How is crystallisation used to purify solids?

Crystallisation is used to purify solids by dissolving the impure solid in a solvent and then allowing it to recrystallise.

Crystallisation is a separation technique that is used to purify a solid substance. This process involves two main steps: dissolution and crystallisation. The impure solid is first dissolved in a suitable solvent in which it is highly soluble. The solution is then heated to increase the solubility of the solid and to dissolve any impurities. This is followed by filtering the solution to remove any undissolved impurities.

The next step is the actual crystallisation process. The hot solution is allowed to cool slowly. As it cools, the solubility of the solid decreases and it begins to come out of the solution, forming crystals. These crystals are pure solid particles as the impurities remain dissolved in the solvent. The rate of cooling is crucial in this process. Slow cooling allows for the formation of large, well-formed crystals, while rapid cooling results in small, less pure crystals.

The crystals are then separated from the remaining solution by filtration. The solid crystals are collected and dried, resulting in a purified solid. This process is particularly useful in the purification of substances in the chemical industry, where high purity is often required.

In summary, crystallisation is a useful method for purifying solids. It utilises the differences in solubility of the solid and its impurities in a particular solvent, and the decrease in solubility of the solid upon cooling, to separate the pure solid from its impurities.