Fractional crystallisation

Crystals start to form when a material in a solution is no longer soluble in the surrounding solvent. This might be because the solution has cooled, or solvent has evaporated making the solution more concentrated. If crystallisation happens quickly, the crystals formed tend to be small. You may have already found this if you have tried the Global Experiment. Slow crystallisation allows larger crystals to form.

Fractional crystallisation is a powerful purification technique. The same process that defines the structure of rocks can be used to separate and purify dissolved solids. This experiment, which you can try, demonstrates the principle of how the process works.