Whenever an unsaturated solution is present in contact with a solid salt, the equilibrium state will sooner or later be achieved by dissolution of all or part of the solid salt. Dissolution of a solid salt does not take place uniformly from the surface of salt crystals. Likewise, precipitation does not occur uniformly throughout a solution. Precipitation proceeds through two stages called nucleation and crystal growth. Nucleation is the process of formation of tiny crystalline nuclei in the solution, while crystal growth is the ordered growth of these nuclei into larger well-formed crystals.
Crystal growth can occur only after nuclei are formed.
Supersaturated solutions are solutions in which significant nucleation has not yet taken place, which explains why the addition of a crystal which can serve as a nucleus, or dust or a scratch which can serve as a nucleus as well, produces rapid crystal formation in supersaturated solutions. The condition of a precipitated salt left in contact with the saturated solution from which it precipitated is observed to change. Over a period of minutes to hours or days the smaller crystals dissolve while the larger crystals grow larger still. Chemists take advantage of this process of conditioning or digestion of precipitates in order to obtain larger crystals which can be filtered from the remaining solution, or supernatant, more easily. Solutions of gases will approach equilibrium much more rapidly when the solution is stirred to distribute the dissolved gas throughout the solvent and when the area of the gas-solution interface is increased.