Crystallization separates a solid dissolved in a liquid that has formed a solution. To crystallize the solid, the solution is heated in an open container to allow the liquid to evaporate, leaving the crystalline solid behind.
Crystallization can be naturally or artificially induced. In addition to crystals that precipitate from a liquid solution, crystallization can also occur in a molten liquid that is cooled or a gas that is deposited on a surface. Crystallization occurs in two major steps: nucleation and crystal growth.
During the nucleation phase, the solvent molecules dispersed in the solution are attracted to one another, forming nanometer-sized clusters. Halting the process at this stage and stabilizing the nanoclusters is a commonly used technique in the synthesis of nanoparticles. Leaving the process to continue yields stable nuclei that persist without dissolving back into the solution. The critical size that these nuclei need to reach to become stable depends on system conditions such as the solution temperature and solvent concentration.
The crystal growth stage involves the subsequent attraction of additional solvent molecules to the nuclei that have previously succeeded in achieving critical size. This process continues so long as the solution is supersaturated: having more solvent dissolved than it can sustain at these particular conditions.