Crystallisation is used to separate homogeneous mixtures, specifically solutions where the solute can form crystals. During crystallisation, the solution is heated to dissolve the solute completely, and then the solution is cooled slowly. As the temperature decreases, the solubility of the solute decreases, causing it to come out of the solution and form crystals. These crystals can then be separated from the remaining liquid, resulting in the purification of the solute.