Teacher Assessment Scheme for Chemistry Practical
Suggested Assessment Points for Ability Area A
(Modified by the courtesy of MsCaroleKPLEE from a questionnaire prepared by her in 1987)
1.Operation of a single piece of apparatus
1.1Choice and use of specific apparatus
a)When a large quantity of liquid or solid is being heated, a boiling tube is used instead of a test tube.
b)For accurate measurement of volumes, a burette or pipette is used instead of a measuring cylinder or beaker with graduations.
c)When measuring the temperature or boiling point of a liquid, the bulb of the thermometer is dipped into the liquid.
d)When measuring the temperature of a vapour (e.g. in distillation), the bulb of the thermometer is placed in a suitable position so as to record the temperature of the vapour actually distilled over and collected.
e)When a water condenser is used (e.g. in distillation and refluxing), the rubber tubings are connected correctly for flow of water.
f)Apparatus are clamped securely with a stand when necessary.
1.2Use of Bunsen burner
a)When lighting the Bunsen burner, the air-hole of the burner is closed.
b)When in normal heating, the airhole of the burner is partly opened.
c)When strong heating is required (e.g. in flame tests or when rapid heating of the solution is required), the air-hole of the burner is completely opened.
d)When the Bunsen burner is not in used, the air-hole is closed to obtain a yellow wavy flame.
e)To decrease the intensity of the flame, the gas supply is adjusted instead of adjusting the air-hole.
f)When refluxing, the flame of the Bunsen burner is adjusted such that the condensation ring of the liquid rises up at most 1/5 of the length of the condenser.
g)When heating an object, the object is placed at the top of the central blue cone of the non-luminous flame.
h)When heating a liquid in a test tube, the test tube is moved to and fro through the flame and pointed towards an open area.
i)When an object to be heated is stationary or when even heating is required, a rotary movement of a non-luminous flame is used by revolving the Bunsen burner around the object.
1.3Use of stirrer
a)A glass rod (not spatula) is used for stirring solutions in beakers or in test tubes.
b)Thermometer is used as a stirrer only in experiments requiring measurement of temperature.
c)When a magnetic stirrer is used, the speed is adjusted to ensure smooth stirring and no spillage of solution.
1.4Handling of a pipette and pipette filler
a)A pipette filler is used when filling a pipette with the solution.
b)The bulb of the pipette is not touched with the hand, when filling with solution.
c)The tip of the pipette is dipped well below the surface of the solution to be filled.
d)The liquid level in the pipette is judged by placing the liquid surface at eye level and aligning the lowest meniscus level with the graduation mark.
e)The last drop of solution in the pipette is allowed to drain out (and not blown out) by touching the liquid surface or the side of the container for a few seconds.
f)Care is taken to prevent the solution from getting into the pipette filler.
g)The pipette filler is manipulated correctly by following the numbers on the pipette filler in sequence.
1.5Handling of a burette
a)The burette is clamped in an upright position.
b)The space between the tip and the stopcock of the burette is filled with solution.
c)After filling the burette, the filter funnel (if used) is removed.
d)Air bubbles are not trapped in the liquid column.
e)Reading of the burette is taken with the liquid surface at eye level.
1.6Correct use of a polarimeter
a)The polarimeter tube is cleaned with distilled water.
b)The polarimeter is securely placed between the polarizer and analyzer.
c)The ‘zeropoint’ of the instrument is determined by rotating the analyzer until maximum darkness is obtained.
d)To determine the angle of rotation, the analyzer is rotated on both sides to obtain a clear view.
e)The polarimeter tube is rinsed several times with portions of the new solution each time the solution is changed.
1.7Use of a colorimeter
a)The instrument is switched on for 5 minutes for warming up before use.
b)A suitable filter (complementary to the colour of the solution) is correctly selected.
c)The range is adjusted by choosing the most concentrated solution as 100% absorbance and pure distilled water as 0% absorbance.
d)The cell/test tube is rinsed each time with the liquid to be used.
e)For solutions involving organic solvents, a glass cell instead of perspex cell is used.
f)The reference cell/test tube is placed in the colorimeter each time for adjustment before taking the reading for the sample.
g)The meniscus of the solution in the cell/test tube is above the beam of light.
1.8Use of a hydrogen electrode
a)A steady but slow stream of hydrogen is bubbled through the cell until a constant e.m.f. is obtained.
b)The hydrogen electrode is rinsed with distilled water before dipping into the test solution.
c)The hydrogen electrode is not left lying on the bench after use.
1.9Use of a pH meter
a)The dial of the pH meter is set to the temperature of the solution.
b)The pH meter is adjusted with buffer solutions of known pH.
c)The electrode is rinsed with distilled water and with the solution to be studied.
d)The electrode is not left lying on the bench after use.
1.10Use of an ammeter/voltmeter
a)Electrical wires are connected properly.
b)The correct range of meter readings is selected.
c)The correct reading from meter scale is taken.
2.Carrying out experimental procedures
2.1Skills in filtering
Filtration under atmospheric pressure by using filter funnel with a fluted filter paper
a)Mixtures of solid and liquid are swirled or stirred before pouring onto the fluted filter paper in the filter funnel.
b)A glass rod is used to guide the flow.
c)The level of liquid in the filter funnel is below the top of the filter paper.
Suction filtration using a Buchner funnel and filter pump
a)Mixtures of solids and liquids is swirled or stirred before pouring onto the filter paper in the Buchner funnel.
b)The size of filter paper disc is correctly selected. (Diameter should be slightly less than the inside diameter of Buchner funnel and should not be folded up against the side of the funnel.)
c)The perforated plate of the Buchner funnel is covered completely before the mixture of solid and liquid is poured in.
d)To prevent water being drawn into the Buchner funnel, the pump is not turned off while filtering.
2.2Liquid-liquid extraction using separating funnel
a)The stopcock is greased with a suitable lubricant, e.g. vaseline or silicone stopcock grease.
b)The stopper at the top of the funnel is not greased.
c)The funnel is shaken gently with the stopper firmly held, and inverted occasionally with the stopcock opened in order to relieve excess pressure.
d)When two sharply defined layers have formed, the lower layer is run off and separated as completely as possible.
2.3Running a chromatogram (paper, thinlayer column)
a)Plates are handled by the edges or by the undersurface which is not coated with absorbent.
b)Spots of solution are applied with drawn capillary tubes.
c)Neighbouring spots are separated at a distance from each other.
d)A filter paper is lined inside the jar to saturate the atmosphere with the solvent.
e)The solvent level in the chromatographic tank is below the spots on the plate.
f)The tank is covered when running the chromatogram.
g)The plate is taken out immediately once the solvent front has ascended to the position which has been initially drawn across the plate.
2.4Titration
a)The volumetric flask is washed with distilled water before filling with the solution.
b)The solute is dissolved in a beaker before filling into the volumetric flask.
c)The solution is added with a dropper or a glass rod to make up to the graduation mark on the volumetric flask.
d)The solution is mixed by inverting the volumetric flask upside down several times.
e)The burette is rinsed with distilled water, then with the liquid to be used.
f)The sample to be titrated is placed in a conical flask.
g)The stopcock of the burette is controlled correctly to adjust the rate of addition of liquid.
h)The conical flask is swirled to ensure thorough mixing.
i)A white paper/tile is placed under the conical flask so that the change of colour of the indicator is easily detected.
j)Any titrant adhered to the inner wall of the conical flask is washed down with distilled water.
2.5Distillation (liquidliquid, liquid-solid, including purification of a liquid)
a)The flask is at most 1/2 to 2/3 filled with the solution to be distilled.
b)An air condenser is used when the boiling point of a liquid is greater than 150C.
c)The water condenser is connected for correct flow of water in and out of it.
d)Boiling chips are added to the solution to prevent bumping. (They should never be added to hot liquids.)
e)When boiling commences, the rate of heating is reduced and the source of heat adjusted so that the distillate is collected at the rate of one or two drops per second.
f)For fractional distillation, the receiver is changed after the collection of each portion of liquid.
2.6Crystallization and recrystallization
a)Suitable choice of solvent for crystallization is used.
b)The substance to be recrystallized is dissolved in a minimum amount of hot solvent.
c)The hot solution is poured into a conical flask through a shortstem filter funnel (with fluted filter paper placed on it).
d)The filtered solution is covered with a watchglass or filter paper to allow slow and undisturbed cooling.
e)When decolorizing charcoal is used, the solution is warmed and filtered when hot.
3.Reading and measurement
a)The no parallax method at eyelevel is used when reading the scale of an instrument.
b)The range and interval of the scale is noted before the reading is taken.
c)The correct number of significant figures and the unit of the reading are recorded.
d)When reading the scale of a thermometer and burette, the apparatus is in a vertical position.
e)When determining the melting point of a solid, the temperatures are taken when the solid starts softening until it completely liquefies.
f)When determining the boiling point of a liquid using the ignition tube method, the temperature is taken when the bubbles cease and the liquid starts to suck back into the tube.
g)A stopwatch is used correctly to start and stop counting the time when doing kinetic experiments.
4.Observations
a)An appropriate background (usually white) is used to judge the colour change of a solution or the formation of a precipitate.
b)Liberated gases are identified accurately by using test papers or collecting if necessary.
c)Changes such as effervescence, colour change, precipitate formation or heat change are detected correctly.
d)When viewing the flame colour of potassium compounds, a cobalt blue glass is used.
e)Colours in flame test are observed correctly.
5.General bench performance
a)Written or oral instructions are followed closely.
b)All the tasks are carried out in a correct sequence.
c)Working time is used efficiently.
d)Apparatus are handled and set up smoothly.
e)The working area is kept tidy.
f)The available bench space is utilized sensibly.
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