What happens when a saturated solution is cooled?

When a saturated solution is cooled, the dissolved component begins to separate from the solution because the solubility of most solid compounds decreases as the temperature drops. The substance continues to separate until the solution reaches a new equilibrium, becoming saturated again at the lower temperature.12

This process is a fundamental technique used in crystallization for the purification of substances. Because different compounds have varying solubilities, cooling a mixture can often cause the less soluble component to crystallize out of the solution while more soluble components remain dissolved.1

For effective purification, this process should occur slowly; rapid solidification can lead to the entrapment of impurities within the crystal matrix. In some cases, a seed crystal may be added to the solution to provide a surface for nucleation and initiate the growth of the solid.13