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AQA · OCR A · practical chemistry

A-level chemistry practical techniques

Clear apparatus and technique diagrams for five practical methods that students are expected to recognise, describe or justify.

These are reference diagrams, not substitutes for recalling the method in an exam. On revision pages you may see a diagram link beside a practical technique; opening it brings up the same labelled image without taking you away from the topic.

Measurement

Calibrating and using a pH meter

Calibrate before measuring an unknown. Use buffer solution(s) of known pH as required by the meter, rinse the probe with deionised water between solutions, blot it gently and wait for a stable reading in the sample.

Five-step diagram showing calibration of a pH meter with a buffer of known pH, rinsing the probe with deionised water, blotting gently, immersing it in a sample and waiting for a stable pH reading.
Calibrate with buffer solution(s) of known pH as required by the meter, rinse the probe with deionised water, blot gently, then measure the sample and wait for a stable reading.

Rinsing prevents carry-over from one solution to the next. Blotting avoids diluting the next solution with rinse water while reducing the risk of damaging the probe by wiping it hard.

Mass transfer

Weighing by difference

Weigh the container and solid before transfer, transfer the solid, then reweigh the container and anything left in it. The difference is the mass that actually reached the receiving vessel.

Four-step diagram showing weighing by difference: record the initial mass of a weighing boat and solid, transfer the solid, record the final mass, then calculate the mass transferred from the difference.
The mass actually transferred is the initial mass of the container plus solid minus the final mass of the container plus any solid left behind.

This is useful whenever a little solid may remain in the weighing boat or bottle: the calculation uses the amount transferred, not the amount you hoped to transfer.

Solid purification

Filtration under reduced pressure

Use a Büchner funnel and side-arm flask connected to a vacuum source. The solid is retained on the filter paper while liquid is drawn rapidly into the flask.

Filtration under reduced pressure apparatus showing a Buchner funnel with filter paper and residue, a side-arm Buchner flask connected to a vacuum pump, and filtrate in the flask.
A Büchner funnel and side-arm flask connected to a vacuum source collect the solid on the filter paper while the filtrate is drawn into the flask.

When collecting recrystallised product, wash the crystals with a small amount of cold solvent and keep the suction on to help remove liquid from the solid.

Liquid separation

Simple distillation

Heat the mixture so the more volatile component forms vapour, pass that vapour through the water-cooled condenser and collect the condensed liquid as distillate.

Simple distillation apparatus showing a heated round-bottom flask with anti-bumping granules, thermometer, sloping water-cooled condenser, water inlet and outlet, and a receiver collecting distillate.
In simple distillation, vapour leaves the heated flask, passes through the condenser and is collected as distillate. Cooling water enters the condenser at the lower connector and leaves at the upper connector.

Cooling water enters at the lower connector and leaves at the upper connector so the condenser jacket stays full. The thermometer bulb should measure the vapour temperature at the still head.

Organic preparation

Heating under reflux

Reflux lets a reaction mixture boil for an extended time without continually losing volatile solvent or reactants. Vapour condenses in the vertical condenser and returns to the reaction flask.

Heating under reflux apparatus showing a round-bottom flask with reaction mixture and anti-bumping granules beneath a vertical water-cooled condenser, with water entering at the bottom and leaving at the top.
A vertical condenser returns condensed vapour to the reaction flask during reflux. Cooling water enters at the lower connector and leaves at the upper connector, and the heated apparatus is not sealed.

Cooling water enters at the bottom and leaves at the top. Do not seal a heated reflux apparatus: pressure must not be allowed to build up.

Practise the decisions around the apparatus

The practical question sets test why a step is used, what can go wrong and how the method changes with the chemical context.

Open practical chemistry Practise organic methods