Question

Give a brief description of the principles of the crystallization and give examples.

Answer 1

Hint: In crystallization, the impure compound is converted into its crystals. It is based on the difference in the solubilities of the compound and the impurities in a suitable solvent. Crystallization of sugar and benzoic acid are some examples of the crystallization process.

Complete step by step answer:
Crystals are the purest form of a substance having definite geometrical shapes. The process by which an impure compound is converted into its crystal is known as crystallization.
This is one of the most commonly used techniques for the purification of solid organic compounds. It is based on the difference in the solubilities of the compound and the impurities in a suitable solvent. The impure compound is dissolved in a suitable solvent in which it is sparingly soluble at room temperature but appreciably soluble at a higher temperature. The solution is concentrated to get nearly a saturated solution. When this saturated solution is cooled crystals of pure substance will separate which are removed by filtration. The filtrate example, mother liquor contains the impurities along with a small quantity of the compound. If the compound is highly soluble in one solvent and too little soluble in another solvent, then crystallization can be carried out in a mixture of these solvents taken in a suitable ratio.

Example: Crystallization of benzoic acid: Suppose we have a mixture of benzoic acid and naphthalene. This mixture can be purified by treating the impure solid with hot water. Benzoic acid will dissolve while naphthalene remains insoluble. The hot solution is filtered and then allowed to cool when crystals of benzoic acid separate. The crystals are separated by filtration and dried.

Note: The solvent should not react chemically with the impure substance and dissolve more of the substance on heating than at the room temperature so that the excess of the substance is thrown out upon cooling.