Crystallisation is a separation technique used to separate mixtures of solids from liquids. The mixture is heated to dissolve the solid, and then cooled to cause the solid to crystallise out of solution. The crystals are then filtered from the liquid.

Crystallisation is used to separate mixtures of solids that have different solubilities. For example, salt can be separated from water by crystallisation, because salt is more soluble in hot water than in cold water. When the salt water solution is heated, the salt dissolves. When the solution is cooled, the salt crystallises out of solution.

Crystallisation can also be used to separate mixtures of solids that have different melting points. For example, sugar can be separated from water by crystallisation, because sugar has a higher melting point than water. When the sugar water solution is heated, the sugar dissolves. When the solution is cooled, the sugar crystallises out of solution.