(a) Crystallisation : It is based on the difference in solubility of the compound and the impurities in a suitable solvent. While at room temperature, the compound is sparingly soluble and crystallizes out of solution but the impurities do not. As a result, they remain in solution and the compound is obtained as a crystal.
The impure compound is dissolved in a solvent and heated. At elevated temperature the compound dissolves as do the impurities. This solution is then gradually cooled.Being less soluble at room temperature it precipitates out in the form of crystals and pure compound is obtained.
(b) Distillation : This method is used to separate either :
(i) Volatile liquids from non-volatile impurities; and
(ii) Two liquids with different boiling points.
The liquid mixture such as that of chloroform and aniline is taken in a round bottom flask fitted with a condenser.
Upon heating, the vapours of lower boiling liquid are formed first and collected through the condenser. The vapours of the higher boiling liquid are formed later. Thus, the two are separated.
(c) Chromatography : (i) It is applicable for the separation of virtually all inorganic and organic materials, except very insoluble polymers.
(ii) In this technique, the mixture of compounds which needs to be separated is applied onto a stationary phase, which may be a solid or a liquid. Another phase which may be a pure solvent, a mixture of solvents or a gas is allowed to more slowly over the stationary phase.
(iii) The components of the mixture which have different solubility in the moving phase, start moving. Since, they have different solubility, they move to different lengths on the stationary phase and become stable there.
(iv) Thus, the different components of the mixture are separated.