Crystallization is mainly a property of pure substances. It is based on the principles of solubility of a pure compound in a suitable solvent. Solutes are usually more soluble in solvents which are at high temperature than they are in cold or at room temperatures.

If a saturated high temperature solution is allowed to cool, the solute particles tend to lose solubility and are nolonger soluble in the solvent. It thus forms crystals of the pure compound. Impurities are not at all there in the growing crystals and the pure solid crystals can be separated from the dissolved impurities by filtration.

This is the main principle behind crystallization.