Supersaturation

Supersaturation is a condition in which a solvent contains a higher concentration of dissolved solute than its equilibrium solubility would permit at a given temperature and pressure. In liquids, this means the solution would be saturated at that temperature, yet more solute remains dissolved. In vapors, supersaturation describes a state where the vapor contains more molecules than the equilibrium amount allowed, often relative to temperature and pressure.

Supersaturation is typically formed by processes that disturb equilibrium, such as cooling a saturated solution, evaporating solvent, or increasing solute concentration more quickly than crystallization can occur. The state is metastable: without a nucleation site or seeds, the excess dissolved material may persist; given time or the introduction of a seed, crystallization or condensation proceeds to restore equilibrium.

Nucleation is central to the transition from supersaturated to stable. Homogeneous nucleation occurs within the bulk solution or vapor, while heterogeneous nucleation happens on surfaces or impurities, which lowers the energy barrier to phase change. Once nucleation starts, rapid crystal growth or droplet formation can follow.

Applications and examples span chemistry, geology, and meteorology. In crystallization processes, controlled supersaturation drives crystal growth and purity. In foods and beverages, supersaturated solutions (for example, sugar syrups) are used to form crystals. Carbonated drinks are stabilized in a supersaturated state of CO2 under pressure. In the atmosphere, supersaturation of water vapor is a prerequisite for cloud formation and precipitation, given the presence of condensation nuclei.

Measurement of supersaturation is often expressed as a ratio S = actual concentration or partial pressure divided by the solubility or saturation value at the same temperature. Supersaturation signifies a driving force toward phase change, tempered by nucleation kinetics and impurities.