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Experimental Techniques in Chemistry The guide also points out relevant planning and interpretative skills. The A Teacher’s Guide programmes are unable to show these, yet they also form an important aspect Hong Kong Examinations Authority, 1987 of teaching.

Introduction Assessment Points

This guide has been written to complement the four video programmes on The main purpose of the guide is to assist teachers with the assessment of experimental techniques produced by a working party of the Hong Kong practical work. In the guide, assessment points related to assessment of Ability Examinations Authority. The titles of the video programmes are: Area A have been given separately, under the criteria:

1. Carrying Out the Flame Test 1. Operation of a single piece of apparatus 2. Use of Quickfit Apparatus 2. Carrying out experimental procedures 3. Acid-Base Titration 3. Reading and measurements of different instruments/ apparatus 4. Active Ingredients in Aspirin Tablets 4. Observational skills 5. General bench performance The guide indicates the teaching points illustrated in the programmes and considers the areas where assessment would be appropriate. Both the teacher’s These criteria have been chosen, based on the reactions of teachers to the guide and the programmes have been produced by a working party, in assessment plan but forward by one member of the working party. In the plan, response to concerns expressed by teachers on the teaching and assessment manipulative skill assessment can be divided into five aspects (1 to 5 above), required in A-level Chemistry practical work. although not all five would be applicable to a given experimental situation. Further details of the plan can be obtained from the working party. It is expected that teachers will wish to show each programme either at one time or in parts, and they have been produced with this in mind. The Assessment points for Ability Area B have also been given, although programmes illustrate techniques that are expected of students and hence lay interpretative skill assessment is not possible in some cases. Attention is drawn stress on the manipulation of apparatus, carrying out experimental procedures, to the assessment of planning skills as this is an area where problem-solving taking readings, making observations, safety aspects and general bench skills in practical work can be assessed cognitively by writing out a plan, or, in performance. terms of manipulative skills, by carrying out the experiment planned.

Teaching points It is the wish of the working party that teachers should take note of the assessment guidelines proposed. There is the implicit suggestion that if all Each guide lists the teaching points related to the experimental techniques. The teachers follow such guidelines, then this would serve as a powerful indicator techniques shown in the programmes may not concur with those currently of standards across Hong Kong schools and hence help to increase the used in your teaching and the working party is aware of other alternatives in reliability of assessment under the Teacher Assessment Scheme. experimental techniques which are equally acceptable. These notes, whilst not claiming to be exhaustive, are written to cover major teaching criteria and it is hoped that all teachers will cover these in their practical lessons. It must be remembered that assessment of manipulative and observational skills is not appropriate before students have received the relevant teaching.

1 Programme 1 : Carrying Out The Flame Test (i) moistening the wire with concentrated hydrochloric acid placed in a watch Main Objective: To identify criteria for the assessment of techniques in glass/test-tube (When using carrying out the Flame Test. concentrated hydrochloric acid, care should be taken to keep one’s face away 1 Duration: 4 /2 minutes from the acid fumes, and to limit the time the stopper is out of the bottle. Any Criteria for Assessment: spillage should be cleaned immediately with a moist cloth.) A. Manipulative skills, observation and general bench performance (ii) introducing the moistened wire into the tip of the blue cone of a roaring Bunsen 1. Operation of a single piece of apparatus flame (iii) repeating (i) and (ii) if necessary until Assessment Point Teaching Point there is no flame coloration Students should be able to (b) conduct the Flame Test conduct the Flame Test by (a) use the use the Bunsen Burner by (i) putting a small amount of the powered sample onto a or other (i) regulating the gas flow suitable container (ii) adjusting the air-hole (ii) moistening the cleaned wire with a (iii) using a non-luminous flame in fresh supply of concentrated conducting the Flame Test hydrochloric acid (b) use a piece of cobalt blue view a flame through the cobalt blue glass (iii) dipping the tip of the moistened wire glass while introducing the platinum/nichrome into the powdered sample, so that some wire into the flame of it adheres to the wire (iv) introducing the tip of the wire to the

base of a quiet, non-luminous Bunsen 2. Carrying out experimental procedures flame, gradually moving the wire to a

hotter part of the flame if no colour is Assessment Point Teaching Point observed Students should be able to (v) repeating (ii) to (iv) for further confirmation, if necessary (a) clean a platinum / clean a platinum/nichrome wire by nichrome wire properly

2 3. Observational skills (e) take safety precautions in take safety precautions in using the Bunsen using the Bunsen Burner Burner by Assessment Point Teaching Point (i) not leaving a non-luminous flame Students should be able to unattended (ii) adjusting to a luminous flame when the (a) observe and record the observe and record the characteristic flame burner is not in use characteristic flame colours, eliminating those that come from (iii) turning off the gas supply should colours impurities “striking back” occur, and relighting observe that some flame colours may only when the burner is cool (i) show up later than others

(ii) be more persistent than others B. Planning the experiment and interpretation of data

4. General bench performance Assessment Point Teaching Point

Students should be able to Assessment Point Teaching Point (a) identify the common Students should be able to cations by their flame (a) take safety precaution take safety precaution by colour, volatility and when carrying out Flame (i) wearing safety goggles persistency, etc. Tests (ii) paying attention to the handling of (b) decide whether cobalt concentrated hydrochloric acid, so as to blue glass should be used minimize the amount of acid fumes and in the identification of to mop up any spillage immediately potassium ions in the presence of sodium ions (b) utilise available bench space sensibly

(c) keep the working area tidy (d) use the time available efficiently

3 Programme 2: Use of Quickfit Apparatus (c) control the rate of flow of (iv) controlling the rate of flow of water so water in the that there are no air bubbles, the tubing Main Objective: To identify criteria for assessing basic techniques in the is not dislodged, and the rate is geared to assembly and use of quickfit apparatus. the boiling point of the distillate

1 (d) check that the side arm of check that when a T-shaped adapter is used Duration: 8 /2 minutes a T-shaped adapter is for connecting a receiver flask, the side arm Criteria for Assessment pointing upwards of the adapter should be pointing upwards

A. Manipulative skills, observation and general bench performance 2. Carrying out experimental procedures

1. Operation of a single piece of apparatus Assessment Point Teaching Point

Students should be able to Assessment Point Teaching Point (a) assemble the apparatus in assemble the apparatus by Students should be able to a systematic manner, (i) considering the position of clamping, lubricate the glass joint evenly, using only a without the need to distance from the sink/ burner, etc. small amount of lubricant frequently dismantle and (ii) starting from the base and building up re-adjust the position of clamp the apparatus from there the apparatus (i) at a suitable position (ii) with the movable parts uppermost (b) introduce chemicals transfer chemicals into the reaction flask (iii) firmly but not too tight without having any without having any chemicals sticking to the chemicals left at the joints joints (a) place the adjust the position of the thermometer by (c) assemble the apparatus check, after assembling the apparatus, that bulb in the correct unscrewing the adapter, sliding the correctly position thermometer through the ring and then (i) the system is not a closed one retightening it (ii) the joints are well fitted (iii) the system is not tilted set up a water condenser by (i) checking that the rubber tubing is of (d) add anti-bumping use anti-bumping granules by sufficient length granules correctly (i) adding only a few (ii) fitting the rubber tubing by lubricating (ii) adding them only when the liquid is with a little water cold (b) run water into the (iii) running water into the condenser from (e) avoid over-heating adjusting the rate of heating by condenser from the lower the lower end, and end (i) controlling the size of the flame/ temperature of the heater (ii) removing the burner at suitable time(s)

4 5. General bench performance

(f) collect the distillate collect the distillate Assessment Point Teaching Point properly (i) at a suitable rate (ii) within a suitable temperature range Students should be able to (iii) in a receiver that is adequately (a) avoid having avoid having unnecessary apparatus lying on supported and correctly positioned unnecessary apparatus the bench, e.g. the boxes for holding the lying on the bench quickfit sets, unused rubber tubing, etc. dismantle the apparatus as soon as it is cool enough to be handled B. Planning the experiment and interpretation of data clean the apparatus properly Assessment Point Teaching Point 3. Reading and measurement of different instruments/apparatus Students should be able to Assessment Point Teaching Point (a) plan how to carry out an plan how to carry out an experiment by experiment efficiently Students should be able to (i) deciding on the appropriate quantities of chemicals to be used (a) read and record the read and record the temperature/ (ii) choosing suitable apparatus for carrying temperature/ temperature range of the distillate/ various out a particular process temperature range of the distillate fractions (to the nearest °C) (iii) designing a workable procedure distillate/ various distillate fractions (to the consider the various facilities required, e.g. nearest °C) location of the sink, heating devices, etc., to avoid unnecessary dismantling and 4. Observational skills re-adjustment

Assessment Point Teaching Point Students should be able to (a) detect any irregularities detect any irregularities as they arise, e.g. as they arise (i) leakage at the joints (fumes, bubbling at the joints) (ii) lack of reaction in the flask

5 Programme 3: Acid-Base Titration (b) The filler

Main Objective: To identify criteria for assessing basic techniques in Assessment Point Teaching Point carrying out acid-base titrations. Students should be able to Duration: 19 minutes (a) use a pipette filler use a pipette filler properly (i) whenever a pipette is to be filled Criteria for Assessment: (ii) by following the correct sequence of opening the valves for a filler with A. Manipulative skills, observation and general bench performance valves (iii) and ensure that no liquid enters the 1. Operation of a single piece of apparatus bulb

(a) The pipette (b) deliver the exact quantity deliver the bulk of the liquid into a conical of liquid from a pipette flask, the final portion being drained out by Assessment Point Teaching Point touching, for a few seconds, the bottom of the flask in a tilted position, by touching the Students should be able to liquid surface, or by touching the side of the (a) rinse a pipette rinse a pipette with a small amount of the conical flask solution to be titrated - draining it only through the tip (c) The (b) keep the tip of the pipette keep the tip of the pipette well below the well below the surface of surface of the solution during pipetting Assessment Point Teaching Point the solution during wipe the outside of the pipette Students should be able to pipetting (a) use a burette use a burette by ensuring that (c) control the amount of control the amount of solution in the pipette solution in the pipette by regulating the pressure inside it through (i) after filling the burette, the filter minute movement of the forefinger on the (if used) is removed top end of the pipette stem (ii) there are no air bubbles trapped in the liquid column, and the space from ensure that the bulb of the pipette is not held below the stopcock to the tip is filled in the hand, especially when filling with with solution solution (iii) the burette is clamped in an upright position, and the burette reading is taken with the meniscus at eye level

6 2. Carrying out experimental procedures (ii) pipetting a fixed volume of solution to a clean conical flask (Not rinsed with the Assessment Point Teaching Point solution) (iii) adding a suitable quantity of the Students should be able to indicator to be used (a) prepare a solution in a prepare a solution in a by (iv) placing the conical flask on a white tile, volumetric flask without (i) dissolving the solute in the minimum and then lowering the burette to a spillage and without amount of solvent in a beaker position such that the tip is just below overshooting the (ii) washing the volumetric flask with the the rim of the conical flask graduation mark solvent before filling with the solution (v) adjusting the rate of addition of liquid (iii) transferring the solution to the by controlling the stopcock or the tap, volumetric flask without spillage, and so that the liquid can be added without overshooting the graduation continuously, or one drop at a time, as mark appropriate (iv) making up to the mark by adding the (vi) swirling the conical flask during the last drops with a dropper, a , addition of liquid from the burette to or from the ensure thorough mixing (v) ensuring homogeneous mixing of the (vii) washing down any liquid adhering to prepared solution by inverting the the inner wall of the conical flask with volumetric flask several times distilled water

(b) prepare the prepare the pipette/burette for use by 3. Reading and measurement of different instruments/apparatus pipette/burette for use (i) rinsing first with distilled water, and then with the liquid to be used (ii) draining the liquid out of a pipette Assessment Point Teaching Point through the jet Students should be able to (iii) draining the liquid from a burette by inverting it with the tap closed, then (a) view the meniscus on a view the meniscus on a graduation mark at opening the tap while draining the graduation mark at eye eye level liquid from the column level (b) record burette readings record burette readings to two decimal (c) carry out the titration carry out the titration process by accurately places, the second decimal place being process (i) transferring the solution from a obtained by estimation volumetric flask to a beaker rinsed with

the solution

7 4. Observational skills (c) obtain and record results obtain and record results to an acceptable to an acceptable degree of degree of accuracy by Assessment Point Teaching Point accuracy (i) selecting consistent titres for calculating Students should be able to the average titre, e.g. ±0.01 cm3 , or according to a (a) detect the end-point of a detect the end-point of a titration by noting specified level of accuracy titration the correct colour changes in the solution (ii) calculating molarity to 4 decimal places (iii) calculating percentage purity to the 5. General bench performance nearest 1% (iv) determining masses to ±0.001g Assessment Point Teaching Point Students should be able to (a) transfer liquids without spillage (b) prevent the pipette from rolling on the bench

B. Planning the experiment and interpretation of data

Assessment Point Teaching Point Students should be able to (a) plan an experiment plan an experiment involving titration involving titration techniques by choosing the appropriate techniques apparatus, materials, indicator(s), and method of working (b) interpret experimental interpret the experimental data obtained, and data make use of the data to perform relevant calculations

8 Programme 4: Active Ingredients In Aspirin Tablets (b) dissolve a solid in the dissolve a solid by

minimal amount of (i) choosing an appropriate solvent Main Objective: To identify criteria for assessing basic techniques in solvent (ii) using the minimal amount of solvent crystallisation and melting point determination. (iii) crushing the solid to a powder form in a

mortar and pestle if necessary 1 Duration: 12 /2 minutes (iv) taking any necessary precautions

against fire hazards when flammable Criteria for Assessment: solvents are used

1. Operation of a single piece of apparatus (c) filter a hot, saturated filter a hot, saturated solution by

solution (i) using a piece of folded Assessment Point Teaching Point (ii) ensuring that the crystals do not form Students should be able to during the process (a) use a mortar and pestle use a mortar and pestle to grind crystals into (d) carry out the process of carry out the process of suction filtration by a powder suction filtration (i) using a Hirsh/Buchner funnel, (b) use filter paper in fold a fluted filter paper moistening the filter paper with solvent filtration to ensure proper suction (ii) using a Buchner flask and a suction (c) use a filter funnel use a filter funnel for filtering hot solutions pump (d) use a melting point use a melting point apparatus (iii) ensuring that the filtrate is not sucked apparatus into the pump by adjusting the direction of the tip of the funnel (iv) washing the crystals with the minimal 2. Carrying out experimental procedures amount of cold solvent

Assessment Point Teaching Point (e) prepare a melting point prepare a melting point tube by tube (i) using a capillary tube of suitable length Students should be able to (ii) sealing one end by heating in the flame (a) set up a water bath set up a water bath by considering with a rotating motion (i) the size of the container (ii) the quantity of water in the bath (iii) the rate of heating necessary to maintain the bath at a suitable temperature

9 (f) determine the melting determine the melting point of a purified point of a purified sample by sample (i) drying the crystals thoroughly in a desiccator/oven before introducing into the melting point tube (ii) packing the solid into a melting point tube by gently tapping it, or by dropping it through a

(Sept 98)

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