Most products contain impurities. One way to purify these products is by recrystallization. Recrystallization begins with dissolving the impure product in just enough hot solvent to form a saturated solution, where as much solute is dissolved in the solvent as possible. Any additional solute will not seem to dissolve.
A hot solvent is used because solubility typically increases with temperature. As temperature increases, the amount of solute that can be dissolved in the solvent increases. As the solution cools, the solubility of the product decreases, and solute molecules come together to form small stable crystals called nuclei.
This is the first step of crystallization, called nucleation. Additional crystal growth happens on the nuclei because solute molecules have a greater affinity for joining existing solute crystals than forming new crystals. Soluble impurities are left in solution.
Crystallization can happen spontaneously or can be encouraged by scratching the inside of the flask, agitating the solution, or adding a seed crystal of the compound, all of which provide a surface for further growth.
Recrystallization into large, pure, regularly shaped crystals only works when an
