Editing Supersaturation (section)

== Occurrence and examples ==
=== Solid precipitate, liquid solvent ===
[[File:Crystallized sugar, multiple crystals and a single crystal grown from seed.jpg|thumb|Crystallized sugar ([[rock candy]]) is made by adding a [[seed crystal]] to a supersaturated solution of table sugar and water. The multiple crystals on the right were grown from a sugar cube, while the left was grown from a single seed taken from the right. A red dye was added to the solution for the left crystal, but was insoluble with the solid sugar, and only traces remain while the rest precipitated out.]] 
A solution of a chemical compound in a liquid will become supersaturated when the temperature of the [[saturated solution]] is changed. In most cases solubility decreases with decreasing temperature; in such cases the excess of solute will rapidly separate from the solution as [[crystals]] or an [[amorphous]] powder.<ref>{{Cite journal|title = Mechanism of precipitate formation during spontaneous crystallization from supersaturated aqueous solutions|date = 2014|journal = Russian Chemical Reviews|doi = 10.1070/rc2014v083n04abeh004399|volume=83 |issue = 4|pages=343–364|bibcode = 2014RuCRv..83..343L |last1 = Linnikov|first1 = O. D.| s2cid=95096197 }}</ref><ref>{{Cite journal|title = Crystallization of molecular systems from solution: phase diagrams, supersaturation and other basic concepts | journal= Chemical Society Reviews |doi=10.1039/c3cs60359h |pmid=24457270 |volume=43 |issue=7 |pages=2286–2300| date=2014-03-10 |last1=Coquerel |  first1=Gérard | s2cid=205855877 }}</ref><ref>{{Cite journal|title = Effect of processing conditions on the crystallinity and structure of carbonated calcium hydroxyapatite (CHAp) |journal=CrystEngComm |doi=10.1039/c4ce00119b|volume=16|issue=19  |pages=3950–3959|date=2014-04-15  |last1=Kareiva  |first1=Aivaras |last2=Yang |first2=Jen-Chang |last3=Yang  |first3=Thomas Chung-Kuang |last4=Yang |first4=Sung-Wei |last5=Gross |first5=Karlis-Agris  |last6=Garskaite |first6=Edita|bibcode=2014CEG....16.3950G }}</ref> 
In a few cases the opposite effect occurs. The example of [[sodium sulfate]] in water is well-known and this was why it was used in early studies of solubility.

[[Recrystallization (chemistry)|Recrystallization]]<ref>{{Cite book|title = Industrial Crystallization |publisher= Springer|doi= 10.1007/978-1-4615-7258-9|year= 1976|isbn= 978-1-4615-7260-2|last1= Mullin|first1= J.|editor1-first= J. W|editor1-last= Mullin}}</ref><ref>{{cite journal|title = Supersaturation operation for quality control of crystalline particles in solution crystallization|doi=10.1016/j.apt.2012.04.009|volume=23|issue = 3|journal=Advanced Powder Technology|pages=273–278|date = May 2012|last1 = Takiyama|first1 = Hiroshi}}</ref> is a process used to purify chemical compounds. A mixture of the impure compound and solvent is heated until the compound has dissolved. If there is some solid impurity remaining it is removed by [[filtration]]. When the temperature of the solution is subsequently lowered it briefly becomes supersaturated and then the compound crystallizes out until chemical equilibrium at the lower temperature is achieved. Impurities remain in the [[supernatant]] liquid. In some cases crystals do not form quickly and the solution remains supersaturated after cooling. This is because there is a thermodynamic barrier to the formation of a crystal in a liquid medium. Commonly this is overcome by adding a tiny crystal of the solute compound to the supersaturated solution, a process known as "seeding". Another process in common use is to rub a rod on the side of a glass vessel containing the solution to release microscopic glass particles which can act as nucleation centres. In industry, [[centrifugation]] is used to separate the crystals from the supernatant liquid.

Some compounds and mixtures of compounds can form long-living supersaturated solutions. [[Carbohydrate]]s are a class of such compounds; The thermodynamic barrier to formation of crystals is rather high because of extensive and irregular [[hydrogen bonding]] with the solvent, water. For example, although [[sucrose]] can be recrystallised easily, its hydrolysis product, known as "[[inverted sugar syrup|invert sugar]]" or "golden syrup" is a mixture of [[glucose]] and [[fructose]] that exists as a viscous, supersaturated, liquid. Clear [[honey]] contains carbohydrates which may crystallize over a period of weeks.

Supersaturation may be encountered when attempting to crystallize a protein.<ref>{{Cite web|title = 1 Introduction to protein crystallisation|url = http://www.xray.bioc.cam.ac.uk/xray_resources/whitepapers/xtal-in-action/node3.html|website = www.xray.bioc.cam.ac.uk|access-date = 2015-04-21|archive-date = 2015-04-18|archive-url = https://web.archive.org/web/20150418205429/http://www.xray.bioc.cam.ac.uk/xray_resources/whitepapers/xtal-in-action/node3.html|url-status = dead}}</ref>

=== Gaseous solute, liquid solvent ===
The solubility of a gas in a liquid increases with increasing gas pressure. When the external pressure is reduced, the excess gas comes out of solution.

Fizzy drinks are made by subjecting the liquid to [[carbon dioxide]], under pressure. In [[champagne]] the CO<sub>2</sub> is produced naturally in the final stage of [[fermentation]]. When the bottle or can is opened some gas is released in the form of bubbles.

Release of gas from supersaturated tissues can cause an [[underwater diving|underwater diver]] to suffer from [[decompression sickness]] (a.k.a. the bends) when returning to the surface. This can be fatal if the released gas obstructs critical blood supplies causing ischaemia in vital tissues.<ref>{{cite report|title=Evidence Report: Risk of Decompression Sickness (DCS) |first1=Johnny |last1=Conkin |first2=Jason R. |last2=Norcross |first3=James H. III |last3=Wessel |first4=Andrew F. J. |last4=Abercromby |first5=Jill S. |last5=Klein |first6=Joseph P. |last6=Dervay |first7=Michael L. |last7=Gernhardt |work=Human Research Program Human Health Countermeasures Element |publisher=National Aeronautics and Space Administration |location=Houston, Texas  |url=https://archive.org/details/NASA_NTRS_Archive_20140003729}}</ref>

Dissolved gases can be released during [[oil exploration]] when a strike is made. This occurs because the oil in oil-bearing rock is under considerable pressure from the over-lying rock, allowing the oil to be supersaturated with respect to dissolved gases.

=== Liquid formation from a mixture of gases ===
A [[cloudburst]] is an extreme form of production of liquid water from a supersaturated mixture of air and water vapour in the [[atmosphere of earth|atmosphere]]. Supersaturation in the vapour phase is related to the [[Surface tension#Influence of particle size on vapor pressure|surface tension]] of liquids through the [[Kelvin equation]], the [[Gibbs–Thomson effect]] and the [[Poynting effect#Chemistry and thermodynamics|Poynting effect]].<ref>George N. Hatsopoulos & Joseph H. Keenan (1965), ''Principles of General Thermodynamics'' - John Wiley & Sons, Inc., New York, London, Sydney. Chapter 28, pages 303-309</ref>

The International Association for the Properties of Water and Steam ([[IAPWS]]) provides a special equation for the [[Gibbs free energy]] in the metastable-vapor region of water in its ''Revised Release on the IAPWS Industrial Formulation 1997 for the Thermodynamic Properties of Water and Steam''. All thermodynamic properties for the metastable-vapor region of water can be derived from this equation by means of the appropriate relations of thermodynamic properties to the Gibbs free energy.<ref>''Revised Release on the IAPWS Industrial Formulation 1997 for the Thermodynamic Properties of Water and Steam'', IAPWS R7-97(2012) [http://www.iapws.org/relguide/IF97-Rev.pdf#page=17]</ref>