Crystallization
Crystallization is a physicochemical process in which the atoms, ions or molecules of a gas, a liquid or a solution arrange themselves into a rigid, geometric crystal lattice to form a crystalline solid.
In the laboratory and in industry it is used mainly as a method for separating and purifying solid substances.
Stages of the process
- Supersaturation: the necessary condition; the solution must contain more solute than it can dissolve at a given temperature.
- Nucleation: the particles come together to form the first small nuclei or seeds of the crystal.
- Crystal growth: molecules from the solution add to the lattice of the initial nucleus and the crystal grows. Slow cooling or evaporation favours larger, purer crystals.
Main crystallization methods
- By cooling: a hot, concentrated solution is prepared; as the temperature falls, the solubility of the solid decreases and it precipitates as crystals.
- By evaporation of the solvent: the liquid is allowed to evaporate slowly (for example, in a laboratory crystallizing dish) until the concentration exceeds the solubility limit.
- By changing the solvent: a second solvent in which the solid is insoluble is added to force it to precipitate.
- By sublimation: direct passage from gas to solid (deposition, or reverse sublimation).
Common examples
- Obtaining common salt (sodium chloride) by evaporating seawater.
- The formation of frost or snowflakes.
- The purification of drugs and organic compounds in the chemical and pharmaceutical industries.