Crystallisation method can be used for the purification of those mixtures which
i. Contains insoluble and/or soluble impurities.
ii. Have crystalline nature.
iii. Cannot be separated by filtration as some impurities are soluble.
By the technique of crystallization, pure solids are separated from impurities. For example, salt obtained from sea is separated from impurities; crystals of alum (Phitkari) are separated from impure samples.