Supersaturation

The term supersaturation refers to a solution that contains more of the dissolved material than could be dissolved by the solvent under normal circumstances. It can also refer to a vapor of a compound that has a higher (partial) pressure than the vapor pressure of that compound.

Condensation
Small particles (seeds) can trigger the separation of the dissolved material from the solvent or condensation of the vapor. Seeds triggering the condensation of vapor are referred to as condensation nuclei, as in the case of water vapor. In the solid form these seeds can lead to the formation of crystallites or even large single crystals. Supersaturation is a measure of the deviation of a dissolved salt crystal from its equilibrium value.

Preparation
Supersaturated solutions are prepared or result when some condition of a saturated solution is changed, for example temperature, volume (as by evaporation), or pressure.

Examples
Carbonated water is a supersaturated solution of carbon dioxide gas in water. At the elevated pressure in the bottle, carbon dioxide can dissolve in water more than at atmospheric pressure. At atmospheric pressure, the carbon dioxide gas escapes very slowly from the supersaturated liquid. This process may be accelerated by the presence of nucleation sites within the solution, such as small bubbles, caused by shaking the bottle, or another solute, such as sugar powder or a widget. A Diet Coke and Mentos eruption is a rather extreme example. Some beverage products such as ales and stouts e.g. Guinness rely on this effect to produce the 'head' on the surface of the poured product. This has led to the invention of the widget, a device developed to produce enhanced bubble seeding in liquids, especially with dual supersaturated gas phases (carbon dioxide and nitrogen) (see patents by Fitzpatrick and Kuzniarski).

Scuba divers' tissues become supersaturated with breathing gases during a dive. If the diver ascends too fast, these gases form bubbles, resulting in decompression sickness.

In air that is supersaturated with water, water droplets may precipitate upon being disturbed. This can be observed in a cloud chamber. In the more general context a precipitate may form.

Supersaturated solutions of sugar and water are commonly used to make Rock candy.