Editing Ice (section)

==Physical properties==
{{Further|Water (properties)#Density of water and ice}}
[[File:Ice Ih Crystal Lattice.png|thumb|The three-dimensional crystal structure of H{{sub|2}}O ice I<sub>h</sub> (c) is composed of bases of H{{sub|2}}O ice molecules (b) located on lattice points within the two-dimensional hexagonal space lattice (a).<ref name="Physics of Ice">Physics of Ice, V. F. Petrenko, R. W. Whitworth, Oxford University Press, 1999, {{ISBN|9780198518945}}</ref><ref>{{cite journal|doi = 10.1063/1.1749327|title = A Theory of Water and Ionic Solution, with Particular Reference to Hydrogen and Hydroxyl Ions|year = 1933|author1=Bernal, J. D. |author2=Fowler, R. H. |journal = The Journal of Chemical Physics|volume = 1|issue = 8|page = 515|bibcode = 1933JChPh...1..515B }}</ref>]]

Ice possesses a regular [[crystalline]] structure based on the [[molecule]] of water, which consists of a single [[oxygen]] atom [[covalently]] bonded to two [[hydrogen atom]]s, or H–O–H. However, many of the physical properties of water and ice are controlled by the formation of [[hydrogen bond]]s between adjacent oxygen and hydrogen atoms; while it is a weak bond, it is nonetheless critical in controlling the structure of both water and ice.<ref name=bjerrum>{{cite journal|last=Bjerrum|first=N|title=Structure and Properties of Ice|journal=Science|date=11 April 1952|volume=115|issue=2989|pages=385–390|doi=10.1126/science.115.2989.385|pmid=17741864|bibcode = 1952Sci...115..385B }}</ref>

An unusual property of water is that its solid form—ice frozen at [[atmospheric pressure]]—is approximately 8.3% less dense than its liquid form; this is equivalent to a volumetric expansion of 9%. The [[density]] of ice is 0.9167<ref name=CRC>{{cite book |chapter=Properties of Ice and Supercooled Water |first=Allan H. |last=Harvey |editor1= Haynes, William M. |editor2 = Lide, David R. |editor3=Bruno, Thomas J. |title = CRC Handbook of Chemistry and Physics |edition = 97th |location = Boca Raton, FL |publisher = CRC Press |year = 2017 |isbn = 978-1-4987-5429-3}}</ref>–0.9168<ref name=Voitkovskii/>&nbsp;g/cm<sup>3</sup> at 0&nbsp;°C and standard atmospheric pressure (101,325&nbsp;Pa), whereas water has a density of 0.9998<ref name=CRC/>–0.999863<ref name=Voitkovskii/>&nbsp;g/cm<sup>3</sup> at the same temperature and pressure. Liquid water is densest, essentially 1.00&nbsp;g/cm<sup>3</sup>, at 4&nbsp;°C and begins to lose its density as the water molecules begin to form the [[Hexagonal (crystal system)|hexagonal]] [[crystal]]s of [[ice crystals|ice]] as the freezing point is reached. This is due to hydrogen bonding dominating the intermolecular forces, which results in a packing of molecules less compact in the solid. The density of ice increases slightly with decreasing temperature and has a value of 0.9340&nbsp;g/cm<sup>3</sup> at −180&nbsp;°C (93&nbsp;K).<ref>{{RubberBible86th}}</ref>

When water freezes, it increases in volume (about 9% for fresh water).<ref name="LibreTexts">{{Cite web |last1=Richardson |first1=Eliza |title=Ice, Water, and Vapor |url=https://geo.libretexts.org/Bookshelves/Oceanography/Essentials_of_Oceanography_(Richardson)/03%3A_From_Rock_to_Salt/3.02%3A_Ice_Water_and_Vapor |website=LibreTexts Geosciences |date=20 June 2021 |language=en |access-date=26 April 2024 }}</ref> The effect of expansion during freezing can be dramatic, and ice expansion is a basic cause of [[freeze-thaw]] weathering of rock in nature and damage to building foundations and roadways from [[frost heaving]]. It is also a common cause of the flooding of houses when water pipes burst due to the pressure of expanding water when it freezes.<ref name="Akyurt2002">{{cite journal |last1=Akyurt |first1=M. |last2=Zaki |first2=G. |last3=Habeebullah |first3=B. |title=Freezing phenomena in ice–water systems |journal=Energy Conversion and Management |date=15 May 2002 |volume=43 |issue=14 |pages=1773–1789 |doi=10.1016/S0196-8904(01)00129-7 |bibcode=2002ECM....43.1773A }}</ref>

Because ice is less dense than liquid water, it floats, and this prevents bottom-up freezing of the bodies of water. Instead, a sheltered environment for animal and plant life is formed beneath the floating ice, which protects the underside from short-term weather extremes such as [[wind chill]]. Sufficiently thin floating ice allows light to pass through, supporting the [[photosynthesis]] of bacterial and algal colonies.<ref>{{cite web|url=http://www.haydenplanetarium.org/tyson/read/1998/05/01/water-water|title=Water, Water|author=Tyson, Neil deGrasse|publisher=haydenplanetarium.org|url-status=live|archive-url=https://web.archive.org/web/20110726095920/http://www.haydenplanetarium.org/tyson/read/1998/05/01/water-water|archive-date=26 July 2011}}</ref> When sea water freezes, the ice is riddled with brine-filled channels which sustain [[Sympagic ecology|sympagic organisms]] such as bacteria, algae, [[copepod]]s and [[annelid]]s. In turn, they provide food for animals such as [[krill]] and specialized fish like the [[bald notothen]], fed upon in turn by larger animals such as [[emperor penguins]] and [[minke whales]].<ref>[http://www.acecrc.sipex.aq/access/page/?page=d664da82-b244-102a-8ea7-0019b9ea7c60 Sea Ice Ecology] {{webarchive|url=https://web.archive.org/web/20120321204557/http://www.acecrc.sipex.aq/access/page/?page=d664da82-b244-102a-8ea7-0019b9ea7c60 |date=21 March 2012 }}. Acecrc.sipex.aq. Retrieved 30 October 2011.</ref>

[[File:Frozen Wappinger Creek.JPG|thumb|Frozen waterfall in southeast [[New York (state)|New York]]]]

When ice melts, it absorbs as much [[Heat|energy]] as it would take to heat an equivalent mass of water by {{cvt|80|C|F}}.<ref name="ESA" /> During the melting process, the temperature remains constant at {{cvt|0|C|F}}. While melting, any energy added breaks the hydrogen bonds between ice (water) molecules. Energy becomes available to increase the thermal energy (temperature) only after enough hydrogen bonds are broken that the ice can be considered liquid water. The amount of energy consumed in breaking hydrogen bonds in the transition from ice to water is known as the ''[[heat of fusion]]''.<ref name="ESA">{{Cite web |title=Ice – a special substance |date=16 April 2013 |url=https://www.esa.int/SPECIALS/Eduspace_Global_EN/SEMHE7TWLUG_0.html |publisher=[[European Space Agency]] |language=en |access-date=26 April 2024 }}</ref><ref name="LibreTexts" />

As with water, ice absorbs light at the red end of the spectrum preferentially as the result of an overtone of an oxygen–hydrogen (O–H) bond stretch. Compared with water, this absorption is shifted toward slightly lower energies. Thus, ice appears blue, with a slightly greener tint than liquid water. Since absorption is cumulative, the color effect intensifies with increasing thickness or if internal reflections cause the light to take a longer path through the ice.<ref name=color>{{cite book|author1=Lynch, David K. |author2=Livingston, William Charles |title=Color and light in nature|url=https://books.google.com/books?id=4Abp5FdhskAC&pg=PA161|year=2001|publisher=Cambridge University Press|isbn=978-0-521-77504-5|pages=161–}}</ref> Other colors can appear in the presence of light absorbing impurities, where the impurity is dictating the color rather than the ice itself. For instance, [[iceberg]]s containing impurities (e.g., sediments, algae, air bubbles) can appear brown, grey or green.<ref name=color/>

Because ice in natural environments is usually close to its melting temperature, its hardness shows pronounced temperature variations. At its melting point, ice has a [[Mohs hardness]] of 2 or less, but the hardness increases to about 4 at a temperature of {{convert|-44|C||sp=us}} and to 6 at a temperature of {{convert|-78.5|C||sp=us}}, the vaporization point of solid [[carbon dioxide]] (dry ice).<ref>{{cite journal |last1=Walters |first1=S. Max |title=Hardness of Ice at Low Temperatures |journal=Polar Record |date=January 1946 |volume=4 |issue=31 |pages=344–345 |doi=10.1017/S003224740004239X|s2cid=250049037 |doi-access=free |bibcode=1946PoRec...4..344. }}</ref>

==={{anchor|Phases}}Phases===
{{Main|Phases of ice}}

[[File:Phase diagram of water.svg|thumb|[[Semi-log plot|Log-lin]] pressure-temperature [[phase diagram]] of water. The [[Roman numeral]]s correspond to some ice phases listed below.]]
[[File:3D representation of several phases of water.jpg|thumb|An alternative formulation of the phase diagram for certain ices and other phases of water<ref>{{cite journal |last1=David |first1=Carl |title=Verwiebe's '3-D' Ice phase diagram reworked |journal=Chemistry Education Materials |date=8 August 2016 |url=https://opencommons.uconn.edu/chem_educ/94/ }}</ref>]]

Most liquids under increased pressure freeze at ''higher'' temperatures because the pressure helps to hold the molecules together. However, the strong hydrogen bonds in water make it different: for some pressures higher than {{convert|1|atm|MPa|abbr=on}}, water freezes at a temperature ''below'' {{cvt|0|C|F}}. Ice, water, and [[water vapour]] can coexist at the [[triple point]], which is exactly {{cvt|273.16|K|C}} at a pressure of 611.657&nbsp;[[Pascal (unit)|Pa]].<ref>{{cite journal |last1=Wagner |first1=Wolfgang |last2=Saul |first2=A. |last3=Pruss |first3=A. |title=International Equations for the Pressure Along the Melting and Along the Sublimation Curve of Ordinary Water Substance |journal=Journal of Physical and Chemical Reference Data |date=May 1994 |volume=23 |issue=3 |pages=515–527 |doi=10.1063/1.555947 |bibcode=1994JPCRD..23..515W }}</ref><ref>{{cite journal|doi=10.1256/qj.04.94 | volume=131 | issue=608 | title=Review of the vapour pressures of ice and supercooled water for atmospheric applications | year=2005 | journal=Quarterly Journal of the Royal Meteorological Society | pages=1539–1565 | last1 = Murphy | first1 = D. M.| bibcode=2005QJRMS.131.1539M | s2cid=122365938 | url=https://zenodo.org/record/1236243 | doi-access=free }}</ref> The [[kelvin]] was defined as {{sfrac|1|273.16}} of the difference between this triple point and [[absolute zero]],<ref>{{cite web|url=http://www1.bipm.org/en/si/base_units/|title=SI base units|publisher=Bureau International des Poids et Mesures|access-date=31 August 2012|url-status=live|archive-url=https://web.archive.org/web/20120716202131/http://www.bipm.org/en/si/base_units/|archive-date=16 July 2012}}</ref> though this definition [[2019 revision of the SI|changed]] in May 2019.<ref>{{cite web |url=https://www.bipm.org/utils/common/pdf/SI-statement.pdf |title=Information for users about the proposed revision of the SI |publisher=Bureau International des Poids et Mesures |access-date=6 January 2019 |archive-date=21 January 2018 |archive-url=https://web.archive.org/web/20180121160000/https://www.bipm.org/utils/common/pdf/SI-statement.pdf |url-status=dead }}</ref> Unlike most other solids, ice is difficult to [[Superheating|superheat]]. In an experiment, ice at −3&nbsp;°C was superheated to about 17&nbsp;°C for about 250 [[picosecond]]s.<ref>{{cite journal|journal=Nature|volume=439|pages=183–186|year=2006| doi=10.1038/nature04415|pmid=16407948|title=Ultrafast superheating and melting of bulk ice|bibcode=2006Natur.439..183I|last1=Iglev|first1=H.|last2=Schmeisser|first2=M.|last3=Simeonidis|first3=K.|last4=Thaller|first4=A.|last5=Laubereau|first5=A.|issue=7073|s2cid=4404036}}</ref>

Subjected to higher pressures and varying temperatures, ice can form in nineteen separate known crystalline phases at various densities, along with hypothetical proposed phases of ice that have not been observed.<ref name="Metcalfe-2021">{{cite news|last1=Metcalfe|first1=Tom|date=9 March 2021|title=Exotic crystals of 'ice 19' discovered|language=en|work=Live Science|url=https://www.livescience.com/exotic-ice-19-discovered.html}}</ref> With care, at least fifteen of these phases (one of the known exceptions being ice X) can be recovered at ambient pressure and low temperature in [[metastable]] form.<ref>{{cite journal|last=La Placa|first=S. J.|author2=Hamilton, W. C.|author3=Kamb, B.|author4=Prakash, A.|year=1972|title=On a nearly proton ordered structure for ice IX|journal=Journal of Chemical Physics|volume=58|issue=2|pages=567–580|doi=10.1063/1.1679238|bibcode = 1973JChPh..58..567L }}</ref><ref>{{cite journal|last=Klotz|first=S.|author2=Besson, J. M.|author3=Hamel, G.|author4=Nelmes, R. J.|author5=Loveday, J. S.|author6=Marshall, W. G.|year=1999|title=Metastable ice VII at low temperature and ambient pressure|journal=Nature|volume=398|issue=6729|pages=681–684|doi=10.1038/19480|bibcode = 1999Natur.398..681K |s2cid=4382067}}</ref> The types are differentiated by their crystalline structure, proton ordering,<ref>{{cite web|url=https://www.uwgb.edu/dutchs/Petrology/Ice%20Structure.HTM|title=Ice Structure|last=Dutch|first=Stephen|publisher=University of Wisconsin Green Bay|access-date=12 July 2017|url-status=dead|archive-url=https://web.archive.org/web/20161016143124/http://www.uwgb.edu/dutchs/petrology/Ice%20Structure.HTM|archive-date=16 October 2016}}</ref> and density. There are also two metastable phases of ice under pressure, both fully hydrogen-disordered; these are Ice IV and Ice XII. Ice XII was discovered in 1996. In 2006, Ice XIII and Ice XIV were discovered.<ref>{{cite journal |last1=Salzmann |first1=Christoph G. |last2=Radaelli |first2=Paolo G. |last3=Hallbrucker |first3=Andreas |last4=Mayer |first4=Erwin |last5=Finney |first5=John L. |title=The Preparation and Structures of Hydrogen Ordered Phases of Ice |journal=Science |date=24 March 2006 |volume=311 |issue=5768 |pages=1758–1761 |doi=10.1126/science.1123896 |pmid=16556840 |bibcode=2006Sci...311.1758S |s2cid=44522271 }}</ref> Ices XI, XIII, and XIV are hydrogen-ordered forms of ices I{{sub|h}}, V, and XII respectively. In 2009, ice XV was found at extremely high pressures and −143&nbsp;°C.<ref>{{cite magazine|url=http://www.sciencenews.org/view/generic/id/47258/title/A_very_special_snowball|title=A Very Special Snowball|author=Sanders, Laura|magazine=Science News|date=11 September 2009|access-date=11 September 2009|url-status=live|archive-url=https://web.archive.org/web/20090914174027/http://www.sciencenews.org/view/generic/id/47258/title/A_very_special_snowball|archive-date=14 September 2009}}</ref> At even higher pressures, ice is predicted to become a [[metal]]; this has been variously estimated to occur at 1.55&nbsp;TPa<ref>{{cite journal |last1=Militzer |first1=Burkhard |last2=Wilson |first2=Hugh F. |title=New Phases of Water Ice Predicted at Megabar Pressures |journal=Physical Review Letters |date=2 November 2010 |volume=105 |issue=19 |page=195701 |doi=10.1103/PhysRevLett.105.195701 |pmid=21231184 |arxiv=1009.4722 |bibcode=2010PhRvL.105s5701M |s2cid=15761164 }}</ref> or 5.62&nbsp;TPa.<ref>{{cite journal|author=MacMahon, J. M.|title=Ground-State Structures of Ice at High-Pressures|doi=10.1103/PhysRevB.84.220104|arxiv=1106.1941|bibcode=2011PhRvB..84v0104M|year=1970|journal=Physical Review B|volume=84|issue=22|pages=220104|s2cid=117870442}}</ref>

As well as crystalline forms, solid water can exist in amorphous states as [[amorphous solid water]] (ASW) of varying densities. In outer space, hexagonal crystalline ice is present in the [[ice volcano]]es,<ref>{{cite news|url=https://www.nytimes.com/2004/12/09/science/09ice.html|title=Astronomers Contemplate Icy Volcanoes in Far Places|author=Chang, Kenneth|work=The New York Times|date=9 December 2004|access-date=30 July 2012|url-status=live|archive-url=https://web.archive.org/web/20150509123243/http://www.nytimes.com/2004/12/09/science/09ice.html|archive-date=9 May 2015}}</ref> but is extremely rare otherwise. Even icy moons like [[Ganymede (moon)|Ganymede]] are expected to mainly consist of other crystalline forms of ice.<ref>{{cite journal |url=http://www.jhuapl.edu/techdigest/TD/td2602/Prockter.pdf |title=Ice in the Solar System |author=Prockter, Louise M. |journal=Johns Hopkins APL Technical Digest |volume=26 |issue=2 |year=2005 |page=175 |url-status=dead |archive-url=https://web.archive.org/web/20150319063545/http://www.jhuapl.edu/techdigest/TD/td2602/Prockter.pdf |archive-date=19 March 2015 |access-date=21 December 2013 }}</ref><ref name=showman1997>{{Cite journal | doi = 10.1006/icar.1997.5778| title = Coupled Orbital and Thermal Evolution of Ganymede| journal = Icarus| volume = 129| issue = 2| pages = 367–383| year = 1997| last1 = Showman | first1 = A. | bibcode = 1997Icar..129..367S| url = http://www.lpl.arizona.edu/~showman/publications/showman-etal-1997.pdf}}</ref> Water in the [[interstellar medium]] is dominated by amorphous ice, making it likely the most common form of water in the universe.<ref name="stanley">{{cite journal|last1=Debennetti|first1=Pablo G. |last2=Stanley |first2=H. Eugene |year=2003 |title=Supercooled and Glassy Water |journal=Physics Today |volume=56 |issue=6 |pages=40–46 |bibcode=2003PhT....56f..40D|doi=10.1063/1.1595053 |url=http://polymer.bu.edu/hes/articles/ds03.pdf |access-date=19 September 2012 }}</ref>  Low-density ASW (LDA), also known as hyperquenched glassy water, may be responsible for [[noctilucent clouds]] on Earth and is usually formed by [[vapor deposition|deposition]] of water vapor in cold or vacuum conditions.<ref>{{cite journal |last1=Lübken |first1=F.-J. |last2=Lautenbach |first2=J. |last3=Höffner |first3=J. |last4=Rapp |first4=M. |last5=Zecha |first5=M. |title=First continuous temperature measurements within polar mesosphere summer echoes |journal=Journal of Atmospheric and Solar-Terrestrial Physics |date=March 2009 |volume=71 |issue=3–4 |pages=453–463 |doi=10.1016/j.jastp.2008.06.001|bibcode=2009JASTP..71..453L }}</ref> High-density ASW (HDA) is formed by compression of ordinary ice I{{sub|h}} or LDA at GPa pressures. Very-high-density ASW (VHDA) is HDA slightly warmed to 160&nbsp;K under 1–2&nbsp;GPa pressures.<ref>{{cite journal|doi=10.1039/b108676f|title=A second distinct structural "state" of high-density amorphous ice at 77 K and 1 bar|year=2001|author1-link=Thomas Loerting|last1=Loerting|first1=Thomas|last2=Salzmann|first2=Christoph|last3=Kohl|first3=Ingrid|last4=Mayer|first4=Erwin|last5=Hallbrucker|first5=Andreas|s2cid=59485355|journal=Physical Chemistry Chemical Physics |volume=3 |pages=5355–5357 |issue=24 |bibcode=2001PCCP....3.5355L }}</ref>

Ice from a theorized superionic water may possess two crystalline structures. At pressures in excess of {{convert|500000|bar|psi}} such ''superionic ice'' would take on a [[body-centered cubic]] structure. However, at pressures in excess of {{convert|1000000|bar|psi}} the structure may shift to a more stable [[face-centered cubic]] lattice. It is speculated that superionic ice could compose the interior of ice giants such as Uranus and Neptune.<ref name=Phys.org-2013-04-25>{{cite news |website=Phys.org |url=http://phys.org/news/2013-04-phase-dominate-interiors-uranus-neptune.html |title=New phase of water could dominate the interiors of Uranus and Neptune |first=Lisa |last=Zyga |date=25 April 2013}}</ref>

===Friction properties===
[[File:2011 Figure Skating WC Takahiko Kozuka.jpg|thumb|[[Takahiko Kozuka]] figure skating - an act which is only possible due to ice's low frictional properties]]
Ice is "[[slipperiness|slippery]]" because it has a low coefficient of friction. This subject was first scientifically investigated in the 19th century. The preferred explanation at the time was "[[Pressure melting point|pressure melting]]" -i.e. the blade of an ice skate, upon exerting pressure on the ice, would melt a thin layer, providing sufficient lubrication for the blade to glide across the ice.<ref name="Rosenberg-2005" /> Yet, 1939 research by Frank P. Bowden and T. P. Hughes found that skaters would experience a lot more friction than they actually do if it were the only explanation. Further, the optimum temperature for figure skating is {{convert|-5.5|C|F K|0}} and {{convert|−9|C|F K|0}} for hockey; yet, according to pressure melting theory, skating below {{convert|−4|C|F K|0}} would be outright impossible.<ref name="Chang2006" /> Instead, Bowden and Hughes argued that heating and melting of the ice layer is caused by friction. However, this theory does not sufficiently explain why ice is slippery when standing still even at below-zero temperatures.<ref name="Rosenberg-2005">{{cite journal |last1=Rosenberg |first1=Robert |title=Why Is Ice Slippery? |journal=Physics Today |date=2005 |volume=58 |issue=12 |pages=50–54 |doi=10.1063/1.2169444 |bibcode=2005PhT....58l..50R |doi-access=free }}</ref>

Subsequent research suggested that ice molecules at the interface cannot properly bond with the molecules of the mass of ice beneath (and thus are free to move like molecules of liquid water). These molecules remain in a semi-liquid state, providing lubrication regardless of pressure against the ice exerted by any object. However, the significance of this hypothesis is disputed by experiments showing a high [[coefficient of friction]] for ice using [[atomic force microscopy]].<ref name=Chang2006>{{cite news |first=Kenneth |last=Chang |title=Explaining Ice: The Answers Are Slippery |work=[[The New York Times]] |date=21 February 2006 |access-date=8 April 2009 |url=https://www.nytimes.com/2006/02/21/science/21ice.html?pagewanted=all |url-status=live |archive-url=https://web.archive.org/web/20081211055112/http://www.nytimes.com/2006/02/21/science/21ice.html?pagewanted=all |archive-date=11 December 2008 }}</ref> Thus, the mechanism controlling the frictional properties of ice is still an active area of scientific study.<ref>{{cite journal |last1=Canale |first1=L. |title=Nanorheology of Interfacial Water during Ice Gliding |journal=[[Physical Review X]] |date=4 September 2019 |volume=9 |issue=4 |page=041025 |doi=10.1103/PhysRevX.9.041025 |arxiv=1907.01316 |bibcode=2019PhRvX...9d1025C |doi-access=free }}</ref> A comprehensive theory of ice friction must take into account all of the aforementioned mechanisms to estimate friction coefficient of ice against various materials as a function of temperature and sliding speed. 2014 research suggests that frictional heating is the most important process under most typical conditions.<ref name=Makkonen2014>{{cite journal|last1=Makkonen|first1=Lasse|last2=Tikanmäki|first2=Maria|title=Modeling the friction of ice|journal=Cold Regions Science and Technology|date=June 2014|volume=102|pages=84–93|doi=10.1016/j.coldregions.2014.03.002|bibcode=2014CRST..102...84M }}</ref>