Crystallization occurs in two main steps. The first is nucleation, the formation of a crystalline phase from either a supercooled liquid or a supersaturated solvent. The second step is known as crystal growth, which involves an increase in particle size and results in a crystalline state. An important feature of this step is that loose particles form layers on the crystal’s surface and become lodged in open voids such as pores, cracks, etc.
Most minerals and organic molecules crystallize easily, and the resulting crystals are generally of good quality, i.e., without visible defects. However, larger biochemical particles, such as proteins, are often difficult to crystallize. The ease with which molecules crystallize depends heavily on the strength of either atomic forces (in the case of mineral substances), intermolecular forces (in the case of organic and biochemical substances), or intramolecular forces (in the case of biochemical substances).
Crystallization is also a chemical solid–liquid separation technique in which mass transfer of a solute from the liquid solution to a pure solid crystalline phase occurs. In chemical engineering, crystallization takes place in a crystallizer. Crystallization is therefore related to precipitation, although the result is not amorphous or disordered, but a crystal.
When a saturated solid-liquid solution is cooled slowly, the solid solute settles into a highly ordered arrangement of its constituent particles—atoms, molecules, or ions.
Crystal growth refers to the subsequent increase in size of nuclei that have reached the critical cluster size. Crystal growth is a dynamic process occurring at equilibrium, in which solute molecules or atoms precipitate out of solution and then redissolve into it. Supersaturation is one of the driving forces of crystallization, as the solubility of a species is an equilibrium process quantified by Ksp. Depending on the conditions, either nucleation or growth may predominate over the other, thereby determining crystal size.
