Crystals start to form as a saturated solution is being cooled because the solubility of most solids decreases as temperature decreases. This means that as the temperature of a saturated solution decreases, the amount of solid that can be dissolved in the solution also decreases. When the amount of solid in the solution exceeds the amount that can be dissolved, the excess solid comes out of solution and forms crystals. The process of cooling a saturated solution to cause crystallization is called crystallization.

Crystallization is important in many industries, such as the pharmaceutical industry, the food industry, and the chemical industry. It is used to produce a variety of products, including crystals of sugar, salt, and other food additives; crystals of pharmaceuticals; and crystals of semiconductors used in electronics.