Describe the method, which can be used to separate two compounds with different solubilities in a solvent S.
Crystallisation is the method used to separate two compounds with different solubilities in a solvent. The impure compound is dissolved in a solvent in which it is sparingly soluble at room temperature but appreciably soluble at higher temperature. The solution is concentrated to get a nearly saturated solution.
On cooling the solution, the pure compound crystallises out and is removed by filtration. The filtrate (mother liquor) contains the impurities and a small quantity of the compound. If the compound is highly soluble in one solvent and very little soluble in another solvent, crystallisation can be carried out in a mixture of these solvents. Repeated crystallisation becomes necessary for the purification of compounds containing impurities of comparable solubilities.
Explanation
The textbook describes crystallisation as a technique based on the difference in the solubilities of the compound and the impurities in a suitable solvent. By heating the mixture in the solvent, the desired compound dissolves, and upon cooling, it crystallises out, leaving the more soluble impurities in the mother liquor.