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Crystallization
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Crystallization is the solidification of atoms or molecules into a highly structured form called a crystal. Usually, this refers to the slow precipitation of crystals from a solution of a substance. However, crystals can form from a pure melt or directly from deposition from the gas phase. It can also refer to the solid-liquid separation and purification technique in which mass transfer occurs from the liquid solution to a pure solid crystalline phase.
Although crystallization may occur during precipitation, the two terms are not interchangeable. Precipitation simply refers to formation of an insoluble (solid) from a chemical reaction. A precipitate may be amorphous or crystalline. Two events must occur for crystallization to occur. First, atoms or molecules cluster together on the microscopic scale in a process called nucleation. If the clusters become stable and sufficiently large, crystal growth may occur. Atoms and compounds can generally form more than one crystal structure (polymorphism). The arrangement of particles is determined during the nucleation stage of crystallization. This may be influenced by multiple factors, including temperature, concentration of the particles, pressure, and the purity of the material. In a solution in the crystal growth phase, equilibrium is established in which solute particles dissolve back into the solution and precipitate as a solid.
If the solution is supersaturated, this drives crystallization because the solvent cannot support continued dissolving. Sometimes having a supersaturated solution is insufficient to induce crystallization. It may be necessary to provide a seed crystal or a rough surface to start nucleation and growth.
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