Science Fair Project Encyclopedia
Recrystallization is an essentially physical process that has meanings in chemistry and geology.
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 and Y have approximately equal solubilities, then X will precipitate while Y stays in solution. The precitipate now has a much higher purity than the original mixture. The cost of this purification method is the loss of the part of compound X that stays in solution.
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.
In metallurgy, recrystallization is the growth of particular grain fragments in a metal or alloy at the expense of others. This occurs when the metal or alloy is severely worked, as by cold rolling. Recrystallization results in greater, strain-free grains.
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