All subjects · Chemistry · WCH15 (Unit 5)

Practical techniques and preparing aspirin (Core Practical 16)

Making an organic compound pure takes as much skill as making it. Core Practical 16 prepares aspirin, purifies it by recrystallisation and checks its purity.

Techniques for liquids

Heating under reflux: heat for a long time without losing volatile substances.
Distillation: separate a liquid by boiling point. Steam distillation separates a volatile compound that does not mix with water (such as an essential oil or phenylamine) at below 100 °C, so it does not decompose.
Solvent extraction: shake with an immiscible solvent in which the product is more soluble, and separate the layers in a separating funnel.
Washing: with water to remove soluble impurities, and with sodium carbonate or hydrogencarbonate solution to remove acids (release the CO2 pressure).
Drying: anhydrous MgSO4, Na2SO4 or CaCl2.

Recrystallisation

1. Dissolve the impure solid in the minimum volume of hot solvent (the product should be much more soluble hot than cold).

2. Filter while hot, if needed, to remove insoluble impurities.

3. Cool slowly so crystals form; soluble impurities stay in solution.

4. Filter under reduced pressure (Buchner funnel), wash with a little cold solvent and dry.

Using too much solvent leaves product in solution and lowers the yield.

Checking purity

Measure the melting temperature in a capillary tube in a melting point apparatus. A pure solid melts sharply (within about 1 °C) at the data-book value; impurities lower the melting point and make it melt over a range. Thin-layer chromatography showing a single spot also suggests a pure product. Liquids are checked by boiling temperature.

Core Practical 16: aspirin

Warm 2-hydroxybenzoic acid with ethanoic anhydride and a few drops of concentrated acid catalyst:

C7H6O3 + (CH3CO)2O -> C9H8O4 + CH3COOH

Add cold water, cool in ice, and filter off the crude aspirin under reduced pressure. Recrystallise from a mixture of ethanol and water, warming gently in a water bath (ethanol boils at 78 °C and is flammable). Dry the crystals, weigh them and find the percentage yield, then measure the melting temperature and run a TLC plate.

Worked example

2.00 g of 2-hydroxybenzoic acid (Mr 138.0) gives 1.45 g of aspirin (Mr 180.0). Calculate the percentage yield.

n = 2.00 / 138.0 = 0.01449 mol
Theoretical mass = 0.01449 x 180.0 = 2.61 g
% yield = 1.45 / theoretical x 100

Answer: 55.6%

Key idea: Reflux, distil (or steam distil), extract, wash, dry. Recrystallise: minimum hot solvent, hot filter, cool, filter under reduced pressure, wash with cold solvent, dry. Pure solids melt sharply at the data value. CP16: aspirin from 2-hydroxybenzoic acid and ethanoic anhydride.

Check yourself

3.00 g of 2-hydroxybenzoic acid (Mr 138.0) gives 2.50 g of aspirin (Mr 180.0). Calculate the percentage yield.
Show answer
63.9 %. Theoretical = 3.00 / 138.0 x 180.0 = 3.91 g; yield = 2.50 / 3.91 x 100 = 63.9%.
Calculate the atom economy for making aspirin: C7H6O3 (Mr 138.0) + C4H6O3 (Mr 102.0) -> C9H8O4 (Mr 180.0) + CH3COOH (Mr 60.0).
Show answer
75.0 %. 180.0 / (138.0 + 102.0) x 100 = 75.0%.
Why is the minimum volume of hot solvent used in recrystallisation?
- To make the solvent boil faster
- To dissolve the insoluble impurities
- So that as much product as possible crystallises on cooling
- To stop the crystals melting
Show answer
So that as much product as possible crystallises on cooling. Excess solvent keeps more product dissolved when cold, lowering the yield.
Why are the crystals washed with cold rather than hot solvent?
- To dry the crystals
- To remove soluble impurities without dissolving much of the product
- To raise the melting point
- To react with the impurities
Show answer
To remove soluble impurities without dissolving much of the product. The product is much less soluble in cold solvent.
A sample of aspirin melts over a range several degrees below the data-book value. What does this show?
- The sample is pure
- The thermometer is broken
- The sample is a different polymorph only
- The sample is impure
Show answer
The sample is impure. Impurities lower and broaden the melting range.
When is steam distillation used?
- To dry an organic liquid
- To separate a volatile, water-immiscible compound that might decompose at its boiling point
- To separate two miscible liquids with close boiling points
- To dissolve a solid in water
Show answer
To separate a volatile, water-immiscible compound that might decompose at its boiling point. It distils the compound below 100 °C.
In recrystallisation, why is the hot solution filtered before cooling?
- To remove soluble impurities
- To speed up crystallisation
- To recover the solvent
- To remove insoluble impurities
Show answer
To remove insoluble impurities. Soluble impurities stay in the solution after cooling; insoluble ones are filtered off hot.

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Last updated 7 November 2026. Spotted a mistake? Tell us.