Editing Supersaturation (section)

=== 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>