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.