When a solution that has had more solute dissolved at a higher temperature is now cooled, it becomes supersaturated at the lower temperature. Because that supersaturated solution holds more solute than is stable at the lower temperature, crystals start to form.

How can you cause a supersaturated solution to crystallize?

The recrystallization of the excess dissolved solute in a supersaturated solution can be initiated by the addition of a tiny crystal of solute, called a seed crystal. The seed crystal provides a nucleation site on which the excess dissolved crystals can begin to grow.

Does supersaturation crystallize?

At low supersaturation, crystals can grow faster than they nucleate, resulting in a larger crystal size distribution. However, at higher supersaturation, crystal nucleation dominates crystal growth, ultimately resulting in smaller crystals.