen and then, once the FLAMMABLE) glow is experienced, close their eyes. The demonstrator should look away once the burning ribbon enters the gas jar. Phosphorus A fume cupboard or a well-ventilated room is essential. No heating is required with white phosphorus (TOXIC and HIGHLY FLAMMABLE and a teacher-only demonstration.) Red phosphorus (HIGHLY FLAMMABLE) needs heating to activate it. The white smoke of phosphorus pentoxide (CORROSIVE) can be choking. Sodium A teacher-only demonstration. The sodium (use a freshly cut cube with sides about 0.5 cm) must be free of (CORROSIVE and crust and heated quite strongly on the deflagrating spoon. A black coating appears and then disappears as the HIGHLY carbon from the oil burns away to leave the shining molten sodium burning with a yellow flame. After insertion FLAMMABLE) of the spoon into the gas jar, the yellow flame burns brighter but sometimes goes out, forming a crust over the metal which then stops further reactions taking place. Remove the deflagrating spoon before any water is added and place the cooled deflagrating spoon in a basin of water to destroy any unreacted sodium (warn the technician that the deflagrating spoon may be hazardous). Sulfur Use a piece of roll sulfur, about 0.5 cm across, in the spoon. Sulfur dioxide (TOXIC) is a choking gas and can affect pupils with asthma. Zinc Use a piece of granulated zinc, about 1 cm across.

Basic procedure Samples to burn should be small but never as a fine powder or dust. The usual method involves placing the material on a deflagrating spoon, heating it in a Bunsen flame so that it is reacting with the oxygen in the air and then placing the spoon in the gas jar of dry oxygen. If the substance is held with tongs before it is

plunged into a gas jar, then leather gauntlet gloves should be worn to prevent sparks causing burns. Many teachers will wish to add water to the jar to dissolve the oxide formed and so enable the acidity/alkalinity of the solution to be revealed.

Table 13.4 indicates the safe maximum amounts of substances commonly reacted with oxygen. Eye protection must be worn for all the reactions in the table.

Chlorine See above for the reaction with hydrogen and also section 13.2.5 (Reactions of metals with ! Mainly chemistry 1320 © CLEAPSS 1992

halogens) for other reactions.

13.2.4 Heating substances Substances are heated so that new chemicals can be prepared, information can be obtained ! about the internal structure of materials and reactions can be speeded up. As some remarkable effects can be obtained, it is tempting to make the experiments bigger and more spectacular. A new COSHH assessment is imperative every time the procedure used is different from that which is either in the Hazcards or in a previous assessment.

Eye protection should be worn whenever a substance is heated. ! The heating of materials can be carried out in the open or in containers such as test tubes and beakers.

In the open Improvised Bottle tops and tobacco tins are suitable containers but, prior to any practical work, they containers should be heated strongly in a fume cupboard to remove lacquers etc, used to coat the interior of the lid. The tin lid can be placed on a pipe-clay triangle supported by a tripod which in turn is placed on a heat-proof mat. Flat sheets of mild steel, supported on a tripod, can also be used.

As pupils will always take as much material as possible to heat, the amount used needs to be carefully controlled by supplying the right number of suitably-sized samples. (It is wise to have some spares available.) Metal powders should not be used; metal turnings or small pieces of foil are quite suitable. The action of heat on group I metals and calcium must be demonstrated by the teacher. Some of the problems that arise are summarised in Table 13.5.

In test tubes When a substance is heated in a test tube, the walls confine any gases or vapours emitted and ! can create a hazardous situation.

Test tube holders Test tubes should be held at the open end with a proper test tube holder, not folded paper (which can catch fire) or tongs (test tubes slip out of their grasp). Experience has shown that the ‘clothes peg’ type holder (see diagram) seems to be the best value, but it must be treated as a consumable.

The ‘wire pattern’ is safe only when new, for after some use, the ring is inclined to fall off and corrosion can cause sticking at the joint. The ‘brass strip’ pattern loses its spring and comes off the handle. However, for the cost of one ‘brass strip’ type, two ‘wire patterns’ or seven ‘clothes peg’ types can be bought ! There is also a d-i-y version which consists of a tool clip (Terry clip) on the end of a piece of dowel or a length of aluminium rod. Some schools find this useful but it requires maintenance to ensure the clip does not work loose.

Table 13.5 Heating substances

Substance Amount and technique © CLEAPSS 1992 1321 Mainly chemistry

Ammonium In a fume cupboard, place a heap large enough to cover a 10p piece on a heat-resistant mat and heat the dichromate edge of the pile with a Bunsen flame directed downwards. Alternatively, use the following arrangement as (OXIDISING AND suggested by SSERC. EXPLOSIVE)

Mineral wool Aluminium lid

Hot glass rod

Ammonium dichromate(VI)

Calcium carbonate Place a marble chip (1 × 0.5 × 0.5 cm) on the corner of a tripod and heat strongly with a Bunsen flame to produce calcium oxide (CORROSIVE). The hot chip will glow brightly (limelight).

Calcium (HIGHLY (Teacher demonstration only.) Use a small turning or one granule on the corner of a tripod with a safety FLAMMABLE) screen as this metal spits when it burns. Ignition may be assisted by scraping the surface before heating if the piece is large enough to hold.

Candle wax This goes molten before it catches fire. If too much is used, the molten liquid can slide off a flat piece of metal and on to the bench or the hands of a pupil. Use pieces about 10 mm × 10 mm × 1 mm.

Iodine (HARMFUL) Heat 0.05 g in a fume cupboard. See the Hazcard.

Lithium (CORROSIVE (Teacher demonstration only.) May explode when heated, particularly on wet days. Use safety screens. AND HIGHLY Always test a sample before the lesson and heat no more than a rice-grain sized piece. See the Hazcard. FLAMMABLE)

Plastics See section 13.6.3 (Burning plastics).

Potassium (Teacher demonstration only if at all.) If done, use a safety screen as hot metal is often ejected and heat (CORROSIVE AND no more than a rice-grain sized piece. See the Hazcard. HIGHLY FLAMMABLE)

Sodium (CORROSIVE (Teacher demonstration only.) Use safety screens. Heat no more than a rice-grain sized piece. See the AND HIGHLY Hazcard. FLAMMABLE)

Sugar Causes a toffee-like smell in the room. Limit the amount used.

Sulfur Heat a small amount (0.1 g) in a fume cupboard on a tin lid. Sulfur dioxide is a TOXIC gas and will affect those with breathing difficulties. The sulfur left often makes a sticky mess on the lid. See the Hazcard.

Other substances that can be used are aluminium foil, charcoal, copper turnings, graphite, ice, iron filings, sand, silicon, sodium chloride and zinc granules. Safe use by Pupils must be shown how to heat a substance in a test tube safely. The test tube should pupils be held at an angle and directed along the bench, not pointing towards themselves nor at their neighbour. It is also important to stipulate the amount of material for heating. The instruction ‘heaped spatula amounts’ is too vague and can lead to spills as well as dangerous reactions. A certain mass should be quoted or, if weighing is too time consuming, the test tube should be filled to a designated height. Diagrams on worksheets should reflect the amount used as shown below. Mainly chemistry 1322 © CLEAPSS 1992

He a t He a t A diagram showing a A diagram showing far too realistic amount of solid much solid in a test tube in a test tube - an example that might be copied by pupils

In their eagerness to see what happens, pupils tend to put test tubes directly into a hot Bunsen flame, whereupon the thermal shock leads to the glass cracking. This poor technique can also lead to missed observations. Heating should start in a small flame with both the gas-control tap and the air-hole half open. Later, the gas pressure can be increased and the air-hole fully opened. Heating liquids Heating liquids in a test tube often leads to ‘bumping’, resulting in hot liquids spitting out of the end. For smooth boiling, an anti-bumping granule should be added to the liquid. A specified volume (or height) should be quoted for filling a test tube with liquid; it should not be more than one quarter full. Again, heating should be a gradual process. Test tube arrangements Figure a Figure b

Position of clamp or Mineral wool plug Heat holder

Position of clamp or holder Heat

Figure a shows the arrangement for heating a solid in a test tube. The position of the clamp or test tube holder should be as far from the hot part of the test tube as possible. The addition of a loose plug of mineral wool stops any spitting or dusts being emitted.

Figure b shows apparatus for heating hydrated salts and collecting the water of crystallisation or for heating solids and testing the gases evolved. The solid to be heated is spread out as much as possible over the bottom of the test tube so that all the solid decomposes. © CLEAPSS 1992 1323 Mainly chemistry

SSERC has devised apparatus in which bubble caps (used in home wine-making kits) are attached to boiling tubes, held vertically. The bubble caps contain a liquid reagent which is used to identify the gas or check its pH1.

Position of clamps

Heat Mineral wool plug

Figure c

Figure c shows the arrangement for collecting a gas over water. The use of an inverted measuring cylinder allows the pupil to find the volume of gas obtained on heating a known mass of solid. Gas jars, boiling tubes or test tubes can, of course, also be used to collect the gas. The plug of mineral wool stops the solid or dust from spitting out into the delivery tube and contaminating the water in the trough. Suck-back is a common problem, especially when the reaction is over; see ‘Cracking’ reactions, in section 13.2.1 (Fossil fuel experiments), for further information. Problems Some problems that arise when heating substances in test tubes are summarised in Table 13.6. (Figures a, b and c refer to the diagrams above.)

In beakers, evaporating basins and crucibles Beakers Beakers should be filled only to one third of their capacity if a liquid is to be boiled. Once any dissolved air has been expelled from a boiling liquid, it often begins to ‘bump’, throwing hot liquid out of the container. The addition of a few anti-bumping granules, or some broken porous pot, will produce smoother boiling. Evaporating A round-bottom evaporating basin is notoriously unstable on a gauze; it should be heated basins on a pipe-clay triangle. The flat-bottom variety can be heated on a gauze. Evaporating salt When salt solutions are boiled either in an evaporating basin or a beaker, then, as the solutions crystals begin to appear, hot material spits out of the container. The Bunsen burner should immediately be turned off at the gas-tap, so that the hand is not put at risk, and the apparatus allowed to cool.

To obtain the crystals of a salt from a solution, the solution should be heated in the evaporating basin, with occasional stirring, until the solid appears evenly at

Table 13.6 Heating substances in test tubes

Substance Amount and technique

1 SSERC, Standard Grade Chemistry Practical Guide Volume 1, SSERC, 1988, Topic 2, p5. Mainly chemistry 1324 © CLEAPSS 1992

Ammonium chloride Use 0.1 g of material per group of pupils in a well-ventilated laboratory. It could also be (HARMFUL) demonstrated on a larger scale, eg, 1 g. Otherwise use the fume cupboard. If apparatus in figure a is used, the ammonia diffuses out of the test tube before the hydrogen chloride and can be detected with moist red litmus. Hydrated salts Use the design in figure b. Hydrated salts of cobalt chloride, copper sulfate, magnesium sulfate, sodium sulfate and zinc sulfate are all suitable. Some sulfates decompose at very high tempera- tures producing oxides of sulfur. Iodine (HARMFUL) Use small crystals (0.05 g) in a test tube. Use the arrangement in figure a. Consult the Hazcard. Nitrates Nitrates, except those of sodium and potassium, give off nitrogen dioxide (VERY TOXIC) when heated. Use a maximum of 0.1 g of solid if it is to be heated in a well-ventilated laboratory by pupils. Using the arrangement in figure c cuts down the nitrogen dioxide fumes because they are soluble in water. It is also possible to trap nitrogen dioxide in a test tube surrounded with salt/ice mixture. Lead nitrate(V) (TOXIC) produces lead oxide which reacts with glass. Dispose of the test tubes after use. Whilst sodium and potassium nitrate do not produce toxic gases when heated, the products formed (nitrites) are both oxidising and toxic. Potassium manganate(VII) Fine particles of manganese(IV) oxide (HARMFUL) and unreacted potassium manganate(VII) are (permanganate) (HARMFUL expelled from the test tube and can make pupils cough. Use the arrangement in figure a. and OXIDISING) Sulfur Use 0.05 g. Heat very slowly to get amber molten sulfur. More heating will break up the sulfur molecules to produce the darker plastic sulfur. Sulfur boils at 444 °C and the vapour will catch fire producing a TOXIC gas. It is not worthwhile cleaning tubes used for sulfur but they can be used another time for this activity. If more is used, this experiment must be done in a fume cupboard. Other substances that can be heated in test tubes are calcium carbonate, calcium hydroxide, magnesium carbonate, sodium hydrogen carbonate and zinc carbonate. the sides. Then the evaporating basin should be transferred to a boiling water-bath for further evaporation. This last operation is time consuming and boring to pupils, so, if time and space allow, it is often better to leave the concentrated solutions to cool overnight, labelling the apparatus with the names of chemicals and pupils (together with any safety warnings), and to look at the results during another lesson. Slower evaporation of solvents leads to larger and more beautiful crystals. Crucibles A crucible must be heated on a pipe clay triangle (not on a gauze). Always start the Bunsen on a low flame with the air hole half open and then gradually increase the heating rate. Plenty of time must be allowed in lessons for the crucible and its contents to cool down. Moving hot Beakers can be moved on to a heat-proof mat using beaker tongs, a ‘hot hand’1 or containers wearing ‘Rigger’ gloves (which are a cheap alternative and are available from d-i-y stores and some scientific suppliers). Hot evaporating basins and crucibles are best left to cool on the tripod.

13.2.5 Metals The chemistry of metals is an essential feature of any GCSE and A-level science course. Hazcards must be consulted before any chemical procedure is carried out but the following paragraphs illustrate some of the problems associated with various reactions and possible solutions to them.

Do not mix: ! magnesium powder with ammonium dichromate, silver nitrate or sulfur;

1 This is a moulded silicone rubber hand protector for gripping hot beakers and conical flasks available from most suppliers. © CLEAPSS 1992 1325 Mainly chemistry

aluminium powder with lead oxides or copper oxides.

Reaction of metals with water See also the relevant Hazcards for information about hazardous metals. !

Sodium and Only rice-grain sized samples should be used, removing as much liquid paraffin as potassium possible with a paper tissue. A wide trough filled with cold water must be used, adding a drop of detergent to help to remove any remaining film of oil which covers the metal. The teacher and the class should protect their eyes with goggles and a safety screen should be used. Pieces of molten metal (especially if potassium is being reacted) and concentrated alkali are sometimes ejected from the trough during the reaction. No attempt should be made to constrain the metals, even in a sodium spoon; they must be allowed to roam freely over the surface of the water.

When the demonstration is over, the knife used for cutting the metal, the tile on which the metal is cut and the tissue should be put into the trough of water. This ensures that no alkali metal will react during the clearing up operations which are often carried out by technicians. Calcium and A large excess of water in a 250 or 400 ml beaker should be used, as, otherwise, the heat lithium from the reaction can cause it to accelerate out of control. In these cases it is safe to collect the hydrogen in an inverted boiling tube but these reactions are not a recommended method of preparing the gas.

Reaction of metals with steam

Do not attempt to react sodium, potassium, calcium, lithium or

! powdered magnesium with steam.

The reaction may be carried out with aluminium, zinc and iron with the apparatus shown in the diagram below.

Mineral wool Metal soaked with powder Hydrogen water

Heat

Aluminium, zinc The metal should be heated first and then the Bunsen moved down towards the mineral and iron wool to vaporise the water. Once heating is started, it must be continued or water will suck back from the gas collecting trough. When enough gas has been collected, lift the delivery tube out of the water to prevent ‘suck-back’ before any experiments are carried out on the gas. Magnesium The reaction between magnesium and steam is suitable only for demonstration with a safety screen and with eye protection for both pupils and demonstrator. The apparatus is set up as shown in the diagram below. Again, the metal should be heated first until it just Mainly chemistry 1326 © CLEAPSS 1992

catches fire and then the Bunsen burner moved to the mineral wool soaked in water. The hydrogen gas is ignited as it comes out the end of the glass tube. The boiling tube is ruined by this experiment due to the formation of black magnesium silicates; no attempt should be made to clean it.

Coiled Mineral wool magnesium soaked with ribbon water Hydrogen which can be burnt

Heat

Reaction of metals with dilute acids The most convenient acids to use are 1 M hydrochloric acid, 0.5 M sulfuric acid and 1 M ethanoic acid, which reacts more slowly than the others. Typical reactions are described in Table 13.7. Dilute nitric acid does not react in the same way as other acids; nitrogen oxides (TOXIC) are produced and not hydrogen. Use moderate sized pieces of metal, eg, foil, granules or turnings, not powder, dust or filings (except for iron when filings may be used). The test tube should not be filled more than one- fifth full.

Table 13.7 The reactions of metals with dilute acids

Metal Comments Calcium and lithium These metals react extremely exothermically. Not more than 0.2 g of calcium or 0.02 g of lithium should be used and then only by teachers or senior pupils. Magnesium The reaction is very fast and highly exothermic; the acid could rise out of a test tube. It is better to use ribbon in about 2 cm lengths rather than turnings. Aluminium With dilute hydrochloric acid the reaction with aluminium foil is very slow at first but then it accelerates so that the acid bubbles over and the test tube gets very hot. The foil should be about 1.5 cm × 1.5 cm. Zinc The reaction is quite slow, especially with granulated zinc, but the addition of a small volume of copper(II) sulfate solution produces copper which acts as a catalyst. Iron Lubricating oils on the surface of the iron and impurities in tit (such as sulfides) can lead to some smelly vapours being produced. The solution may be gently warmed to achieve a better reaction. Filtration results in a green solution of iron(II) sulfate. Lead There is no reaction with dilute sulfuric acid. However, with hydrochloric acid, a reaction can be just seen on heating to boiling. When left to cool, white crystals of lead chloride appear.

Do not react, sodium, potassium, or other very reactive metals with

! strong acids. © CLEAPSS 1992 1327 Mainly chemistry

Reaction of metals with sulfur Do not react lithium, sodium, potassium, calcium, magnesium or

! other very reactive metals with sulfur.

Zinc Eye protection and a safety screen must be used. The class should be placed well away ! (demonstration from the reaction. Prepare, in separate containers, 4 g of zinc and 2 g of flowers of sulfur. only) Mix them gently, stirring with a clean spatula. Make a small heap (no larger than a 10p piece) of the mixture on a ceramic-centred gauze. Heat from below and retire to a safe distance. It is best to do this before a break so that the room can cleared of fumes (which may affect children with breathing difficulties) before the next lesson. Copper and Iron The experiments should be carried out in borosilicate test tubes (eg Pyrex) as the reaction is exothermic. For 1 g of sulfur, use 1.75 g of iron or 2 g of copper for each reaction. If too much sulfur is used, sulfur vapour will boil off and ignite at the mouth of the test tube.

Reaction of metals with halogens

Do not attempt:

! potassium, barium and antimony with any halogen; sodium, aluminium and mercury with bromine.

Reactions with chlorine (TOXIC) Dry chlorine gas, made following the instructions in section 13.2.2 (Gas preparation), is ! placed in a sealed gas jar. This should be done in a fume cupboard. Eye protection is important both for the demonstrator and class for all these reactions. Sodium The metal is cleaned, placed on a clean deflagrating spoon (see step 1 below), heated until it just catches fire and inserted into the gas jar of chlorine (step 2).

If an old deflagrating spoon is used, a mixture of iron(III) chloride (IRRITANT) and sodium chloride is produced, giving rise to brown fumes. Care must be taken when disposing of the solid as there may be unreacted sodium present, protected by a crust of sodium chloride. After cooling, the deflagrating spoon must be immersed in a large basin of cold water before any other washing up is done.

Do not taste the product! ! Mainly chemistry 1328 © CLEAPSS 1992

Magnesium About 8 cm of magnesium ribbon is tied to a deflagrating spoon. It is set alight and plunged into the gas jar of chlorine. Alternatively, the deflagrating spoon can be filled with magnesium turnings. These are heated until they glow whereupon the spoon is inserted into the gas jar. Aluminium This interesting chloride (it sublimes at 180 °C and reacts with water) can be prepared as described above, but a purer product is made using the following arrangement.

This preparation must be done in a working fume cupboard. The tube entering the bottle has a large diameter, otherwise the solid would condense in the tubing and cause a blockage. The addition of water to aluminium chloride (CORROSIVE) produces fumes of hydrogen chloride (CORROSIVE). Dutch metal (a The foil is placed in the gas jar of chlorine using tongs. The seal should be replaced copper/zinc alloy) immediately to avoid CORROSIVE zinc chloride fumes escaping. Antimony powder Older text books often describe this reaction: a pinch of antimony powder produces clouds of highly poisonous antimony chloride when sprinkled into a gas jar of chlorine. Topics in Safety1 recommends that antimony should not be used in schools.

Reactions with bromine (CORROSIVE) Iron Eye protection and gloves must be worn for this demonstration. The apparatus shown below should be set up in an efficient fume cupboard, using no more than 1 ml of ! bromine. The iron wool is first heated strongly and then the bromine may be warmed if necessary. Fumes of iron(III) bromide will be produced.

1 Association for Science Education, Topics in Safety, ASE, 1988, ISBN 0863571042, p52. © CLEAPSS 1992 1329 Mainly chemistry

Reactions with iodine (HARMFUL) Aluminium Iodine and aluminium mixtures do not react until moistened (or are in the presence of a salt with water of crystallisation) when a vigorous reaction starts. A fume cupboard must ! be used and iodine will condense on to the walls of the cupboard during the reaction, so it must be left on until signs of discoloration have disappeared. Mercury A fume cupboard is essential as warming is necessary to start the reaction and mercury vapour will be emitted. Only 1-2 g of iodine and one drop of mercury may be used.

Tin To make tin(IV) iodide, SnI4 (melting point 143.5 °C), 4 g of iodine is dissolved in 10 ml of 1, 1, 1-trichloroethane (HARMFUL) in a pear-shaped flask equipped with a reflux condenser as described in section 13.7.2 (Refluxing). An excess of tin (1.2 g) is added and the flask is heated until the reaction begins. When no more iodine vapour is produced in the condenser, the reaction vessel is cooled and the orange crystals filtered off.

Thermit reaction Please see CLEAPSS Guides L195, and SRA026 Magnesium reduction of copper(II) oxide The following reaction is safe if the guidelines below are followed. Do not try to extend the ! reaction to other metal oxides. It is best done at the end of a lesson as there will be fumes of magnesium oxide produced. Pupils and teacher must wear eye protection and stand as far from the reaction as possible. Safety screens should also be used. The amounts of each chemical used must be small, eg, enough to cover a 10p piece. The class should be standing at the back of the laboratory before the chemicals are mixed. The teacher, wearing a face mask, should gently mix the magnesium powder and copper oxide and place them in a crucible on a pipe-clay triangle, supported on a tripod, placed over a large sheet of hardboard to protect the bench. A Bunsen burner flame is then put under the crucible and the teacher retires to a safe position. A very vigorous reaction takes place after about 1 - 2 minutes (be patient!). If nothing happens, do not return to the apparatus immediately but leave it for up to 15 minutes. When the reaction is over or to be abandoned, the gas should be turned off at the main stopcock if it is at all possible, and the apparatus allowed to cool before the crucible and contents are placed in water.

13.2.6 Acids, bases and salts

Concentrated acids Sulfuric acid Schools are not advised to keep fuming sulfuric acid, (oleum), which is a solution of sulfur ! trioxide in concentrated sulfuric acid. Nitric acid Commercial concentrated nitric acid (CORROSIVE) is 70% w/v. Nitric acid made from sodium nitrate (OXIDISING) and concentrated sulfuric acid (CORROSIVE) will be nearly ! 100% pure (OXIDISING and CORROSIVE).

Nitric acid preparation Precautions This experiment must be carried out in a fume cupboard, using all-glass apparatus; cork and rubber are chemically attacked by nitric acid. The operator should wear eye ! protection and suitable nitrile or rubber gloves; both concentrated sulfuric acid and nitric acid are CORROSIVE. It is imperative that the correct chemicals are used. Mistakes have been made! The preparation This experiment used to be done in a glass retort but Quickfit apparatus would be equally suitable (see the diagram overleaf). 10 g of sodium or potassium nitrate(V) is placed in ! the pear-shaped flask and 10 ml of concentrated sulfuric acid in the dropping funnel. The acid is added to the flask slowly and the flask is warmed gently. The nitric acid will be distilled over, along with brown nitrogen dioxide gas (TOXIC), a product of the Mainly chemistry 1330 © CLEAPSS 2004

decomposition of nitric acid.

Reaction with sawdust This is the one reaction that it is reasonably safe to do with pure nitric acid. A good working fume cupboard is imperative. Wear suitable gloves; skin turns yellow on contact with the acid. In the working fume cupboard, 2-4 ml of pure nitric acid is added to a heap of dried sawdust on a heat-proof mat. After a while, fumes of nitrogen dioxide will appear as the acid oxidises the cellulose in the wood. 100% nitric acid should not be retained for any further experiments so dispose of it by adding ! it slowly to water, with stirring and while adding anhydrous sodium carbonate (IRRITANT). The resulting solution may be poured down the foul-water drain with plenty of water. Preparation of nitric acid

Concentrated alkalis Most concentrated alkalis are CORROSIVE solids. Absorption of water and carbon dioxide because of poor storage (eg, leaving the screw tops loose) can produce solid blocks of the chemical in the bottle. Breaking down a solid block of sodium hydroxide is a risky procedure. Instead, seriously ! consider disposing of the chemical and purchasing fresh material. Ammonia solution and its organic substituted homologues, such as ethylamine solution, are liquids. The concentration of these solutions decreases during poor storage. Screw tops should be securely tightened otherwise white solid deposits occur on the bottles in the store.

Preparation of salts

Chemical problems CLEAPSS has received several calls describing the following activity. “A class of pupils neutralised sulfuric acid in various ways (using excess zinc metal or zinc or copper oxide / carbonate). However, when pupils evaporated the water, clouds of choking white fumes were given off.” © CLEAPSS 2004 1329 Mainly chemistry

There are at least two possible causes for this incident, in which TOXIC and CORROSIVE sulfur dioxide and/or trioxide gas or sulfuric acid vapour are produced. If pupils have not neutralised the acid completely, sulfuric acid (still present in the filtrate) may be concentrated, vaporised and finally decomposed during any heating. Instructions in some publications for making zinc or copper(II) sulfate are misleading because they omit, or do not stress strongly enough, the need to warm the reactions to achieve completion, when using the oxides. The safest procedure is to add the oxide a little at a time, warm the solution and wait for the oxide to dissolve visibly before adding more. If zinc granules are used, the reaction is also particularly slow. In that case, it is better to add small amounts of zinc powder to warm 1 M sulfuric acid. (The reaction can be accelerated by adding a few drops of copper(II) sulfate solution but this might be too much of a chemical distraction. However, the zinc granules could be pre- dipped in copper(II) sulfate solution so the pupils do not notice.) If carbonates are used in place of oxides, there is no need to warm the reactants. Also, never heat solutions of zinc or copper(II) sulfate (and many other salts) to dryness, except over a water bath. Many sulfates decompose, producing toxic fumes, if heated strongly. In preparing salts, pupils can concentrate a solution by evaporating some of its water but then leave it to crystallise slowly in a safe place - this gives much better crystals. If pupils cannot be trusted to stop heating when some liquid remains, do not allow them to attempt heating at all. Much depends on which salt is used but, of those likely to be met in school science, only potassium and sodium salts can be safely evaporated to dryness. Even then, there is a real risk of hot particles spitting as the last traces of water evaporate. Table 13.7A discusses some of the problems that arise with each of the salts that are commonly prepared.

Table 13.7A Preparation of various salts

Reaction Salt Comments Insoluble metal Copper(II) In general, oxides need extra heating while carbonates react at oxide or carbonate sulfate room temperature. However, with carbonates there is a lot of froth- with an acid ing and the chemistry is more complicated. Saturated copper sulfate is 1.2 mol dm-3 at room temperature. An example of how this proc- edure could be carried out with small volumes of acid is discussed below. Metal with an acid Iron(II) sulfate There is an obnoxious smell produced when the iron is warmed with the acid, possibly caused by impurities in the iron. Placing a cotton wool or Superwool1 plug in the neck of the boiling tube reduces the smell. An example of how this procedure could be carried out with small volumes of acid is also discussed below. Do not be concerned if the reaction has not completely stopped fizzing. Iron(II) sulfate solutions are more stable to air oxidation in acidic rather neutral or alkaline solutions. Magnesium This reaction does not need heat. In fact, the reaction needs cooling sulfate down in a water bath at room temperature. The hydrated salt conv- erts from a glassy crystal to a powder if left in the atmosphere. Zinc sulfate This needs a lot of heat to complete the reaction but the addition of copper(II) sulfate solution produces copper which then acts as a catalyst. The hydrated salt converts from a glassy crystal to a pow- der if left in the atmosphere.

1 See section 9.11.3 (Asbestos substitutes) for details of this safer mineral fibre. Mainly chemistry 1332 © CLEAPSS 2004

Soluble carbonate Ammonium The pH of this salt solution is These salts need a pH meter or or hydroxide with sulfate about 4.5 so the addition of 1 M an indicator to assess the end- an acid ammonia solution to 1 M sulfuric point. The indicator then needs acid can be stopped after the pH to be removed using activated just exceeds 4.5. Ammonia carbon. An example of how this evaporates away when the solu- procedure could be carried out tion is concentrated. to prepare potassium nitrate Potassium - with small volumes of solutions nitrate is discussed below. Titration methods are not suit- Sodium - chloride able at the concentrations used in these procedures. Sodium sulfate The hydrated salt converts from a glassy crystal to a powder if left in the atmosphere. Table 13.7A Preparation of various salts (continued)

Reaction Salt Comments Precipitation of an Barium sulfate The precipitate is very fine. Heating to boiling begins to coagulate insoluble salt the solid but even then some particles pass through the filter paper. This can lead to an interesting discussion on pore size. Although filter paper can be obtained with a smaller pore size, the rate of filtration is slower and the paper becomes quickly blocked up. An example of how this procedure could be carried out to prepare barium sulfate with small volumes of solutions is discussed below. Lead Lead chromate(VI) (TOXIC) is used extensively, for example, to paint chromate(VI) yellow lines on the road. There is only limited evidence that it is a carcinogen (unlike potassium chromate and some other chromate pigments), probably because it is much less soluble. It could be possible for classes at key stage 4 to prepare and isolate lead chromate(VI). CLEAPSS can provide a special risk assessment for this procedure. Logistical problems Salt preparation places heavy demands on the use of equipment, not only during the lesson because samples of solutions are often left out in evaporating basins or other containers in the laboratory for a week. During this time, the chemical can undergo chemical changes, [iron(II) salts oxidise to iron(III)] or water of crystallisation is lost (sodium sulfate-7-water goes powdery). The procedures also take time to complete. Filtering and boiling are long and boring activities, especially if volumes in excess of 20 ml are used. Preparation of copper(II) sulfate-5-water Procedure Part 1 • Wear eye protection. • Place a boiling tube in a 250 ml beaker, half-filled with boiling hot water from a kettle. • Pour 15 ml of 1.2 M sulfuric acid (IRRITANT) into a boiling tube. • Weigh out between 1.8 and 2.0 g of copper(II) oxide (HARMFUL). • Divide the copper(II) oxide into four portions and add one portion to the hot sulfuric acid. • Remove the boiling tube and agitate it. Return it to the hot water. • When the solution clears, add the next portion. • With the 3rd portion, replace the now-cooling water in the beaker with more boiling water from the kettle. • After the 4th portion, the solution should not clear. Leave the mixture for 5 minutes, agitating it at times. © CLEAPSS 2004 1331 Mainly chemistry

• Set up a filter funnel in a 100 ml conical flask with a folded piece of Whatman No. 1 filter paper. • Filter the contents of the boiling tube. • Once filtered, place the conical flask on a tripod and gauze and heat. Boil for 3 minutes only. (NEVER ALLOW THE SOLUTION TO BOIL DRY.) • Using a pen, write your initials on the base of a 55 mm-diameter plastic Petri dish. • Turn off the Bunsen burner. Once the liquid has stopped boiling and spitting, wrap a damp cloth or dry paper towel around the neck of the conical flask and place the flask on a bench mat. (TAKE CARE.) • Pour the hot contents into the Petri dish. (TAKE CARE.) • Do not put the lid on the dish. Leave for 5 minutes - you might see crystals appearing. • Place the Petri dish on a tray. This will be stored until the next lesson. Part 2 • Weigh a specimen tube. • Use a spatula to transfer the contents of the Petri dish into the specimen tube. • Reweigh the specimen tube. • What mass of copper(II) sulphate-5-water was obtained? Disposal • The filter paper and its contents can be placed in the normal waste. • The prepared copper(II) sulfate can be placed into a labelled bottle and recycled for further use, eg, electrolysis or displacement reactions.

Preparation of iron(II) sulfate-7-water Procedure Part 1 • Wear eye protection. • Place a boiling tube in a 250 ml beaker, half-filled with boiling hot water from a kettle. • Pour 15 ml of 1.4 M sulfuric acid (IRRITANT) into the boiling tube. • Weigh out between 1.8 and 2.0 g of iron powder. • Add the iron to the acid, insert a plug of cotton wool or Superwool into the neck of the boiling tube and place the boiling tube in the beaker of hot water. • When the reaction subsides, place the hot beaker on a tripod and gauze and heat the water with a Bunsen burner flame until the water boils. Turn off the Bunsen burner. • Set up a filter funnel in a 100 ml conical flask with a folded piece of Whatman No 1 filter paper. • When the reaction in the boiling tube has subsided, use a damp cloth or dry paper towel to lift the boiling tube out of the hot beaker. (TAKE CARE.) • Filter the contents of the boiling tube. • Once filtered, place the conical flask on a tripod and gauze and heat to boiling. • Boil for 1 minute only. (NEVER ALLOW THE SOLUTION TO BOIL DRY.) • Turn off the Bunsen burner. Once the liquid has stopped boiling and spitting, wrap a damp cloth or dry paper towel around the neck of the conical flask and place the flask on a bench mat. (TAKE CARE.) • Using a pen, write your initials on the base of a 55 mm-diameter plastic Petri dish. • Pour the hot contents into the Petri dish. (TAKE CARE.) • Do not put the lid on the dish. Leave for 5 minutes - you might see crystals appearing. • Place the Petri dish on a tray. This will be stored until the next lesson. Part 2 • Weigh a specimen tube. • Use a spatula to transfer the contents of the Petri dish into the specimen tube. Mainly chemistry 1334 © CLEAPSS 2004

• Reweigh the specimen tube. • What mass of iron(II) sulfate-7-water was obtained? Disposal • The filter paper and its contents can be placed in the normal waste. • The prepared iron(II) sulfate can be placed into a labelled bottle and recycled for further use, eg, precipitation or displacement reactions.

Preparation of potassium nitrate Procedure Part 1 • Wear goggles or a face shield. • Place 10 ml of 2 M potassium hydroxide solution (CORROSIVE) in a 100 ml beaker (Beaker 1). • Add 2 drops of methyl orange. The solution should turn yellow. • Add 1 ml of 2 M potassium hydroxide solution (CORROSIVE) to a test tube (Test tube X) followed by a drop of methyl orange to identify the solution, and 10 ml of water to dilute the solution. • Place 10 ml of 2 M nitric acid (CORROSIVE) in another beaker (Beaker 2) and add 2 drops of methyl orange. The solution should turn red. • Add 1 ml of 2 M nitric acid (CORROSIVE) in another test tube (Test tube Y) followed by a drop of methyl orange to identify the solution, and 10 ml of water to dilute the solution. • Add the contents of Beaker 1 to Beaker 2. Stir the mixture and examine the colour of the solution. Is it red or yellow? • If it is red, use Test tube X. If it is yellow, use Test tube Y. • Use a dropper pipette to add drops of the chosen reagent to Beaker 2 until an orange colour is obtained. Remember to stir the solution each time to ensure mixing. • Add 0.1 g of activated charcoal. • Set up a tripod and gauze with a Bunsen burner beneath. Heat the beaker and its contents to boiling. • Set up a filter funnel in a 100 ml conical flask with a folded piece of Whatman No 1 filter paper. • Turn off the Bunsen burner. Use a damp cloth or dry paper towel to hold the beaker and filter the hot solution. (TAKE CARE: THE EQUIPMENT IS VERY HOT.) • Relight the Bunsen burner and boil the contents of the conical flask for 5 minutes. (NEVER ALLOW THE SOLUTION TO BOIL DRY.) • Turn off the Bunsen burner. Wrap a damp cloth or dry paper towel around the neck of the conical flask and place the flask on a bench mat. (TAKE CARE: THE EQUIPMENT IS VERY HOT.) • Using a pen, write your initials on the base of a 55 mm-diameter plastic Petri dish. • Once the liquid has stopped boiling and spitting, pour the hot contents into the Petri dish. (TAKE CARE: THE EQUIPMENT IS VERY HOT.) • Do not put the lid on the dish. (Crystals will not form quickly.) • Place the Petri dish on a tray. This will be stored until the next lesson. Part 2 • Weigh a specimen tube. • Use a spatula to transfer the contents of the Petri dish into the specimen tube. Mainly chemistry 1333A © CLEAPSS 2004

• Reweigh the specimen tube. • What mass of potassium nitrate was obtained? Disposal • The filter paper and its contents can be placed in the normal waste.

Preparation of barium sulfate Procedure Part 1 • Wear eye protection. • Place 10 ml of 0.5 M barium chloride solution (HARMFUL) in a 100 ml beaker. Place this on a bench mat. • Place a 25 ml measuring cylinder on the bench mat and add 15 ml of 0.5 M magnesium sulfate solution. • Add the contents of the measuring cylinder to the beaker. Stir the mixture with a stirring rod. • Place the beaker on a tripod and gauze and heat the slurry to boiling. (TAKE CARE AS THE SOLUTION WILL ‘BUMP’ WHEN IT REACHES BOILING POINT.) • Fold a filter paper into four sections, place it in a funnel in the usual manner and dampen it with a little water so that it sticks to the funnel. Place the funnel into a labelled 250 ml conical flask. • Turn off the gas supply. Hold the beaker with a damp cloth or dry paper towel and pour the slurry of barium sulfate from the beaker, down the stirring rod into the funnel. • Add about 5 ml of water to the beaker, stir and pour this into the filter paper. • Leave the material to filter and dry at least overnight. Part 2 • Wear eye protection. • Find the mass of a specimen tube. • Remove the filter paper from the funnel and crush the dried product by folding the filter paper and pressing it with a spatula, taking care to prevent dust escaping. • Angle the filter paper above the specimen tube and tap the paper with a spatula to shake the powder into the tube. • Find the mass of the specimen tube and the barium sulfate. Disposal • The filter paper containing barium sulfate can be placed in the normal waste. • Any contaminated glassware should be rinsed with tap water before cleaning. • The filtrate can be poured down the sink and flushed with water.

13.3 Gas cylinders [The information previously included here has been updated and moved to sections 9.9 and 11.2.]

13.4 Gas syringes See also information on gas syringes in section 10.10.4 (Syringes). Mainly chemistry 1334hello © CLEAPSS 2004

In the past, gases were handled in glass tubes trapped over mercury. In schools, gas syringes are much more convenient but they still need care and maintenance. Plastic syringes Plastic syringes are robust and relatively inexpensive but have various disadvantages for handling gases, depending on the design. They should be stored with the barrels and pistons separate, to prevent them sticking together. They usually need to be used with a manometer or other atmospheric pressure indicator. In some cases the markings rub off; this can be overcome by scratching the necessary marks with a sharp point when the syringe is new.

This page has been deliberately left blank in order to maintain the existing pagination. The text resumes on page 1335.

Mainly chemistry 1334A © CLEAPSS 2004

Glass syringes Glass syringes are more useful, but must be very carefully handled and stored to prevent breakage. The plastic bags in which the syringes arrive should not be discarded: they are necessary to protect the syringes from dust, which gets between the ground glass surfaces and scratches the glass. Syringes with ‘Luer’ fittings (for needles) are less expensive than those made specially for schools but they need smaller bore tubing than is generally in use in chemistry laboratories. Such tubing can be used as a small sleeve on the Luer fitting and the normal tubing will fit over this; alternatively, a slightly longer piece of small-bore tubing can be connected to the normal tubing via a plastic combination connector, as illustrated in section 10.9.5 (Tubing connectors), available from plastic laboratory-ware suppliers. Washing and Glass syringes should be washed frequently, ideally after each use, to maintain their free cleaning running and gas-tight properties. For the same reason, the pistons should not be put down on the bench as they might pick up small particles. They should be washed in distilled water, then industrial methylated spirit (HIGHLY FLAMMABLE)1. When they are clean and dry, they should be re-assembled, with the numbers on the pistons matched to those on the barrels. Breakages The most fragile part of the syringe is where the nozzle joins the barrel and great care must be taken when fitting tubing. It is advisable to attach a piece of string between the barrel of the syringe and the handle of the piston. The string should be long enough for the piston to give full scale readings but short enough to stop the piston falling out of the barrel. Testing syringes To test syringes for friction and leaking, two syringes are connected together by a short length of tubing. One syringeful of air is passed to and fro and the readings are noted. The piston moving outward should not be touched at all until all the gas has been passed into it. After noting the reading, the piston may be rotated, ‘feeling’ for atmospheric pressure. Another test, to indicate how gas-tight the syringes are at pressures above atmospheric pressure, is done by compressing the gas from 100 ml to 90 ml and holding for ten seconds. After releasing the piston the volume is read again. A new syringe should show no readable difference but, after some use, an error of up to 2% can be expected.

13.4.1 Gas syringe experiments Gas Syringe Experiments by Martin Rogers and published by Heinemann contains many ! examples of gas syringe experiments. The book is now out of print but old copies may still be found in laboratories. Please read this section carefully before attempting any of the experiments.

1 Then rinse with propanone (HIGHLY FLAMMABLE) if they are required again immediately. Mainly chemistry 1336 © CLEAPSS 2005

Molar mass of A measured volume of volatile liquid is injected into a 100 ml gas syringe and allowed to volatile liquids1 come to equilibrium at atmospheric pressure. The amount of volatile liquid that is injected from the hypodermic syringe needs to be calculated carefully to ensure that the final volume reading is below 100 ml. Rate of reaction Again, a preliminary calculation is necessary as the final volume of gas obtained must be experiments measurable if these experiments are to produce meaningful results. The total theoretical volume of gas should be worked out from a known amount of reagents. Percentage The tube containing copper turnings (no need to use the powder) should be made out of oxygen in borosilicate glass or silica. It is still necessary to ‘plug’ the ends of the tube with mineral atmosphere wool to avoid copper turnings being blown into the gas syringes. Gas reactions Potentially explosive gas mixtures must not be used. When filling the syringe with a particular gas, flush it out three or four times before finally filling it. This is a particularly important point in combustion experiments.

Explosions have occurred in experiments involving a mixture of

! hydrogen and chlorine despite all precautions, and hence this mixture should not be used.

Combustion Safety screens must be fitted around the combustion pipette. The cylinders must not be experiment directed at the class who should be 3 m away. The filament is warmed to bright red heat before the combustion gas is passed slowly into the combustion pipette. The flame should be about 1 cm long. If no flame appears, the pressure must be removed from the gas piston and the current to the filament switched off. The apparatus must be cleared of combustion products before the experiment is restarted. Ethyne (acetylene) must, of course, not be used in such experiments. Use at high Sudden changes of temperature must be avoided and the maximum temperature kept as temperatures low as practicable. Most glass syringes leak quite badly at over 150 °C. Use of powders When a gas is passed over a powder (eg, copper(II) oxide), great care is necessary to prevent powder entering the syringes. A small plug of mineral wool will stop the passage of dusts and powders getting into the barrel.

13.5 Molecular models

1 Revised Nuffield Advanced Science, Chemistry Students Book 1, Longman, 1984, ISBN 0582353610, p51. © CLEAPSS 1992 1337 Mainly chemistry

Knowledge of the three-dimensional arrangement of atoms and ions in substances has helped scientists in the past to make important discoveries, eg, the structure of enzymes, stereospecific polymers and the structure of Buckminsterfullerene, the third allotrope of carbon. The National Curriculum demands that pupils should be able explain some of the basic properties of materials such as metals, simple ionic salts, simple organic molecules and polymers, using three-dimensional models. At A-level, more detailed studies of crystal structures, organic and inorganic molecules are required, including the important concept of chirality. An article by Buglass1 suggests an organic chemistry teaching program based on commercial models. For details of the use of molecular models in biology, see section 15.3.2 (Types of models).

13.5.1 Types of molecular model Models for showing molecular structure can be divided into three variants, tangential, skeletal and space-filling, while the PEEL models are quite different. Tangential These are made of spheres, sometimes of different sizes, assembled so that they touch models each other. They are best used to represent the structure of metals (all the spheres have the same radius) and ionic solids (spheres may be of different sizes as ionic radii vary). Data on radii can be found in section 20 (Reference - tables). Two examples of tangential arrangements are shown in the diagram above. Skeletal models These are also called ball-and-spoke models. They consist of drilled spheres, in various colours, to represent atoms of different elements. The spheres are held together at the correct angles by sticks which represent the internuclear distances2. These models can give a false impression of unoccupied space in a molecule but, as the interior visibility is good, the relative positions of the different particles in the structure can be seen easily. Two versions of the same molecule

Skeletal Space-filling

Space-filling A space-filling model is based on van der Waals radii as well as covalent radii. These models models probably give the best representation of a molecule but the interior visibility is non-existent particularly with larger molecules. The following diagram explains some of the terms used in this paragraph.

1 A Buglass, Molecular models in organic chemistry, Education in Chemistry, 17 (No 1), Jan 1980, p15. 2 The spheres therefore represent the nuclei but not to scale. Mainly chemistry 1338 © CLEAPSS 1992

PEEL models These were designed to represent the Probability Envelopes of Electron Location (the position of electron orbitals) in molecules. At the moment, the original PEEL models are no longer available but orbital attachments can be fitted to the Giant-size Molecular Model Kit (see Table 13.10).

13.5.2 Commercial models There are many models and kits available and some are presented in Table 13.10. Only a vague idea of relative cost is given as this will depend on how much is bought. © CLEAPSS 1992 1339 Mainly chemistry

Table 13.10 Commercial molecular models

Name Type of model Supplier / Cost Notes Beever’s miniature models Skeletal. Acrylic spheres held Griffin & George These do not come apart. together with stainless steel Philip Harris Demonstration use only. Good rods. Very accurately made. for seeing unit cells. Various Expensive. lattices available. Desktop molecular modeller Computer simulation of over Oxford University Press This is the schools version of a 60 molecules. Various styles major package. It is IBM PC and editing features are Expensive but no more than a compatible with versions for possible. class set of models. different screen formats. Fieser molecular model Skeletal. Colour-coded plastic Aldrich Chemical Co Ltd As the models show research kit and aluminium parts which movement within the snap together to form bonds. Moderately expensive. molecule, these are ideal for showing conformational and steric effects. Giant-size molecular model kit Skeletal. Colour-coded atom Aldrich Chemical Co Ltd As bond lengths can be as centres held together by long as 27 cm, these are plastic bonds. Two scales Expensive. suitable only for available. Also there are demonstration. orbital attachments. Metal spheres Tangential. 3 mm diameter Griffin & George Demonstration of crystal metal spheres are held in a structure of metals including clear plastic case. Expensive. dislocations etc. Can be displayed on an OHP. Minit system Skeletal. Colour-coded plastic Cochranes of Oxford Ltd Student pocket set, group set, spheres, smaller than the Orbit and biochemistry set available. system, held by plastic Inexpensive. Suitable for class use. connectors of various lengths. Molymod molecular models Skeletal. Colour-coded plastic Griffin & George Sets include introductory, spheres held together by Philip Harris GCSE, organic and inorganic. flexible plastic connectors. Spiring Enterprises Ltd Suitable for class use.

Moderately expensive. Molymod molecular models Space-filling. Colour-coded Griffin & George Sets include organic, complex plastic spheres held together Spiring Enterprises Ltd ions, biochemistry and by short plastic connectors. individual models of polymers, Moderately expensive. soaps, fats and peptides. Suitable for class use and demonstration. Orbit system Skeletal. Colour-coded atom Cochranes of Oxford Ltd Sets include basic, organic centres and green plastic Griffin & George and inorganic, lattices, tubes as bonds. Hogg Laboratory Supplies biochemistry and a Philip Harris demonstration set. Suitable for class use. Inexpensive. Polystyrene spheres Tangential. Plain white Philip Harris Jigs and a wooden triangle to spheres in a wide range of hold the spheres available. diameters. Inexpensive. Adhesive( PVA) also available. Suitable for both class use and demonstration. Scale atoms Space-filling. Bowls and caps Philip Harris Sets include Introductory which fit together. Colour- (class use), Advanced and coded. Moderately expensive. Demonstration. Unit system Skeletal. Colour-coded plastic Hogg Laboratory Supplies More suitable for demon- spheres, held by plastic stration purposes. connectors of various lengths. Expensive. Mainly chemistry 1340 © CLEAPSS 1992

13.5.3 D-i-y models Making models is a time-consuming activity and any mistakes made have a nasty habit of adding to each other. Hard materials such as wooden spheres need skills which come with experience. Softer materials such as polystyrene spheres are easier to work with but holes can get larger with use and the model can fall apart. The following ideas and references will help you find further details.

Table 13.11 A list of D-i-y ideas

Topic Material Reference Notes Orbital repulsion Balloons - See below. Polymers Beads, ‘Poppits’, - Useful for illustrating polymeric chains etc structures. Molecular shape, Ceiling tiles and R Cowin, Useful visual aids and models A whole host of teaching ideas electronegativity, magnetic strip from expanded polystyrene ceiling tiles, using a very cheap material. activation, SSR, 52(No 180), Mar 1971, p606 detergency, polymerisation, orbitals, etc. Molecules Coloured Plasticine - Very cheap and easily put together and cocktail sticks to give a skeletal model. Complex ion models R Cowin, Complex ion models, SSR, 52(No How to support polystyrene models 181), June 1971, p938 on a wooden base. Molecules Foam plastic C Kennard, Easily assembled models, The use of sliced foam plastic Education in Chemistry, 19(No 4), July packing material to form polyhedra 1982, p103 instead of spheres. This shape stacks easier. Proton stabilisation Magnetic boards R Cowin, The use of magnetic boards and Constructing the board and the models to illustrate the concept of proton models, along with their teaching stabilisation, SSR, 52(No 178), Sept 1970, value. p120 Crystal models Acetate M Crowe, Crystal and molecular models, The frame enables the structure to SSR, 52(No 179), Dec 1970, p380 be seen easily. Model making Many Chemistry: Handbook for Teachers, Model A lot of detailed ideas for making making, The Nuffield Foundation, models. Longmans, 1967, p202. This book is now out of print but many schools and teachers centres still have a copy. Models of protein Card and silicone A Davies, Models of protein structure, SSR, Models made of card and silicone structure rubber 71(No 256), Mar 1990, p77 rubber tubing. Organic Paper D Diaper, Organic stereochemistry through A cheap material is converted into stereochemistry paper models, SSR 71(No 255), Dec 1989, various shapes. p94 Molecular models Polystyrene M Byrne, Further notes on the use of Various ideas for teaching. polystyrene molecular models in the teaching of chemistry, SSR 49(No 168), Mar 1968, p474 Molecular models Wood R. Hateley, Molecular models, SSR 43(No Making wooden space-filling 149), Nov 1961, p152 models.

Using balloons Two long sausage-shape balloons can be twisted in the middle and made into a tetrahedral shape. A third balloon can be added to give an octahedral shape. When half of a each balloon is burst in turn, the arrangement of the balloons will change to show the repulsion between the ‘electron orbitals’.

© CLEAPSS 1992 1341 Mainly chemistry

13.6 Plastics As plastics and man-made fibres are extremely common materials, they are included in most GCSE courses. Polymers, a more general name for these materials, have molecules made up from thousands ! of repeated units of smaller molecules, known as monomers1. Unfortunately, many of these monomers have harmful vapours, so they are not suitable chemicals for pupils to work with on the open bench. This makes preparative methods difficult on a class basis. Other hazards may be encountered when commercial plastics are burnt, as they contain many other materials such as fillers, plasticisers, anti-oxidants and dyes. Also, heating plastics in the absence of air, produces a wide range of products; for example, between 225 and 310 °C, polyvinylchloride gives rise to hydrogen chloride gas (CORROSIVE), along with small amounts of methane (HIGHLY FLAMMABLE), benzene, chloroethene (vinyl chloride) (both TOXIC and HIGHLY FLAMMABLE) and methylbenzene (HARMFUL and HIGHLY FLAMMABLE). The following sections illustrate some of the problems in more detail and suggest possible solutions.

13.6.1 Preparations

Poly(phenylethene) (polystyrene) This preparation is used as an example of addition polymerisation. The monomer, ! phenylethene (styrene) is HARMFUL and has a workplace exposure limit (WEL) [see section 7.9 (Control of airborne chemical exposure)] of 250 ppm over a 15 minute period so, as the monomer will be heated, it is important to keep the exposure to its vapour as low as possible. Hence, the experiment is best done as a demonstration in a fume cupboard.

Inhibitor An inhibitor (usually a benzene triol) is added to phenylethene by the suppliers in order to stop polymerisation at room temperature during storage. If a sample of phenylethene is becoming viscous, then that inhibitor has been consumed and polymerisation is taking place. This will continue until the contents within the bottle have completely solidified. The inhibitor can be removed by washing phenylethene with dilute sodium hydroxide, water and finally drying it with anhydrous sodium sulfate. However, this is not usually necessary for school experiments, as the amount of initiator added usually overcomes the small amounts of inhibitor present. Method Polymerisation is a slow process at room temperature so both heat and an initiator are required. The initiator recommended for schools is di(dodecanoyl) peroxide (lauroyl peroxide). 0.2 g of initiator is required for 10 ml of phenylethene in a boiling tube. This mixture should be kept very hot2 for over 30 minutes, by standing the boiling tube in hot water obtained from a kettle, changing the water every 5 minutes The time of heating can be extended until the liquid becomes highly viscous or it solidifies. If the product is solid, the boiling tube is placed in a cloth and the glass is broken. The solid will still contain unreacted phenylethene so gloves should be used during the isolation of the product. Alternatively, the viscous liquid can be poured into a beaker of ethanol (HIGHLY FLAMMABLE), to precipitate the white polymer.

Poly(methyl methacrylate) The monomer has a very irritant vapour, is highly flammable and its boiling point is below that of water. In a few rare cases, people handling the monomer have become sensitised so that further contact brings on an asthma attack.

1 For example, methyl methacrylate is the monomer that makes the polymer Perspex. 2 Heating the monomer in a hot water bath with a Bunsen burner can cause ignition at the mouth of the boiling tube. Mainly chemistry 1342 © CLEAPSS 1992

It is advisable not to polymerise methyl methacrylate to make Perspex. ! Nylon This preparation is used as an example of condensation polymerisation. The preferred solvent for the acid chloride is now cyclohexane (HIGHLY FLAMMABLE) not tetrachloromethane (TOXIC). Full details are given on the Hazcard for hexane-dioyl chloride (CORROSIVE).

Urea-formaldehyde resins A convenient recipe requires 5 g of urea dissolved in 10 ml of 40% methanal solution (TOXIC) to which 5 ml of 1 M sulfuric acid (CORROSIVE) solution has been added. (Do not substitute hydrochloric acid for sulfuric acid as a possibly carcinogenic by-product could be formed.) This exothermic experiment must be carried out in a fume cupboard as methanal vapour is produced. The reaction is often carried out in a polystyrene cup so that the solid resin can be obtained by tearing the cup. Wear protective gloves as there will be unreacted reagents present.

13.6.2 Sources of plastics Examples of plastics can be found in many everyday objects. Some examples are presented in Table 13.12.

Table 13.12 Plastic Sources of samples Sources of High density polythene Carrier bags and food bags (noisier than low density polythene) plastics from Low density polythene Carrier bags, bread bags everyday Nylon Curtain rail fittings, fishing line materials Perspex Car light covers Phenol-formaldehyde Dark-coloured electric fittings Polyester film Carbonated drink bottles Polypropylene Transparent crisp bags Polystyrene Yoghurt cups, egg boxes, disposable cups (do not use expanded polystyrene) Polyvinyl chloride Cling film, many transparent cooking oil or shampoo bottles Urea-formaldehyde Light-coloured electric fittings

13.6.3 Burning plastics Burning small samples of plastics is used as a method of identifying them as shown in Table m 13.13. This must be done in a fume cupboard as the many of the gases produced are toxic. Small samples (10 mm × 5 mm × 1 mm or less) of plastics should be prepared by the technician in advance. Table 13.13 Results of burning plastics

Plastic Observation High and low density polythene Burns with a blue flame with a yellow tip and small amount of smoke. Smell of burning Polypropylene candle after the flame goes out. Nylon Difficult to ignite. Often melts and chars. Odour of burning hair. © CLEAPSS 1992 1343 Mainly chemistry

Perspex Burns readily with a yellow flame. Strong fruity odour after the flame goes out. Phenol-formaldehyde resin Burns with great difficulty. Odour of phenol. Polyester Burns readily with a yellow flame. Strong fruity odour after the flame goes out. Polystyrene Burns readily with an orange-yellow flame with black sooty smuts. Sweetish odour. Polyvinyl chloride Pure PVC burns with difficulty with a yellow smoky flame. Acrid odour of hydrogen chloride (CORROSIVE). Urea-formaldehyde resin Burns with difficulty. Swells and cracks. Fish-like smell.

13.6.4 Heating plastics in the absence of air As commercial plastics contain so many other substances, it is now very debatable whether ! this should be done all. For example, the depolymerisation of polystyrene will yield not only the monomer but benzene (TOXIC AND HIGHLY FLAMMABLE) and methylbenzene (HARMFUL AND HIGHLY FLAMMABLE). It is essential that these experiments should be done in a fume cupboard by an experienced teacher. The method should not be used as a source of monomer.

13.7 Techniques in advanced chemistry The practical preparation of a chemical at A-Level needs careful planning; most take longer than a double period. Some factors to consider are listed below. The choice of If at all possible, the chosen preparations should involve short-term operations; for preparation instance, refluxes can take from 10 minutes to 24 hours! It is convenient to have stages at which the preparation can be stopped and restarted in another lesson. The introduction This may take so long that the practical never gets started in time! Recrystallisation Plenty of time must be allowed for these procedures. It is often convenient for an exercise and drying to be timed so that the drying procedure can take place overnight. Melting point Most organic preparations are concluded by a measurement of the melting point of the determinations product to confirm that the intended substance has been prepared. Any heating should be done slowly so, again, plenty of time must be allowed. Tidying up Preparations can stain and use a lot of glassware and the sooner it is washed, the easier it is to clean. Apparatus should be rinsed by the pupils and handed in for washing so that it can be available for the next lesson. This too takes time. Concluding Discussion is often best left to another period. A pupil’s brain is not very receptive after remarks 90 minutes of practical work! In the following sections, it is assumed that all reactions are carried out at atmospheric pressure and in Quickfit1 apparatus where appropriate. For the most

advanced preparations, including vacuum work, a University textbook2 should be consulted.

13.7.1 Quickfit apparatus The glass in Quickfit kits is borosilicate glass which has a low thermal expansion and consequently high resistance to thermal shock. The advantages of using Quickfit over traditional apparatus are listed below.

1 Quickfit is a registered trademark (J. Bibby Science Products Ltd). 2 Furniss et al, Vogel’s Textbook of Practical Organic Chemistry, Fifth Edition, Longman, 1989, ISBN 0582061083. Mainly chemistry 1344 © CLEAPSS 1992 a) Time is saved preparing apparatus. b) There is no contamination of the chemicals by contact with cork or rubber stoppers. c) By using funnels with taps, corrosive liquids are handled more safely. d) A large number of operations can be carried out with just a few pieces of apparatus, leading to efficient use of equipment. e) Wider glass tubing means that risks from suck-back and blockages are reduced. Unfortunately this equipment is expensive and needs looking after. The joints There are two types of joints used, namely conical and ball-and-socket. Conical joints are more common in schools and the sizes of the some of the joints (specified in British Standard BS572) are shown in Table 13.14.

Table 13.14 Dimensions of interchangeable ground glass conical joints

Size designation Nominal diameter of wide Nominal diameter of narrow Nominal length of end / mm end / mm engagement / mm 12/21 12.5 10.4 21 14/23 14.5 12.2 23 19/26 18.8 16.2 26

To grease or not These joints are precision made and must be kept free of dust, grit and solids from to grease solutions that crystallise on evaporation. For most work at atmospheric pressure, grease is not needed on the joints. If salt solutions or alkalis are expected to come into contact with the joints, then a very thin smear of grease may be applied1. Students tend to apply too much grease which then contaminates the materials and is difficult to clean from the glassware. Disconnecting If joints are separated immediately after a reaction, then problems of seizing up do not seized joints occur. Some methods of undoing a seized-up joint are outlined below. a) Set the joint upright and place some glycerol at the junction to penetrate into the joint. b) If method a does not work, direct hot air from a hair dryer on to the joint. Now gently twist the two pieces of apparatus apart. It is often useful to give the apparatus a small tap on a wooden bench.

The set A modern set of Quickfit should consist of: a 50 ml pear-shaped flask; a ground glass stopper; a stillhead; a condenser2; a receiver adaptor; a separating funnel fitted with a ground socket and a 14/23 cone below the stopcock; a screw-capped adaptor for a thermometer; a calcium chloride guard tube (optional).

1 Use Apiezon grease, L, M or N or silicone vacuum grease. 2 Those fitted with a screw thread connector enable rubber tubing to be connected without risk of breakage to the glass: they may be more expensive but they last longer ! © CLEAPSS 1992 1345 Mainly chemistry

Other additions For demonstration purposes, it may be desirable to have a set of apparatus using flasks with a larger capacity. The apparatus used to illustrate Raoult’s Law, for example, requires the use of two-necked pear-shape flasks. Steam distillation, as discussed in section 13.7.3 (Distillation), requires larger flasks (eg, 250 ml) with wider necks in order to get the material into and out of the flask. Adaptors will be required to adjust the neck size to the 14/23 size on the stillhead.

Plastic clips can be bought which hold the joints together and reduce the risk of breakage.

13.7.2 Refluxing Refluxing allows a reaction to be carried out at a higher temperature than in an open vessel, thus increasing the rate of the reaction but avoiding the loss of material through evaporation. The apparatus for a basic reflux procedure is shown in the diagram.

Water out

Cold water in

Position of the clamp

The flask should be no more than two/fifths full of liquid reagent (otherwise it boils up into the neck of the condenser) and two or three anti-bumping granules should be added before starting to heat. Anti-bumping granules should never be added during a reflux: to do so would cause the solution to froth up the condenser tube.

A Bunsen burner should be used directly only if the boiling point of the substance is known to be above 80 °C. If the boiling point of the liquid is between 50 and 80 °C, a boiling water bath is suitable and it may be heated with a Bunsen burner. If the boiling point of the liquid, eg, ethoxyethane (EXTREMELY FLAMMABLE), is lower than 40 °C then the flask should be surrounded with hot water to avoid flames in the vicinity of the distillation. It should be stressed that many reagents in organic chemistry are flammable and, while electric heating mantles are the preferred method of heating organic solvents, unfortunately, they are expensive. A beaker of cold water will accelerate the cooling of a flask after reflux.

The clamp holding the apparatus should be fitted around the neck of the pearshaped flask. Another clamp on the condenser is not needed and could cause stress in the joints. However, if it is considered wise to support the condenser, an adjustable spring clamp, shown below, may be used; it is particularly suitable for holding the condenser in distillation. Mainly chemistry 1346 © CLEAPSS 1992

Other Figure a shows the arrangement at the top of the condenser when refluxing moisture- arrangements sensitive compounds, eg, ethanoyl chloride.

Rubber Cotton- wool tubing plug Screw-capped adaptor

Inverted funnel Anhydrous calcium chloride

Cold water

Figure a Figure b

Figure b shows an arrangement by which water-soluble gases, eg, hydrogen chloride, evolved in a reaction can be absorbed. This allows work that would otherwise have to be carried out in a fume cupboard to be performed in a well-ventilated laboratory.

The following arrangement is used when a reagent needs to be added whilst a reflux continues. The stillhead allows the reaction to proceed at atmospheric pressure.

© CLEAPSS 1992 1347 Mainly chemistry

Still head adaptor

13.7.3 Distillation Simple distillation The following diagram shows the arrangement for simple distillation.

Cold water in

Water out

Heat

Mainly chemistry 1348 © CLEAPSS 1992

A clamp should be fitted around the neck of the flask and another very carefully on the receiver adaptor. Stress on the glass joints must be avoided. Anti-bumping granules should be added to the flask before distillation, never during it, unless the flask is first cooled down. The thermometer bulb must be at the same level as the stillhead opening. See section 13.7.2 (Refluxing) for comments on heating methods.

Once the liquid boils, the heating should be reduced so that the distillate is produced at one or two drops per second. Inevitably, distillates are collected over a temperature range (eg ±3 °C) and not at a precise boiling point. With mixtures of liquids, fractional distillation is a better procedure but expense and time do not allow for this in most class work; more information can be found in the Chemical Engineering special study in Nuffield Advanced Science, Chemistry1. Steam distillation Steam distillation allows an organic substance to be distilled at a temperature well-below its boiling point and so less decomposition or polymerisation of material will take place. The apparatus as described in Nuffield2 (a 250 ml flask) is quite suitable for class work as it avoids the generation of steam, a lengthy and awkward procedure. As it is a useful technique for extracting volatile natural products from fruits and flowers, it is advisable to use flasks with wide necks in order to add and remove the material more easily. A ground-glass adaptor can be used to attach the stillhead to the flask.

13.7.4 The use of tap funnels A dropping funnel has a tap, a body which is mainly cylindrical and no stopper. It is used to add reagents in a controlled manner, often to reflux reactions or distillations (see earlier diagram). A separating funnel also has a tap but the body has a lower conical section and a stopper is provided. It is used to extract organic reagents from aqueous mixtures or to remove acidic or alkaline gases that dissolve in organic solvents but it can also be used to add reagents to reflux reactions or distillations. It is important to know the density of the solvents used so that the identity of each layer is known. If the density of the solvent is near that of water, the addition of salt often helps in the separation. If there is doubt over which layer is which, then adding a little more of the solvent should identify the layer which increases in volume. It is more efficient to extract three times with 5 ml of a solvent, than once with 15 ml of solvent. Solvent extraction The following instructions allow solvent extraction to be done without incident. a) Place the aqueous solution containing the substance to be extracted in the separating funnel. b) Add the solvent to the separating funnel and fit the stopper. c) Invert the funnel, keeping a finger over the stopper. d) Gently agitate3 the flask and open the stopcock while the flask is inverted to reduce the internal gas pressure. e) Repeat step d) three times until there is no increase in internal pressure. f) Leave the separating funnel with the stopper at the bottom and vertical to allow the layers to separate.

g) Remove the stopper (to let the air in freely) and run one layer at a time into separate containers. Never empty either liquid layer into a sink: it could be the wrong one! Purifying organic To remove acidic substances from organic layers, 0.5 M sodium hydrogen carbonate

1 Revised Nuffield Advanced Science; Chemistry, Chemical Engineering; A Special Study, Longman, 1984, ISBN 0582389259, p27. 2 Revised Nuffield Advanced Science, Chemistry: Students Guide 1, Longman, 1984, ISBN 0582353610, p292. 3 Extremely vigorous shaking can result in emulsions which may take hours to separate. © CLEAPSS 1992 1349 Mainly chemistry

liquids solution is used in the separating funnel. Shaking must be very gentle at first as carbon dioxide is produced and the internal pressure must be released regularly. Alkaline materials are removed similarly but with dilute hydrochloric acid. A final wash with distilled water removes any inorganic impurities. The organic layer is now ready for drying.

13.7.5 Removing water from organic solvents Even though an organic liquid may be immiscible in water, it could still contain small amounts of water as an impurity. The presence of this water may prevent complete separation of a solute from an organic solution or contaminate a distilled product. Therefore, before distillation, the solution should be dried with a chemical which extracts the water but does not dissolve the solvent nor react with the solute. The most effective drying agents are anhydrous magnesium sulfate or sodium sulfate.

Magnesium chlorate must not be used as a drying agent. See Hazcard. ! The method A solvent which contains water is often cloudy as the water forms an emulsion in the liquid. It can be dried by following these instructions. a) Place the wet solvent in a dry conical flask and add a little of the drying agent. (The solid usually adheres to the bottom of the flask.) b) Swirl the mixture and add more drying agent until the solution is clear and the drying agent is moving freely in the liquid. c) Filter the dried liquid from the added drying agent. (In many cases there is not a lot of liquid present and it would all be absorbed into the filter paper. In that case, use a small plug of glass wool in a filter funnel instead of a paper.) d) Add the liquid and the used drying agent to the filter funnel and press lightly on the residue with a glass stopper to squeeze out any more organic liquid.

13.7.6 Recrystallisation Because the crude products from most chemical reactions are contaminated with both reactants and side products, it is necessary purify the material by recrystallisation. The procedure is summarised below. It is a time-consuming activity (not the sort of thing to do in 5 minutes at the end of a double period) and needs patience.

Step Process Comments 1 Dissolve the impure substance in a Addition of a little activated charcoal removes any coloured material that suitable solvent held at a temperature as should not be present. near to its boiling point as possible. 2 Filter the hot solution to remove This should be as rapid as possible. Use a warmed funnel and fluted filter undissolved solids and dust. paper. 3 Cool the solution. Rapid cooling in an ice bath produces smaller crystals. If no solid appears, then try these procedures in order: a) scratch the sides of the vessel with a glass rod; b) seed the solution with some of the solid material; c) cool in a salt/ice mixture. 4 Filter off the crystals, allow to dry, weigh Weigh the filter paper before the filtering. Make sure that the drying cabinet is the material and determine the melting not so hot so that the solid melts. point. Choosing a The correct solvent for this procedure cannot always be predicted by theory; it is usually solvent found by trial and error as in the detailed instructions below. Mainly chemistry 1350 © CLEAPSS 2005

Add 0.1 g of the crude material to a test tube with 1 ml of the solvent under test. Warm gently1 and, if the solute dissolves quickly, try an alternative solvent. If the solid has not dissolved, add solvent 0.5 ml at a time, repeating the warming, until 3 ml has been added. If the solid has still not dissolved, try an alternative solvent. If the solid dissolves, then cool the solution to reform the crystals. If crystals do not form, find an alternative solvent. Common solvents are water, ethanol, propanone and petroleum ether. Mixed solvents If a solid is very soluble in one solvent, insoluble in another and the solvents are miscible, a mixture of the two solvents may have the desired effect, eg, water: ethanol mixtures.

13.7.7 Vacuum filtration Vacuum filtration is a more rapid and efficient technique than gravity filtration. Many different designs of apparatus exist; the diagram shows two of the most common funnels that are used in schools. The Hirsch funnel is to be preferred when only a small quantity of residue is to be collected while the Büchner funnel is best when the filtrate is required or larger volumes are to be separated.

Büchner funnel Hirsch funnel These are fitted by means of a rubber stopper or ring into a thick-walled filtering flask, also known as a Büchner flask. This, in turn, is fitted to a vacuum pump, (often a water pump in schools) with thick-walled pressure tubing. A trap can be placed between the suction flask and the pump (see diagram), although this is not always convenient in a school laboratory. See section 10.6.4 (Water-operated filter pumps). If a rotary vacuum pump is used, a trap is essential.

Three-way tap Connection to the vacuum pump Pressure tubing Optional water Büchner trap flask

The filter flask should be disconnected from the pump before the pump is turned off. This avoids the water being sucked back into the Büchner flask and diluting the filtrate. A three- way tap can also be used to isolate the flask.

1 Take care! If the solvent is highly flammable, use a beaker of hot water. © CLEAPSS 1992 1351 Mainly chemistry

Technique The filter paper should be selected so that it just covers all the perforated holes but is smaller than the inside diameter of the funnel. It should never be folded. The paper is then moistened with the liquid to be filtered and moderate vacuum applied, whereupon the paper should stick firmly to the base of the funnel. A suspension should not be stirred before filtering but the clearer liquid poured off first and then the slurry. Finally, the container is washed out with the solvent being used and the washings added to the funnel. If necessary, the residue may be pressed with a glass stopper. If the suspension contains fine particles, the filtration may take a long time as the pores in the filter paper become clogged. In this case, it is best to do the filtration in separate stages and then combine all the residues and/or filtrates together. A Hirsch funnel is used in a similar manner but with a filter paper of smaller diameter.

13.7.8 Melting point determination The melting point of a substance helps to identify it and check its purity. Melting point tables are readily available in reference books1. The usual method used to determine the melting point involves heating slowly a small amount of the solid contained in a capillary tube closed at the bottom, (called a melting point tube), often in a special melting point apparatus. The temperature range over which melting takes place is noted and, for a pure substance, the melting should occur over a range of not more than one degree. If there is an impurity present, then the melting point will not be sharp2 and it will occur at a lower value than expected. Melting point These are best obtained from suppliers and will usually require the end closing. They can tubes also be made by heating and rotating soda-glass tubing until it begins to soften; then the tubing is pulled apart to make the capillary. It requires practice and patience to get tubes not so fine that they bend but not so wide that they will not fit into commercial melting point apparatus. A length of capillary tubing usually needs to be broken in half by touching it lightly with a file. It is sealed by inserting one end in the extreme edge of a Bunsen flame and rotating it. Prepared melting point tubes can be stored in specimen tubes. Filling the melting The open end of the melting point tube is placed in the solid under examination so that point tube only a few grains are picked up. The powder is then shaken down into the tube by tapping the closed end gently on the bench or holding the tube between fingers and thumb and then hitting the bench with the hand. Measuring the Two methods are available. melting point a) The melting point tube is attached to a thermometer, with care to ensure that the bulb is adjacent to the bottom of the capillary tube, and placed in a heated liquid bath. Thiele’s apparatus is available from suppliers but the home-made version is quite adequate at school level (see diagram). The liquids that can be used are ethan- 1,2-diol (up to about 140 °C)3, liquid paraffin (to 220 °C) or dibutyl phthalate (to 300 °C). b) The melting point tube is inserted into an electrically-heated metal block, available as a ‘melting-point apparatus’ from the main school suppliers.

In both cases, it is usual to heat quickly to 15 degrees below the expected melting point and then heat very slowly, eg, two deg per minute. A note is made of the temperature at which liquid just appears in the melting point tube and then when melting is complete. If the melting point of a solid is unknown, then two melting point tubes need to be made up. One is used to find the approximate melting point, and then, after allowing the apparatus to cool 30 degrees below the reading, the second melting point tube is used to make an accurate determination. The temperature at which the solid solidifies when the

1 Furniss et al, Vogel’s Textbook of Practical Organic Chemistry, Fifth Edition, Longman, 1989, ISBN 0582061083 or Nuffield Advanced Science, Book of Data; Longman, 1984, ISBN 058235448X, p102. 2 A eutectic mixture will also have a sharp melting point. However, its melting point can be changed by further crystallisation. 3 It has the advantage of being soluble in water and hence it is easier to clean the apparatus when cool. Mainly chemistry 1352 © CLEAPSS 2005

apparatus cools down must be ignored because many substances partially decompose on melting, giving rise to impurities, thus altering the melting point.

Thermometer

Rubber tubing Liquid paraffin

Melting point Stirrer tube

Thiele's apparatus (without thermometer and melting point tube) Home-made apparatus

13.7.9 Nitration Gloves and eye protection must always be worn during the preparation and use of nitrating ! mixture (see practical texts for the proportions of concentrated sulfuric acid and concentrated nitric acid required for a particular case). The nitration of benzene must not be carried out in a school laboratory. Methyl benzoate however, is an excellent substitute and can be nitrated in its place1. It is recommended that nitrations of phenol (the products are toxic) and naphthalene (there are impurities present which react to give toxic compounds) are not carried out.

13.7.10 Electrophoresis [The information previously in this section has been expanded and moved to section 11.1.7 (Electrophoresis).]

13.8 Titration Volumetric analysis uses the technique of titration; see section 13.8.6. This involves the addition of a solution from a graduated vessel (a burette; see section 10.10.1) into a conical flask containing another solution, the volume of which has usually been measured with a pipette; see section 10.10.3. All solutions will have been accurately made up in volumetric flasks; see section 10.10.5. The concentration of one of the solutions will be accurately known, ie, it is a standard solution; see section 13.8.4. When sufficient solution is added just to react completely with the pipetted solution, the end point is said to be reached. An indicator is usually added to detect this end point; see section 13.8.5. Suitable There are four types of reactions that can be used in volumetric analysis: reactions a) neutralisation; b) oxidation-reduction (or redox); c) complexometric; d) precipitation.

1 Revised Nuffield Advanced Science, Chemistry: Students Guide 1, Longman, 1984, ISBN 0582353610, p308. CLEAPSS guide L195, Safer chemicals, safer procedures, has details as well. © CLEAPSS 2005 1353 Mainly chemistry

13.8.1 Volumetric flasks [Information previously in this section is now incorporated into section 10.10.5.]

13.8.2 Pipettes [Information previously in this section is now incorporated into section 10.10.3.]

13.8.3 Burettes [Information previously in this section is now incorporated into section 10.10.1.]

13.8.4 Standard solutions A standard solution contains a known mass of solid dissolved in a defined volume of solution; hence its concentration is accurately known. Ideally the solid must be available in a pure state1, must not alter its mass during weighing (ie, not be hygroscopic nor absorb atmospheric carbon dioxide), should have a high molar mass, must dissolve in water and must react quickly and stoichiometrically with the other reagent involved. Standard solutions should always be made up in distilled or deionised water.

Standards for acid/alkali titrations Sodium Anhydrous sodium carbonate (IRRITANT) (molar mass 106 g) should be heated to 260- carbonate 300 °C for over 30 minutes and then allowed to cool in a desiccator to ensure that it is (Na CO ) really dry. The weighing should be carried out quickly to avoid reabsorption of moisture. 2 3 The solution is weakly alkaline and should be titrated against a strong acid using methyl orange as the indicator. One mole of sodium carbonate reacts with two moles of hydrogen ions. Sodium Sodium tetraborate-10-water (molar mass 381 g) should not be heated above 55 °C tetraborate-10- when making up the solution, otherwise the pentahydrate crystallises out on cooling. The water (borax) solution is weakly alkaline and should be titrated against a strong acid using methyl red as the indicator. One mole of sodium tetraborate reacts with two moles of hydrogen ions. (Na2B4O7. 10H2O) Potassium Potassium hydrogen phthalate (molar mass 204 g) forms a weak acidic solution in water hydrogen and can be titrated against any strong alkali with phenolphthalein as the indicator. One phthalate mole of potassium hydrogen phthalate reacts with one mole of hydroxide ions. (KHC8H4O4) Sulfamic acid Sulfamic acid (IRRITANT) (molar mass 97 g) is an expensive chemical but the GPR grade is 99% pure and could be used for school purposes. The solution must not be stored, as (NH2SO3H) hydrolysis forms ammonium hydrogensulfate solution. It can be titrated with both strong and weak alkalis using indicators that change between pH 4 and 9. One mole of sulfamic acid reacts with one mole of hydroxide ions. Standard solutions for redox reactions Sodium Sodium ethandioate solution (HARMFUL) is used to standardise potassium mang- ethandioate anate(VII) solution. It should be dried at 110 °C before being weighed (13.4 g per litre of (Na C O .2H O) solution for a 0.1 M solution). The titration needs to be carried out at 80 °C. One mole of 2 2 2 2 sodium ethandioate reacts with 0.4 moles of potassium manganate(VII).

1 If, when general reagents are used, cloudy solutions are obtained even with distilled water as the solvent, AnalaR® quality (or equivalent) material should be used. For example, it is often quite difficult to obtain a clear solution of sodium carbonate unless AnalaR® quality is used. Mainly chemistry 1354 © CLEAPSS 2005

Potassium Potassium dichromate is sufficiently pure to act as a primary standard itself. It should be dichromate(VI) dried at 150 °C for an hour before being weighed. (K2Cr2O7)

Potassium 0.1 M sodium thiosulfate solution is standardised against potassium iodate solution. iodate(V) (KIO ) Potassium iodate(V) (OXIDISING AGENT) should be dried at 180 °C for one hour before 3 being weighed (3.57 g per litre of solution for a 0.0167 M solution). To generate iodine, 1 g of potassium iodide and 10 ml of 2 M sulfuric acid are added to the measured volume of potassium iodate solution. 0.0167 mole of potassium iodate produces 0.05 mole of iodine which reacts with one mole of sodium thiosulfate.

Standard solutions for EDTA titrations Calcium 0.1 M EDTA solution should be titrated against a standard solution of calcium ions. carbonate Precipitated calcium carbonate is dried at 150 °C for an hour before being weighed (CaCO ) (10.0 g per litre of solution to be made) in a conical flask. Enough hydrochloric acid (2 M) 3 is added to dissolve the calcium carbonate, with a filter funnel placed over the conical flask to stop the spray from coming out of the flask. The underside of the funnel should be rinsed into the flask and the solution made up in a 1 litre volumetric flask. The titration needs to be carried out in a pH 10 buffer1, 1 ml per 20 ml of solution to be titrated.

Standard solutions for precipitation reactions As silver nitrate is extremely expensive, it usual to standardise commercial grade silver nitrate solution against a known solution of sodium chloride, prepared from solid which has been dried for one hour at 120 °C.

13.8.5 Indicators

Acid/alkali indicators CLEAPSS Recipe cards have details for making up solutions of the more common indicators. All the indicators on the cards are suitable for strong acid/strong alkali titrations. However, for weak acid/strong alkali titrations, only phenolphthalein indicator is suitable. For weak alkali/strong acid titrations, bromophenol blue, methyl orange and methyl red are suitable. Universal or full-range indicators2 are not suitable for titration work but are used for the approximate determination of the pH of a solution.

Redox titrations Diphenylamine This is used for potassium dichromate titrations. A 1% solution is made up in concentrated sulfuric acid (CORROSIVE). Use 6 to 8 drops of this indicator followed by 5 ml of phosphoric acid (CORROSIVE). The colour of the solution will go violet-blue at the end point. The necessity of using concentrated sulfuric acid can be avoided if a 0.2% w/v aqueous solution of diphenylamine-p-sulfonic acid (sodium salt) is used instead. Potassium Potassium manganate(VII) acts as its own indicator in these titrations since the intense colour can be visibly detected in very dilute solutions. (0.01 ml of 0.01 M solution added

1 Dissolve 54 g of ammonium chloride (IRRITANT) in 350 ml of concentrated ammonia solution (CORROSIVE) and dilute to 1 litre with distilled water. Use a fume cupboard and wear eye protection. 2 A recipe for a universal range indicator follows. Dissolve 0.05 g of methyl orange, 0.15 g of methyl red, 0.3 g of bromothymol blue and 0.35 g of phenolphthalein in 600 ml of ethanol (HIGHLY FLAMMABLE) and make up to 1 litre with water. The colour changes are: pH up to 3 red; pH 4 orange-red; pH 5 orange; pH 6 yellow; pH 7 yellow-green; pH 8 greenish-blue; pH 9 blue; pH 10 violet; pH 11 reddish-violet. © CLEAPSS 2005 1355 Mainly chemistry manganate(VII) to 100 ml of water can still impart a pink colour!) Starch Starch, used to detect an excess of iodine in iodine/thiosulfate titrations, is prepared by making a paste with 1 g of starch and a small amount of water and then adding 100 ml of boiling water with stirring. Now heat the solution to boiling before allowing to cool to room temperature. Starch must not be added to the iodine solution at the beginning of the titration, otherwise some iodine remains absorbed within the starch even at the end point. It is best to wait until the solution is a very pale straw colour and then add 1 ml of starch solution.

Complexometric titrations EDTA solutions can be used to find the concentration of various ions in solution but the choice of indicator and pH is important; Table 13.15 summarises the conditions. In each case, 10 ml of the metal ion (about 0.1 M) is titrated against 0.1 M EDTA solution. Where a true buffer giving the required pH is not available, a suitable technique is suggested below. Table 13.15 A summary of EDTA titrations Metal pH Indicator Colour change Notes Calcium ions 1. pH 12 buffer (5 ml) 1. Murexide 1. Red changes to purple 1. Titrate rapidly as calcium during the titration. carbonate can precipitate on End point is violet. standing. 2. 2 ml of the 2. Eriochrome black 2. Dark red to blue 2. The colour change is sharper in magnesium -EDTA the presence of magnesium- solution (see below) EDTA. plus pH 10 buffer Cobalt(II) ions Any pH between 4 and 6 Xylenol orange Red-violet to lemon-yellow Dilute to 50 ml. Warm to 40 °C. Hard water pH 10 buffer (2 ml) Patten and Reeder’s Dark red to blue Use 50 -100 ml of tap water. Titrate (Calcium and indicator (HHSNNA) with 0.02 M EDTA solution. magnesium ions) Lead(II) ions Any pH between 4 and 6 Xylenol orange Red-violet to lemon-yellow Dilute to 50 ml. Magnesium ions pH 10 buffer (2 ml) Eriochrome or Dark red to blue Dilute to 50 ml. Warm to 40 °C. Solochrome black Manganese(II) ions pH 10 buffer (2 ml) Eriochrome black Dark red to blue Add solid hydroxyammonium chloride to stop oxidation of the ions. Dilute to 50 ml. Warm to 40 °C. Zinc(II) ions pH10 buffer (2 ml) Eriochrome Black Dark red to blue Dilute to 50 ml.

Eriochrome and The dye (0.1 g) is ground in a mortar and pestle with 20 g of AR sodium chloride. After Solochrome black the buffer is added (see Table 13.15) to the metal ion solution, 0.2-0.4 g of the indicator is added. Solochrome Black 6B is a very similar chemical to Eriochrome Black T but gives sharper colour changes with magnesium and calcium ions. Murexide The dye (0.1 g) is ground in a mortar and pestle with 10 g of AR sodium chloride. After the buffer is added to the metal ion solution, 0.2 - 0.4 g of the indicator is added. Patten and 0.1 g of the dye is ground with 10 g of AR anhydrous sodium sulfate. It is used to detect Reeder’s calcium in the presence of magnesium, especially in hard water. After the addition of the indicator buffer, add 1 g of the ground mixture. (HHSNNA) Xylenol orange Dissolve 0.1 g of the indicator in 100 ml of 50% ethanol (HIGHLY FLAMMABLE). Buffer solutions The following recipes can be used to prepare the buffer solutions required above. pH 4 to 6 ‘buffer’ Mix 100 ml of 0.1 M ethanoic acid with 200 ml of 0.2 M sodium ethanoate solution. pH 10 buffer In a fume cupboard, add 17.5 g of AnalaR® ammonium chloride to 142 ml of concentrated ammonia solution and dilute to 250 ml with distilled water. pH 12 ‘buffer’ Add 1 ml of 2 M sodium hydroxide solution (CORROSIVE) to 50 ml of solution. Magnesium- Dissolve 3 g of EDTA and 2 g of hydrated magnesium sulfate in distilled water and EDTA make up to 100 ml. Mainly chemistry 1356 © CLEAPSS 2005

Precipitation reactions Eosin A 0.1% solution of eosin in 70% ethanol can be used with silver nitrate titrations against bromides and iodides. Near the end point, the precipitate coagulates and the solution becomes red as more silver nitrate is added. The end point is reached when one drop of silver nitrate solution turns the precipitate a pronounced magenta colour. Fluorescein (an A solution is prepared containing 0.2 g of fluorescein dissolved in 100 ml of 70% ethanol adsorption solution (HIGHLY FLAMMABLE). Ten drops of this solution is used per titration. The end indicator) point is reached when the greenish-yellow solution develops a pronounced reddish tint. This is accompanied by coagulation of the silver chloride precipitate. Dichlorofluorescein (a 0.1% solution in 70% ethanol) is suitable for use with more dilute chloride concentrations (less than 0.005 M), eg, drinking water. The solution under titration must be either neutral or only weakly acidic. Potassium A solution is prepared containing 5 g of potassium chromate (IRRITANT) in 100 ml of chromate solution. For the titration, 1 ml of this solution is used. The end point is reached when a indicator faint red-brown colour persists after swirling. It is not suitable for the titrations of the chlorides of copper, nickel, manganese, zinc, aluminium or magnesium; fluorescein should be used instead.

13.8.6 Carrying out a titration a) The burette is rinsed and then filled with one of the reagents (see section 10.10.1). For acid/alkali titrations, it is traditional to place the acid in the burette because alkalis can react with grease on the barrel, which then blocks the jet. However, this is less of an issue now that modern burettes have PTFE barrels. b) The pipette is rinsed and filled with the other reagent (see section 10.10.3) and the contents are emptied into a clean conical flask. (As school titrations are based on aqueous solutions it does not matter if the flask is wet.) c) Other additions (eg, indicator and/or buffer) are made to the solution in the conical flask (see section 13.8.5). d) The titration is carried out using the technique given in section 10.10.1. After a change of colour in the conical flask, the addition is stopped and the rough titration volume of liquid is read to one decimal place. e) The contents of the flask are disposed of as appropriate. The flask is washed twice with tap water and then twice with distilled water. There is no need to dry the flask. f) Steps a, b and c are repeated. The contents of the burette are added to within 1 ml of the volume added in step (d). The solution is then added dropwise from the burette until the required colour in the indicator is reached. The volume delivered by the burette is recorded and read to the nearest 0.05 ml. (It may be possible to read even smaller intervals and a magnifying glass might help.) g) Step f is repeated so that three accurate readings, differing by no more than 0.1 ml, are obtained. These accurate values (not including the rough titration) are averaged to give the mean titration volume. 13.9 Water There are activities in school practical science which can be totally spoilt by impurities in the water. This section discusses the problem and ways of dealing with it.

13.9.1 Water purity Dissolved salts The chemical composition of the water supplied in the United Kingdom varies greatly from one area to another. For instance, parts of the Birmingham area have water supplies (from Wales) with as little as 60 ppm of dissolved solids in every litre (usually described as 60 ppm total dissolved solids or TDS) but, in parts of East Anglia, levels up to 800 ppm TDS can occur. The TDS figure is not the same as ‘total hardness’, a figure which only considers calcium and magnesium salts, but includes chloride, sodium and any other © CLEAPSS 2005 1357 Mainly chemistry

soluble ions. The local Water Authority will supply an analysis form which will either contain the TDS value or enable it to be worked out by adding up the individual components. If you make your own measurement of total hardness, the result can be worked out in ppm in terms of calcium carbonate and the TDS figure can be expected to be perhaps 30% higher than this. The composition of the dissolved solids also varies: in some areas calcium sulfate predominates, in others calcium hydrogencarbonate. From the school chemist’s point of view, the main problem ions in tap water are likely to be calcium, magnesium, chloride, hydrogencarbonate and sulfate. Carbon dioxide Carbon dioxide interferes with all measurements of purity in water. Its solubility is 3.29 × -3 1 10 mol per 100 g of water at 298K. Using the pKa of 6.3 for the dissociation constant of carbonic acid2 into hydrogen and hydrogen carbonate ions, the most acidic pH value that could be obtained is 3.9. Carbon dioxide in water is lost on boiling but is re-absorbed on cooling. Making up Opinions vary on the extent to which untreated tap water may be used in school solutions chemistry (if at all). It is probably not worth the risk of making up bench solutions in tap water where the TDS figure is greater than 300 ppm. In softer water areas, some solutions can be made with tap water but silver nitrate, for instance, can give a turbid solution with quite low chloride levels, down to at least 10 ppm chloride ion.

Table 13.16 highlights some work where water purity is important.

Table 13.16 The importance of water purity

Work Comments Demonstration of Sodium chloride solution should, of course, be neutral. Demonstrating this with distilled water containing neutrality excess carbon dioxide is not very convincing. Boiling the water beforehand (in clean equipment !) is one answer but this is a case where it is simpler to use tap water. Hydrogencarbonate Distilled water with excess dissolved carbon dioxide will cause this to be yellow when made up. It is entirely (‘bicarbonate’) reversible, however, and all that needs to be done to get it to the ‘equilibrium’ state is to bubble air through indicator for a while. Microbiology The media solutions used in microbiological experiments should use pure water. Solutions Silver, barium and lead solutions will go turbid if made up in hard water. Solutions of soluble carbonates, ethandioates fluorides and hydroxides also need to be made up in distilled water. Solutions required for volumetric work must be based upon purified water.

13.9.2 Measuring purity Schools often need to check the purity of the water they are using but, as discussed below, a measurement of pH is not a satisfactory test and only the conductivity is at all reliable. Conductivity Water is a conductor of electricity because a very small proportion of water molec-ule have broken up (dissociated) into hydrogen (H+) and hydroxyl (hydroxide) ions (OH-). Because salts conduct electricity when dissolved in water, conductivity can be used to measure the purity of a sample of water; the values to be expected are listed in Table 13.17.

Table 13.17 The conductivity of ‘pure’ water

Purity level Conductivity Comments / S cm-1

1 Nuffield Advanced Science, Book of Data; Longman, 1984, ISBN 058235448X, p69. 2 Nuffield Advanced Science, Book of Data; Longman, 1984, ISBN 058235448X, p122. Mainly chemistry 1358 © CLEAPSS 2005

Pure water 4.2 × 10-8 It is difficult to demonstrate this theoretical value as impurities (eg carbon dioxide from the air, or ions from glass) increase the conductivity appreciably. Water from a good 0.2 × 10-6 This soon rises to a value of 0.9 × 10-6 S cm-1 as carbon dioxide is absorbed. deioniser Laboratory air often causes a greater rise due to traces of acidic gases etc. The conductivity will also rise as the resin becomes exhausted. Distilled water 2 × 10-6 The high value is caused by dissolved carbon dioxide. If this excess carbon dioxide is removed, a value of 2 × 10-6 S cm-1 or lower would be expected. Water from reverse 1 × 10-6 Like distilled water and in contrast with deionised water, it will be free from bacteria osmosis initially. pH Measuring the pH of water with low levels of dissolved salts with a pH probe is very difficult as the reading will certainly take a long time to settle. The only reliable way to obtain a figure is to allow the water to flow over the probe slowly for a while. The pH obtained for distilled water could be as low as 4.9 (the value for water direct from the CLEAPSS still) and is probably due to dissolved carbon dioxide but is not serious for most school chemistry. Significantly alkaline pH values would not be expected for distilled water and, if real, would indicate a serious contamination problem. Unusual pH values in the output of a deioniser indicate that one of the resin components has become exhausted before the other. This would also show up as a large increase in conductivity due to the rise in the concentration of OH- or H+ ions. Biological The presence of organic matter in deionised water may be important for biological impurities applications. Filter units are available to fit in series with deionisers if removal of organic matter is important. Note also that deionised water is not sterile. It is difficult in any case to keep distilled water sterile even though it is so as it emerges from the still.

13.9.3 Water purification Schools purify water using stills or deionisers1 but a third option, reverse osmosis, is becoming more competitive.

Distillation Distillation involves boiling impure water in a still and condensing the vapour to yield ‘pure’ water. Stills supply water at rates of up to 4 litres an hour. The water will initially be free from organic matter but will contain one to three ppm of dissolved salts and considerable amounts of carbon dioxide1. The residual salts in distilled water are partly leached from the glass and partly carried over in the steam in very fine spray. Stills cost over £350 and running costs are low (if you are not paying the electricity bills directly !). If the TDS value for water is > 300 ppm then a still is probably the best solution for your water purification problems. Storage of the distilled water is essential to provide adequate supplies when needed.

Removing scale As scale begins to cover the heating element, more energy is required to heat the water. Thus in hard water areas it is necessary to remove the scale. Glass stills should be emptied and filled with 2 M hydrochloric acid. Once the scale is removed and the flask is rinsed out, it is advisable not to collect the first litre of distilled water. Metal stills should be washed out using a commercial scale remover (usually methanoic acid or citric acid) used for descaling kettles.

Acidic distilled The carbon dioxide content of distilled water can be considerably higher than equilibrium with air would cause in areas with significant temporary hardness (calcium

1 For a more comprehensive account and a survey of stills consult CLEAPSS guide R10, Water purification. 1 Carbon dioxide can give rise to quite low pH values in distilled water. For pH measurement of distilled water, boil the water in a beaker to expel the carbon dioxide before testing. © CLEAPSS 2005 1359 Mainly chemistry water hydrogencarbonate). This occurs because the internal volume of a still is quickly purged of air by the steam up to the condenser. Temporarily hard water will release carbon dioxide as it boils and this will build up in the cool end of the condenser where the water on the condenser coils readily takes it up. If the level of carbon dioxide on distilled water is considered a problem for an experiment, then boil the water and a allow it to cool in a sealed bottle. Bubbling nitrogen through water can also drive off carbon dioxide.

Deionisers Impure water is passed through a resin (often based upon polystyrene) which exchanges metal ions in the water for hydrogen ions and anions in the water for hydroxyl ions. The hydrogen and hydroxyl ions then combine to form water. A water softener uses a different type of resin which does not purify water but exchanges calcium and magnesium ions for sodium ions. It is of no use to school science labs unless used in tandem with a still (when it can reduce the need for frequent descaling). Deionised water varies in purity as the resin becomes exhausted. A good deioniser will produce 60 - 80% of its output to a standard of 0.5 ppm residual salts or better but the water obtained at the end of the resin’s life may well be worse than that from a still. Water is available ‘on tap’ and the product has little dissolved carbon dioxide. However, in a hard water area, the resin will become exhausted more rapidly than in a soft water area and will need to be replaced or regenerated regularly. Organic material is not removed either and, if it is not regularly used, bacteria will grow in any stagnant water. The cost of a deioniser depends on the size of the cartridge but starting prices begin at about £150, but it is important to remember to put aside money for replacement cartridges each year. If the TDS level is less than 100 ppm, then a deioniser is probably the best solution.

Reverse osmosis The diagram shows that the feed water (ie tap water) is pumped into a pressure vessel containing a semi-permeable membrane. This is a thin polyamide film which strips the water of 90-98% of inorganic ions1 and virtually all organic molecules with a molar mass greater than 100 g. Colloids, bacteria, pyrogens and viruses are also removed.

1 Polyvalent ions are rejected by the membrane more efficiently than monovalent ions. Mainly chemistry 1360 © CLEAPSS 2005

Applied pressure

H O 2

H O 2

H O 2

Pure Feed water water

Semi-permeable membrane

Although this process is still relatively new and expensive where a pump is needed to boost the feed water pressure, running costs are said to be low and it may well become cheaper as the technology becomes less expensive to produce. The membrane should be able to cope with 25,000 litres of water.

Combined systems The systems described above can be worked in tandem for even purer water or lower running costs. These are summarised below. The first system is very attractive in areas with a high TDS level as the deioniser resin lasts for a very long time before regeneration becomes necessary. The second is attractive where the water is so hard that the still needs descaling frequently and the third option may be the preferred one where the incoming water is at high pressure.

Distillation Deionizer

Water Tap Distillation Pure Water softener water Reverse Deionizer osmosis

© CLEAPSS 2005 1361 Mainly chemistry

13.9.4 Water storage Distilled water Water storage is essential with a still as the rate of water output will be of the order of 4 litres per hour or less. Large polythene aspirators are suitable for this use and they should be sited away from direct sunlight to avoid problems with embrittlement of the plastic. For school uses, deterioration of the water purity is not a significant problem. Indeed, water with excess carbon dioxide will lose that excess and approach equilibrium carbon dioxide concentration in storage.

A major problem with water storage from stills is that the still has to be left on for several hours to fill a large aspirator. It is all too easy to leave the still on for too long and allow the aspirator to overflow. More expensive stills have reservoir limit switches and such switches can also be fitted to other models. Deionised water Water may normally be taken from a deioniser as it is required; they often have flow rates as high as a litre per minute. For those deionisers that require the resin cartridge to be returned for regeneration, it is wise to fill a small aspirator for use while the cartridge is away. Alternatively, a spare cartridge could be purchased and used in rotation with the main one.

13.10 Ground-glass jointed apparatus [The information here was previously in section 9.8.3 and has not been revised. It will ultimately be incorporated into a revision of section 13.7.1.] Ground glass joints are labelled using an internationally agreed dual number system where, for example, joints are marked ‘14/23’ meaning that 14 mm is the nominal diameter of the larger end of the joint and 23 mm is nominal length of the joint. It can be cheaper to buy standard packs of flasks etc. The decision to buy kits or not really depends on the systems of storage and distribution with the laboratory. Condensers Modern kits usually have Liebig condensers but ‘cold finger’ condensers are still available and have advantages. With this type, when refluxing and distilling, the apparatus is balanced over the flask and so does not need a separate clamp; secondly, it is very easy to change from reflux to distillation. However, the cold finger types do not seem to be quite as efficient as the Liebig ones. This defect is mainly due to the outside of the condenser being considerably warmed by the convection currents from the distillation flask and from the original source of heat. Quickfit condensers now have short threaded glass outlets onto which a tubing connector with an O-ring seal will screw. Spare connectors of both types are available and some users may find it useful to have several connectors and leave them attached to tubing which can then be screwed onto the item of equipment. Flasks For preparative work 50 ml capacity seems most useful. The 25 ml capacity flask is too small for many preparations although it is a very useful size for re-distillations. A 100 ml flask will be necessary for some experiments, eg steam distillation. Most users find that pear shaped flasks are preferable to round ones because there are fewer problems due to bumping. However, they are more difficult to clean. Flasks with two necks are a great help in some experiments, eg, where liquid additions have to be made while the preparation is refluxing, particularly if a cold finger condenser is used. Thermometers Thermometer pockets are not at all satisfactory. Thermometers with cones are available but are expensive. Bibby (the makers of Quickfit) supply a screw cap adapter which holds an ordinary thermometer with its bulb directly in the vapour or liquid. Tap funnel This is a useful item when liquids are to be added slowly during the preparation. Seized joints Naturally, prevention is better than cure. A touch of suitable grease or the use of an aerosol spray of dry film lubricant and the immediate dismantling and washing of the apparatus after Mainly chemistry 1362 © CLEAPSS 2005 use, usually avoids the problem. Particular care should be taken where alkaline solutions are involved. Joints that have seized should be left under running warm water for a few minutes before another attempt. PTFE sleeves are now available for use with ground glass joints and these are recommended for any use where seizure is likely. A basic kit Cold finger condensers 50 ml pear-shaped flasks (with a large proportion of 2-necked ones) 25 ml round-bottomed flasks (a few, for re-distillations) 100 ml round bottomed flasks (a few, for steam distillations) Calibrated receivers Screw cap adaptors for thermometers Stoppers Tap funnels Drying tube, adaptors with T-connection etc (a few as required) Quick-fit joint clips These plastic clips fit over a joint holding the two parts together. They are extremely useful as they hold the equipment firmly and so reduce the number of support clamps needed.

Storage The storage of large numbers of individual items can be a problem. One useful idea is to line drawers with plastic foam and divide it into sections with more strips of plastic foam to stop items like round bottomed flasks rolling about too much.