Question

Explain the principle of crystallization.

Answer 1

Crystallization - The process of cooling a hot, concentrated solution of a substance to obtain crystals is called crystallization. It is the most commonly used technique for the purification of a solid organic compound.

Principle of Crystallization -

1. It is based on the difference in the solubility of the compounds and the impurities in a suitable solvent.
2. The impure solid compound is dissolved in a solvent in which it is sparingly soluble at room temperature but appreciably soluble at a higher temperature.
3. The solution is then concentrated by heating gently on a water bath to get a nearly saturated solution. Now, on cooling the hot saturated solution, the crystals of the pure solid compound are obtained.
4. Crystals of the pure compound so obtained are removed by filtration.
5. The filtrate (mother liquor) contains dissolved impurities and a small quantity of the compound.
6. Repeated crystallization becomes necessary for the purification of compounds containing impurities of comparable solubility.

Example -
1. Crystals of pure Copper Sulphate$\left({\mathrm{CuSO}}_4\right)$ can be obtained from an impure sample of Copper Sulphate.