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{{short description|Thermodynamic point where three matter phases exist}} {{other uses|Tripoint|Tripoint (disambiguation)}} [[Image:Phase-diag2.svg|class=skin-invert-image|thumb|upright=1.6|A typical [[phase diagram]]. The solid green line applies to most substances; the dashed green line gives the anomalous behavior of water]] In [[thermodynamics]], the '''triple point''' of a substance is the [[temperature]] and [[pressure]] at which the three [[Phase (matter)|phases]] ([[gas]], [[liquid]], and [[solid]]) of that substance coexist in [[thermodynamic equilibrium]].<ref name="gold">{{GoldBookRef | title=Triple point |file=T06502 |year=1994}}.</ref> It is that temperature and pressure at which the [[sublimation (phase transition)|sublimation]], [[Melting|fusion]], and [[vaporisation]] curves meet. For example, the triple point of [[Mercury (element)|mercury]] occurs at a temperature of {{convert|−38.8|°C|°F}} and a pressure of 0.165 [[Milli|m]][[Pascal (unit)|Pa]]. In addition to the triple point for solid, liquid, and gas phases, a triple point may involve more than one solid phase, for substances with multiple [[polymorphism (materials science)|polymorphs]]. [[Helium-4]] is unusual in that it has no sublimation/deposition curve and therefore no triple points where its solid phase meets its gas phase. Instead, it has a vapor-liquid-[[superfluid]] point, a solid-liquid-superfluid point, a solid-solid-liquid point, and a solid-solid-superfluid point. None of these should be confused with the [[lambda point]], which is not any kind of triple point. The first mention of the term "triple point" was on August 3, 1871 by [[James Thomson (engineer)|James Thomson]], brother of [[Lord Kelvin]].<ref>James Thomson (1871) [https://www.nature.com/articles/004288a0 "Speculations on the Continuity of the Fluid State of Matter, and on Relations between the Gaseous, the Liquid, and the Solid States."], ''The British Association Meeting at Edinburgh'' . ''Nature'' '''4''', 288–298 (1871). From Section A on page 291: "This point of pressure and temperature [[James Thomson (engineer)|he]] designates as ''the triple point;'' and he shows how this point belongs to the three important curves, as being their intersection."</ref> The triple points of several substances are used to define points in the [[ITS-90]] international temperature scale, ranging from the triple point of hydrogen (13.8033 K) to the triple point of water (273.16 K, 0.01 °C, or 32.018 °F). Before 2019, the triple point of [[water]] was used to define the [[kelvin]], the [[SI base unit|base unit]] of thermodynamic temperature in the [[International System of Units]] (SI).<ref>[http://www1.bipm.org/en/si/base_units/ Definition of the kelvin] at BIPM.</ref> The kelvin was defined so that the triple point of water is exactly 273.16 K, but that changed with the [[2019 revision of the SI]], where the kelvin was redefined so that the [[Boltzmann constant]] is exactly {{val|1.380649|e=−23|u=J⋅K<sup>−1</sup>}}, and the triple point of water became an experimentally measured constant. == 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}} == Triple-point cells == Triple-point cells are used in the [[calibration]] of [[thermometer]]s. For exacting work, triple-point cells are typically filled with a highly pure chemical substance such as hydrogen, argon, mercury, or water (depending on the desired temperature). The purity of these substances can be such that only one part in a million is a contaminant, called "six nines" because it is 99.9999% pure. A specific [[isotope|isotopic]] composition (for water, [[Vienna Standard Mean Ocean Water|VSMOW]]) is used because variations in isotopic composition cause small changes in the triple point. Triple-point cells are so effective at achieving highly precise, reproducible temperatures, that an international calibration standard for thermometers called [[International Temperature Scale of 1990|ITS–90]] relies upon triple-point cells of [[hydrogen]], [[neon]], [[oxygen]], [[argon]], [[mercury (element)|mercury]], and [[water (molecule)|water]] for delineating six of its defined temperature points. == Table of triple points == This table lists the gas–liquid–solid triple points of several substances. Unless otherwise noted, the data come from the U.S. [[National Bureau of Standards]] (now [[NIST]], National Institute of Standards and Technology).<ref>{{cite book |first1=Yunus A. |last1=Cengel |first2=Robert H. |last2=Turner |title=Fundamentals of thermal-fluid sciences |location=Boston |publisher=McGraw-Hill |year=2004 |page=78 |isbn=0-07-297675-6 }}</ref> {| class="wikitable sortable" style="text-align:center" !width="170"|Substance !!width="160" data-sort-type=number| ''T'' [[[Kelvin|K]]] ([[Celsius|°C]]) !!width="230" data-sort-type=number| ''p'' [[[Pascal (unit)|kPa]]]* ([[Atmosphere (unit)|atm]]) |- |align="left"| [[Acetylene]] | {{convert|192.4|K|C|sortable=on}} || {{convert|120|kPa|atm|abbr=on|sortable=on}} |- |align="left"| [[Ammonia]] | {{convert|195.40|K|C|sortable=on}} || {{convert|6.060|kPa|atm|abbr=on|sortable=on}} |- |align="left"| [[Argon]] | {{convert|83.8058|K|C|sortable=on}} || {{convert|68.9|kPa|atm|abbr=on|sortable=on}} |- |align="left"| [[Arsenic]] | {{convert|1090|K|C|sortable=on}} || {{convert|3628|kPa|atm|abbr=on|sortable=on}} |- |align="left"| [[Butane]]<ref>See [[Butane (data page)]]</ref> | {{convert|134.6|K|C|sortable=on}} || {{convert|7e-4|kPa|atm|abbr=on|sortable=on}} |- |align="left"| Carbon ([[graphite]]) | {{convert|4765|K|C|sortable=on}} || {{convert|10132|kPa|atm|abbr=on|sortable=on}} |- |align="left"| [[Carbon dioxide]] | {{convert|216.55|K|C|sortable=on}} || {{convert|517|kPa|atm|abbr=on|sortable=on}} |- |align="left"| [[Carbon monoxide]] | {{convert|68.10|K|C|sortable=on}} || {{convert|15.37|kPa|atm|abbr=on|sortable=on}} |- |align="left"| [[Chloroform]]<ref>See [[Chloroform (data page)]]</ref><ref>{{cite web |title=Trichloromethane |url=https://webbook.nist.gov/cgi/cbook.cgi?ID=C67663&Mask=4#Thermo-Phase |website=NIST Chemistry WebBook, SRD 69 |publisher=NIST (National Institute of Science and Technology) |access-date=11 May 2024}}</ref> | {{convert|209.61|K|C|sortable=on}} || ? |- |align="left"| [[Deuterium]] | {{convert|18.63|K|C|sortable=on}} || {{convert|17.1|kPa|atm|abbr=on|sortable=on}} |- |align="left"| [[Ethane]] | {{convert|89.89|K|C|sortable=on}} || {{convert|1.1e-3|kPa|atm|abbr=on|sortable=on}} |- |align="left"| [[Ethanol]]<ref>See [[Ethanol (data page)]]</ref> | {{convert|150|K|C|sortable=on}} || {{convert|4.3e-7|kPa|atm|abbr=on|sortable=on}} |- |align="left"| [[Ethylene]] | {{convert|104.0|K|C|sortable=on}} || {{convert|0.12|kPa|atm|abbr=on|sortable=on}} |- |align="left"| [[Formic acid]]<ref>See [[Formic acid (data page)]]</ref> | {{convert|281.40|K|C|sortable=on}} || {{convert|2.2|kPa|atm|abbr=on|sortable=on}} |- |align="left"| [[Helium-4]] (vapor−He-I−He-II)<ref name=Donnelly>{{cite journal| title=The Observed Properties of Liquid Helium at the Saturated Vapor Pressure | first1=Russell J.| last1=Donnelly| first2=Carlo F.| last2=Barenghi| journal=[[Journal of Physical and Chemical Reference Data]]| year=1998| volume=27| issue=6| pages=1217–1274| doi=10.1063/1.556028|bibcode = 1998JPCRD..27.1217D }}</ref> | {{convert|2.1768|K|C|sortable=on}} || {{convert|5.048|kPa|atm|abbr=on|sortable=on}} |- |align="left"| [[Helium-4]] ([[Hexagonal close-packed|hcp]]−[[Body-centered cubic|bcc]]−He-II)<ref name=Hoffer>{{cite journal| title=Thermodynamic properties of <sup>4</sup>He. II. The bcc phase and the P-T and VT phase diagrams below 2 K | first1=J. K.| last1=Hoffer| first2=W. R.| last2=Gardner| first3=C. G.| last3=Waterfield| first4=N. E.| last4=Phillips| journal=[[Journal of Low Temperature Physics]]| date=April 1976| volume=23| issue=1| pages=63–102| doi=10.1007/BF00117245|bibcode = 1976JLTP...23...63H | s2cid=120473493}}</ref> | {{convert|1.463|K|C|sortable=on}} || {{convert|26.036|kPa|atm|abbr=on|sortable=on}} |- |align="left"| [[Helium-4]] (bcc−He-I−He-II)<ref name=Hoffer/> | {{convert|1.762|K|C|sortable=on}} || {{convert|29.725|kPa|atm|abbr=on|sortable=on}} |- |align="left"| [[Helium-4]] (hcp−bcc−He-I)<ref name=Hoffer/> | {{convert|1.772|K|C|sortable=on}} || {{convert|30.016|kPa|atm|abbr=on|sortable=on}} |- |align="left"| [[Hexafluoroethane]]<ref>See [[Hexafluoroethane (data page)]]</ref> | {{convert|173.08|K|C|sortable=on}} || {{convert|26.60|kPa|atm|abbr=on|sortable=on}} |- |align="left"| [[Hydrogen]] | {{convert|13.8033|K|C|sortable=on}} || {{convert|7.04|kPa|atm|abbr=on|sortable=on}} |- |align="left"| [[Hydrogen-1]] (Protium)<ref>{{cite web | url=https://www.britannica.com/science/protium-isotope | title=Protium | isotope | Britannica | date=27 January 2024 }}</ref> | {{convert|13.96|K|C|sortable=on}} || {{convert|7.18|kPa|atm|abbr=on|sortable=on}} |- |align="left"| [[Hydrogen chloride]] | {{convert|158.96|K|C|sortable=on}} || {{convert|13.9|kPa|atm|abbr=on|sortable=on}} |- |align="left"| [[Iodine]]<ref>{{cite book |last=Walas |first=S. M. |title=Chemical Process Equipment – Selection and Design |location=Amsterdam |publisher=Elsevier |year=1990 |page=639 |isbn=0-7506-7510-1 }}</ref> | {{convert|386.65|K|C|sortable=on}} || {{convert|12.07|kPa|atm|abbr=on|sortable=on}} |- |align="left"| [[Isobutane]]<ref>See [[Isobutane (data page)]]</ref> | {{convert|113.55|K|C|sortable=on}} || {{convert|1.9481e-5|kPa|atm|abbr=on|sortable=on}} |- |align="left"| [[Krypton]] | {{convert|115.76|K|C|sortable=on}} || {{convert|74.12|kPa|atm|abbr=on|sortable=on}} |- |align="left"| [[Mercury (element)|Mercury]] | {{convert|234.3156|K|C|sortable=on}} || {{convert|1.65e-7|kPa|atm|abbr=on|sortable=on}} |- |align="left"| [[Methane]] | {{convert|90.68|K|C|sortable=on}} || {{convert|11.7|kPa|atm|abbr=on|sortable=on}} |- |align="left"| [[Neon]] | {{convert|24.5561|K|C|sortable=on}} || {{convert|43.332|kPa|atm|abbr=on|sortable=on}} |- |align="left"| [[Nitric oxide]] | {{convert|109.50|K|C|sortable=on}} || {{convert|21.92|kPa|atm|abbr=on|sortable=on}} |- |align="left"| [[Nitrogen]] |{{convert|63.18|K|C|sortable=on}} || {{convert|12.6|kPa|atm|abbr=on|sortable=on}} |- |align="left"| [[Nitrous oxide]] | {{convert|182.34|K|C|sortable=on}} || {{convert|87.85|kPa|atm|abbr=on|sortable=on}} |- |align="left"| [[Oxygen]] | {{convert|54.3584|K|C|sortable=on}} || {{convert|0.14625|kPa|atm|abbr=on|sortable=on}} |- |align="left"| [[Palladium]] | {{convert|1825|K|C|sortable=on}} || {{convert|3.5e-3|kPa|atm|abbr=on|sortable=on}} |- |align="left"| [[Platinum]] | {{convert|2045|K|C|sortable=on}} || {{convert|2e-4|kPa|atm|abbr=on|sortable=on}} |- |align="left"| [[Radon]] | {{convert|202|K|C|sortable=on}} || {{convert|70|kPa|atm|abbr=on|sortable=on}} |- |align="left"| [[Silane|(mono)Silane]]<ref>{{cite web |url=https://encyclopedia.airliquide.com/silane |title=Silane-Gas Encyclopedia |author=<!--Not stated--> |website=Gas Encyclopedia |publisher=Air Liquide <!-- |access-date={{date}} automatically using date of last page refresh makes no sense for a citation --> }}</ref> | {{convert|88.48|K|C|sortable=on}} || {{convert|0.019644|kPa|atm|abbr=on|sortable=on}} |- |align="left"| [[Sulfur dioxide]] | {{convert|197.69|K|C|sortable=on}} || {{convert|1.67|kPa|atm|abbr=on|sortable=on}} |- |align="left"| [[Titanium]] | {{convert|1941|K|C|sortable=on}} || {{convert|5.3e-3|kPa|atm|abbr=on|sortable=on}} |- |align="left"| [[Uranium hexafluoride]] | {{convert|337.17|K|C|sortable=on}} || {{convert|151.7|kPa|atm|abbr=on|sortable=on}} |- |align="left"| [[Water]]<ref name=Wagner/><ref name=Murphy/> | {{convert|273.16|K|C|sortable=on}} || {{convert|0.611657|kPa|atm|abbr=on|sortable=on}} |- |align="left"| [[Xenon]] | {{convert|161.3|K|C|sortable=on}} || {{convert|81.5|kPa|atm|abbr=on|sortable=on}} |- |align="left"| [[Zinc]] | {{convert|692.65|K|C|sortable=on}} || {{convert|0.065|kPa|atm|abbr=on|sortable=on}} |} Notes: * For comparison, typical atmospheric pressure is 101.325 kPa (1 atm). * Before the new definition of SI units, water's triple point, 273.16 K, was an exact number. == See also == * [[Critical point (thermodynamics)]] * [[Gibbs' phase rule]] == References == {{reflist}} == External links == * {{Commons category-inline}} {{Phase of matter}} {{DEFAULTSORT:Triple Point}} [[Category:Chemical properties]] [[Category:Phase transitions]] [[Category:Thermodynamics]] [[Category:Threshold temperatures]] [[Category:Gases]] [[Category:1873 introductions]]
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