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Triple point
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== Triple point of water == === Gas–liquid–solid triple point === {{see also|Properties of water#Triple point}} [[File:Water-triple-point-20210210.gif|thumb|Water boiling at 0°C using a vacuum pump.]]Following the 2019 revision of the SI, the value of the triple point of water is no longer used as a defining point. However, its empirical value remains important: the unique combination of pressure and temperature at which liquid [[water]], solid [[ice]], and [[water vapour]] coexist in a stable equilibrium is approximately {{val|273.16|0.0001|u=K}}<ref name="SI Brochure 9">{{cite web |title=SI Brochure: The International System of Units (SI) – 9th edition |url=https://www.bipm.org/documents/20126/41483022/SI-Brochure-9-EN.pdf/2d2b50bf-f2b4-9661-f402-5f9d66e4b507 |publisher=BIPM |access-date=21 February 2022}}</ref> and a vapour pressure of {{convert|611.657|Pa|mbar atm}}.<ref name="Wagner">[https://www.nist.gov/srd/upload/jpcrd477.pdf International Equations for the Pressure along the Melting and along the Sublimation Curve of Ordinary Water Substance]. W. Wagner, A. Saul and A. Pruss (1994), J. Phys. Chem. Ref. Data, '''23''', 515.</ref><ref name="Murphy">{{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> Liquid water can only exist at pressures equal to or greater than the triple point. Below this, in the vacuum of [[outer space]], solid ice [[Sublimation (phase transition)|sublimates]], transitioning directly into water vapor when heated at a constant pressure. Conversely, above the triple point, solid ice first melts into liquid water upon heating at a constant pressure, then evaporates or boils to form vapor at a higher temperature. For most substances, the gas–liquid–solid triple point is the minimum temperature where the liquid can exist. For water, this is not the case. The melting point of ordinary ice decreases with pressure, as shown by the [[phase diagram]]'s dashed green line. Just below the triple point, compression at a constant temperature transforms water vapor first to solid and then to liquid. Historically, during the [[Mariner 9]] mission to [[Mars]], the triple point pressure of water was used to define "sea level". Now, [[laser altimetry]] and gravitational measurements are preferred to define Martian elevation.<ref>{{cite book |first=Michael H. |last=Carr |title=The Surface of Mars |url=https://archive.org/details/surfacemars00carr |url-access=limited |publisher=Cambridge University Press |year=2007 |page=[https://archive.org/details/surfacemars00carr/page/n19 5] |isbn=978-0-521-87201-0 }}</ref> === High-pressure phases === At high pressures, water has a complex [[phase diagram]] with 15 known [[Ice#Phases|phases of ice]] and several triple points, including 10 whose coordinates are shown in the diagram. For example, the triple point at 251 K (−22 °C) and 210 MPa (2070 atm) corresponds to the conditions for the coexistence of [[ice Ih]] (ordinary ice), [[ice III]] and liquid water, all at equilibrium. There are also triple points for the coexistence of three solid phases, for example [[ice II]], ice V and ice VI at 218 K (−55 °C) and 620 MPa (6120 atm). For those high-pressure forms of ice which can exist in equilibrium with liquid, the diagram shows that melting points increase with pressure. At temperatures above 273 K (0 °C), increasing the pressure on water vapor results first in liquid water and then a high-pressure form of ice. In the range {{val|251|-|273|u=K}}, ice I is formed first, followed by liquid water and then ice III or ice V, followed by other still denser high-pressure forms. [[File:Phase_diagram_of_water.svg|class=skin-invert-image|thumb|upright=3.3|center|Phase diagram of water including high-pressure forms ice II, ice III, etc. The pressure axis is logarithmic. For detailed descriptions of these phases, see [[Ice#Phases|Ice]].]] {| class="wikitable" |+ The various triple points of water ! Phases in stable equilibrium ! Pressure ! Temperature |- | liquid water, [[ice Ih|ice I<sub>h</sub>]], and water vapor | 611.657 Pa<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> | 273.16 K (0.01 °C) |- | liquid water, ice I<sub>h</sub>, and [[ice III]] | 209.9 MPa | 251 K (−22 °C) |- | liquid water, ice III, and [[ice V]] | 350.1 MPa | −17.0 °C |- | liquid water, ice V, and [[ice VI]] | 632.4 MPa | 0.16 °C |- | ice I<sub>h</sub>, [[Ice II]], and ice III | 213 MPa | −35 °C |- | ice II, ice III, and ice V | 344 MPa | −24 °C |- | ice II, ice V, and ice VI | 626 MPa | −70 °C |} {{clear}}
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