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. Crystallization 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 the formation of an insoluble (solid) from a chemical reaction. A precipitate may be amorphous or crystalline.
The Process of Crystallization
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, the concentration of the particles, pressure, and the purity of the material.
In a solution in the crystal growth phase, an 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.
Examples of Crystallization
A material may crystallize either naturally or artificially and either quickly or over geological timescales. Examples of natural crystallization include:
- Snowflake formation
- Crystallization of honey in a jar
- Stalactite and stalagmite formation
- Gemstone crystal deposition
Examples of artificial crystallization include:
- Growing sugar crystals in a jar
- Production of synthetic gemstones
There are many methods used to crystallize a substance. To a large degree, these depend on whether the starting material is an ionic compound (e.g., salt), covalent compound (e.g., sugar or menthol), or a metal (e.g., silver or steel). Ways of growing crystals include:
- Cooling a solution or melt
- Evaporating solvent
- Adding a second solvent to reduce the solubility of the solute
- Solvent layering
- Adding another cation or anion
The most common process is to dissolve the solute in a solvent in which it is at least partially soluble. Often the temperature of the solution is increased to increase solubility so the maximum amount of solute goes into solution. Next, the warm or hot mixture is filtered to remove undissolved material or impurities. The remaining solution (the filtrate) is allowed to slowly cool to induce crystallization. The crystals may be removed from the solution and allowed to dry or else washed using a solvent in which they are insoluble. If the process is repeated to increase the purity of the sample, it is called recrystallization.
The rate of cooling of the solution and the amount of evaporation of the solvent can greatly impact the size and shape of the resulting crystals. Generally, slower is better: slowly cool the solution and minimize evaporation.