In chemistry, recrystallization is a procedure for purifying compounds. A typical situation is that a desired compound X is contaminated by a small amount of compound Y. A chemist can prepare a saturated solution of the mixture X+Y in a warm solvent and subsequently lower the temperature. For most compounds, the solubility decreases with decreasing temperature. If X is not soluble in the solvent at lower temperatures, and Y is soluble at lower temperatures, then X will precipitate as the temperature decreases, while Y stays in solution. The precipitate now has a much higher purity than the original mixture. The level of purity can then be checked by taking a melting point range of the solid and comparing it to an accepted melting point range if one exists. The cost of this purification method is the inevitable loss of the part of compound X that stays in solution. A typical yield might be 80%.

Recrystallization is an essentially physical process that has meanings in chemistry, metallurgy and geology.

Successful recrystallization depends on finding the right solvent. This is a combination of prediction and trial/error. The solvent must be soluble with X+Y at higher temperatures, and must be insoluble with either X or Y at lower temperatures to force the product to either precipitate out (while the impurity stays in solution), or stay in solution (while the impurity precipitates out). This separates the desired product from the impurity.

In geology, solid-state recrystallization is a metamorphic process that occurs under situations of intense temperature and pressure where grains, atoms or molecules of a rock or mineral are packed closer together, creating a new crystal structure. The basic composition remains the same. This process can be illustrated by observing how snow recrystallizes to ice without melting. As opposed to metasomatism, which is a chemical change caused by metamorphism, recrystallization is a physical process. However, recrystallization can occur when a local migration of chemicals results in the chemical change of the rock or mineral with no external addition of materials.

Limestone is a sedimentary rock that undergoes metamorphic recrystallization to form marble, and clays can recrystallize to muscovite mica.

In metallurgy, recrystallization is the nucleation and growth of new undeformed grains in a deformed metal.

For ice, recrystallization refers to the growth of larger cyrstals at the expense of smaller ones. Some biological antifreeze proteins have been shown to inhibit this process, and the effect may be relevant in freezing-tolerant organisms.

Category:Chemical processes
Category:Geological processes
Category:Materials science