Editing Freezing (section)

==Supercooling==
{{main|Supercooling}}
[[File:SuperCool 2009-01-02.ogv|thumb|Rapid formation of ice crystals in supercool water (home freezer experiment)]]
In spite of the [[second law of thermodynamics]], crystallization of pure liquids usually begins at a lower temperature than the [[melting point]], due to high [[activation energy]] of [[Nucleation#Homogeneous nucleation|homogeneous nucleation]]. The creation of a nucleus implies the formation of an interface at the boundaries of the new phase. Some energy is expended to form this interface, based on the [[surface energy]] of each phase. If a hypothetical nucleus is too small, the energy that would be released by forming its volume is not enough to create its surface, and nucleation does not proceed. Freezing does not start until the temperature is low enough to provide enough energy to form stable nuclei. In presence of irregularities on the surface of the containing vessel, solid or gaseous impurities, pre-formed solid crystals, or other nucleators, [[Nucleation#Heterogeneous nucleation often dominates homogeneous nucleation|heterogeneous nucleation]] may occur, where some energy is released by the partial destruction of the previous interface, raising the supercooling point to be near or equal to the melting point. The melting point of [[water]] at 1 atmosphere of pressure is very close to {{convert|0|°C|°F K|abbr=on}}, and in the presence of [[Nucleation|nucleating substances]] the freezing point of water is close to the melting point, but in the absence of nucleators water can [[Supercooling|supercool]] to {{convert|-40|C|F K}} before freezing.<ref>{{cite journal | vauthors = Lundheim R | title = Physiological and ecological significance of biological ice nucleators | journal = Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences | volume = 357 | issue = 1423 | pages = 937–43 | date = July 2002 | pmid = 12171657 | pmc = 1693005 | doi = 10.1098/rstb.2002.1082 }}</ref><ref>{{cite journal | vauthors = Franks F | s2cid = 25606767 | title = Nucleation of ice and its management in ecosystems | journal = Philosophical Transactions. Series A, Mathematical, Physical, and Engineering Sciences | volume = 361 | issue = 1804 | pages = 557–74; discussion 574 | date = March 2003 | pmid = 12662454 | doi = 10.1098/rsta.2002.1141 | url = http://rsta.royalsocietypublishing.org/content/361/1804/557.long | format = [[PDF]] | bibcode = 2003RSPTA.361..557F | url-access = subscription }}</ref> Under high pressure (2,000 [[Atmosphere (unit)|atmosphere]]s) water will supercool to as low as {{convert|-70|C|F K}} before freezing.<ref>{{Cite journal |vauthors=Jeffery CA, Austin PH |date=November 1997 |title=Homogeneous nucleation of supercooled water: Results from a new equation of state |journal=Journal of Geophysical Research |volume=102 |issue=D21 |pages=25269–25280 |bibcode=1997JGR...10225269J |citeseerx=10.1.1.9.3236 |doi=10.1029/97JD02243}}</ref>