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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.
Small particles (seeds) can trigger the separation of the dissolved material from the solvent or condensation of the vapor. Seeds triggering the conversion 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 state.
Supersaturated solutions are prepared or result when some condition of a saturated solution is changed, for example increasing (or, rarely, decreasing) temperature, decreasing volume of the saturated solvent (as by liquid evaporation), or increasing pressure.
Carbonated water is a supersaturated solution of carbon dioxide gas in water. At the elevated pressure in the bottle, more carbon dioxide can dissolve in water 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 Nicholas Fitzpatrick and Kuzniarski).
Scuba divers' tissues become saturated with breathing gases during a dive. Supersaturation is a theoretical term describing a state in which the tension of a dissolved gas is greater than its inspired partial pressure when the diver ascends, in contrast to Henry's law. If the diver ascends too fast, these gases form bubbles, resulting in decompression sickness. The term was popularized by J.S. Haldane.
Supersaturated solutions of sugar and water are commonly used to make rock candy.
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