Question

Give a brief description of the principle of the following technique taking an example.

Crystallisation 

Answer 1

Crystallisation is one of the most commonly used techniques for the purification of solid organic compounds

Principle: 

It is based on the difference in the solubility of the compound and the impurities in a given solvent. The impure compound gets dissolved in the solvent in which it is sparingly soluble at room temperature but appreciably soluble at a higher temperature. The solution is concentrated to obtain a nearly saturated solution. On cooling the solution, the pure compound crystallizes out and is removed by filtration.

For example, pure aspirin is obtained by recrystallising crude aspirin. Approximately 2 – 4 g of crude aspirin is dissolved in about 20 mL of ethyl alcohol. The solution is heated (if necessary) to ensure complete dissolution. The solution is then left undisturbed until some crystals start to separate out. The crystals are then filtered and dried.