Q.Describe the method which can be used to separate two compounds with different solubilities in a solvent S.

Concept: Fractional crystallisation exploits differences in solubility to separate compounds.

When two compounds have different solubilities in a solvent, the less soluble compound crystallises out first upon cooling or evaporation, while the more soluble one remains in solution.

Method:

1. Dissolve the mixture in hot solvent $S$ until both compounds are in solution.
2. Cool the solution slowly or allow controlled evaporation. The less soluble compound reaches its saturation point first and crystallises out.
3. Filter to collect the crystals of the less soluble compound.
4. The filtrate (mother liquor) contains the more soluble compound, which can be recovered by further evaporation or cooling. …

When two solid compounds share the same physical state but differ in how readily they dissolve in a given solvent, we can turn that difference into a separation tool. The technique is fractional crystallisation (also called recrystallisation when purifying a single substance).

The core idea is simple: solubility usually increases with temperature. If compound A is sparingly soluble in solvent S and compound B is highly soluble, heating the solvent will dissolve both. But when you cool the solution back down, A will reach its saturation point first and crystallise out, leaving B behind in solution.

The Fractional Crystallisation Method

1. Dissolve the mixture in hot solvent S

   Heat the solvent to a temperature where both compounds dissolve completely (or nearly so). You want the minimum volume of hot solvent that achieves this — using excess solvent dilutes the solution and reduces yield.

2. Cool the solution slowly

   As the temperature drops, the solubility of both compounds decreases. The less-soluble compound (let's call it A) will become supersaturated first and begin to crystallise. Slow cooling encourages the formation of pure, well-formed crystals rather than an amorphous precipitate that traps impurities.

3. Filter to collect the crystals

   Once crystallisation is complete (often aided by scratching the flask or seeding with a tiny crystal), filter the mixture. The crystals of A are retained on the filter paper, while the mother liquor (the remaining solution) contains most of compound B.

4. Wash and dry the crystals

   Rinse the crystals with a small amount of cold solvent S to remove adhering mother liquor (which contains B), then dry them. Cold solvent is crucial — hot solvent would redissolve your product.

5. Recover the second compound (optional)

   If you need compound B as well, evaporate or further cool the mother liquor to crystallise it out. Alternatively, concentrate the solution and repeat the process.