The process in which a pure solid is separated from a solution in the form of its crystals is called Crystallisation. Crystallisation is a method which is used to purify the solids. The mixtures containing solids in which the impurities may be either 'less soluble' in the solvent or 'more soluble' in the solvent than the solids, are separated by the technique of crystallisation. For example, the impurities present in impure copper sulphate, common salt and alum, etc., can be separated by this technique crystallisation to obtain the pure substances.