How is recrystallisation used to increase purity?

Recrystallisation is used to increase purity by dissolving a substance in a solvent and then allowing it to crystallise, leaving impurities behind.

Recrystallisation is a common method used in laboratories to purify solid substances. The process begins by dissolving the substance in a suitable solvent at a high temperature. The choice of solvent is crucial as it should be one in which the substance is soluble at high temperatures but less soluble at lower temperatures. This allows the substance to recrystallise as the solution cools.

Once the substance is dissolved, the solution is filtered while it is still hot. This step removes any insoluble impurities. The solution is then allowed to cool slowly. As it cools, the solubility of the substance decreases and it begins to form crystals. These crystals are pure substance, as the impurities remain dissolved in the solvent.

The crystals are then separated from the remaining solution by filtration. This leaves behind a residue of impurities in the solvent, while the pure crystals are collected on the filter paper. The crystals are then dried to remove any remaining solvent.

The key to successful recrystallisation is controlling the rate of cooling. If the solution cools too quickly, the substance will not have time to form well-defined crystals and impurities may become trapped inside them. However, if the solution cools too slowly, the substance may remain dissolved in the solvent. Therefore, it is important to allow the solution to cool slowly at first to form seed crystals, and then cool more rapidly to grow the crystals.

In summary, recrystallisation is a useful technique for increasing the purity of a substance. It relies on differences in solubility at different temperatures and the ability to separate crystals from a solution.