Supersaturaation

Supersaturation is a thermodynamic condition in which a solution contains more dissolved solute than what is predicted to be soluble at a given temperature, pressure, and solvent composition. In such a state the solution is not at equilibrium and is prone to crystallization or precipitation as the excess solute seeks a more stable arrangement.

Supersaturation can be produced by various methods. One common approach is to dissolve solute at a higher temperature and then slowly cool the solution without allowing crystallization. Another method is to evaporate solvent, which concentrates the dissolved material beyond its equilibrium limit. Changing the solvent or adding an anti-solvent that lowers solubility can also induce supersaturation. In vapor systems, supersaturation occurs when the partial pressure of a vapor exceeds the equilibrium vapor pressure over a liquid or solid.

Formation of crystalline phases from a supersaturated solution requires nucleation. This can be homogeneous, occurring within the bulk, or heterogeneous, occurring on surfaces, impurities, or seed crystals. Once nucleation starts, crystallization proceeds, often rapidly, reducing the solute concentration toward solubility at the prevailing conditions.

Supersaturation is exploited in many fields. In chemistry and pharmaceuticals it enables enhanced solubility and controlled crystallization to obtain desired polymorphs. In chemical manufacturing, it underpins precipitation processes and purification. In atmospheric science, supersaturation of water vapor is a key driver of cloud formation and precipitation.

The stability of a supersaturated state depends on temperature, pressure, impurities, agitation, and the presence of nucleation sites. Without barriers to nucleation, supersaturated solutions tend to revert to equilibrium through crystallization or evaporation.