Triple point - wikipedia Triple point For other uses, see Tripoint and Tripoint ( disambiguation ). In thermodynamics, the triple point of a substance is the temperature and pressure at which the three phases ( gas, liquid, and solid ) of that substance coexist in thermodynamic equilibrium. For example, the triple point of mercury occurs at a temperature of − 38.83440 ° C and a pressure of 0.2 m 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 polymorphs. Helium - 4 is a special case that presents a triple point involving two different fluid phases ( lambda point ). The triple point of water is used to define the kelvin, the base unit of thermodynamic temperature in the International System of Units ( SI ). The value of the triple point of water is fixed by definition, rather than measured. 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 ). The term `` triple point '' was coined in 1873 by James Thomson, brother of Lord Kelvin. Contents 1 Triple point of water 1.1 Gas -- liquid -- solid triple point 1.2 High pressure phases 2 Triple point cells 3 Table of triple points 4 See also 5 References Triple point of water Gas -- liquid -- solid triple point Main article : Water ( molecule ) § Triple point A typical phase diagram. The solid green line applies to most substances ; the dotted green line gives the anomalous behavior of water The single combination of pressure and temperature at which liquid water, solid ice, and water vapor can coexist in a stable equilibrium occurs at exactly 273.16 K ( 0.01 ° C ; 32.02 ° F ) and a partial vapor pressure of 611.657 pascals ( 6.11657 mbar ; 0.00603659 atm ). At that point, it is possible to change all of the substance to ice, water, or vapor by making arbitrarily small changes in pressure and temperature. Even if the total pressure of a system is well above the triple point of water, provided that the partial pressure of the water vapor is 611.657 pascals, then the system can still be brought to the triple point of water. Strictly speaking, the surfaces separating the different phases should also be perfectly flat, to negate the effects of surface tension. The gas -- liquid -- solid triple point of water corresponds to the minimum pressure at which liquid water can exist. At pressures below the triple point ( as in outer space ), solid ice when heated at constant pressure is converted directly into water vapor in a process known as sublimation. Above the triple point, solid ice when heated at constant pressure first melts to form liquid water, and then evaporates or boils to form vapor at a higher temperature. For most substances the gas -- liquid -- solid triple point is also the minimum temperature at which the liquid can exist. For water, however, this is not true because the melting point of ordinary ice decreases as a function of pressure, as shown by the dotted green line in the phase diagram. At temperatures just below the triple point, compression at constant temperature transforms water vapor first to solid and then to liquid ( water ice has lower density than liquid water, so increasing pressure leads to a liquefaction ). The triple point pressure of water was used during the Mariner 9 mission to Mars as a reference point to define `` sea level ''. More recent missions use laser altimetry and gravity measurements instead of pressure to define elevation on Mars. High pressure phases At high pressures, water has a complex phase diagram with 15 known phases of ice and several triple points including ten 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 251 -- 273 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. 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. Triple point cells Triple point cells are used in the calibration of thermometers. 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. When it is a water - based cell, a special isotopic composition called VSMOW is used because it is very pure and produces temperatures that are more comparable from lab to lab. Triple point cells are so effective at achieving highly precise, reproducible temperatures, an international calibration standard for thermometers called ITS -- 90 relies upon triple point cells of hydrogen, neon, oxygen, argon, mercury, and 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 comes from the U.S. National Bureau of Standards ( now NIST, National Institute of Standards and Technology ). Substance T ( K ) ( ° C ) p ( kPa ) * ( atm ) Acetylene 0192.4 ! 192.4 K ( − 80.7 ° C ) 7002120000000000000 ♠ 120 kPa ( 1.2 atm ) Ammonia 0195.40 ! 195.40 K ( − 77.75 ° C ) 7000607590000000000 ♠ 6.076 kPa ( 0.05997 atm ) Argon 0083.81 ! 83.81 K ( − 189.34 ° C ) 7001689000000000000 ♠ 68.9 kPa ( 0.680 atm ) Arsenic 1090 ! 1,090 K ( 820 ° C ) 7003362800000000000 ♠ 3,628 kPa ( 35.81 atm ) Butane 0134.6 ! 134.6 K ( − 138.6 ° C ) 6996700000000000000 ♠ 7 × 10 kPa Carbon ( graphite ) 4765 ! 4,765 K ( 4,492 ° C ) 7004101320000000000 ♠ 10,132 kPa ( 100.00 atm ) Carbon dioxide 0216.55 ! 216.55 K ( − 56.60 ° C ) 7002517000000000000 ♠ 517 kPa ( 5.10 atm ) Carbon monoxide 0068.10 ! 68.10 K ( − 205.05 ° C ) 7001153700000000000 ♠ 15.37 kPa ( 0.1517 atm ) Chloroform 0175.43 ! 175.43 K ( − 97.72 ° C ) 6999870000000000000 ♠ 0.870 kPa ( 0.00859 atm ) Deuterium 0018.63 ! 18.63 K ( − 254.52 ° C ) 7001171000000000000 ♠ 17.1 kPa ( 0.169 atm ) Ethane 0089.89 ! 89.89 K ( − 183.26 ° C ) 6996800000000000000 ♠ 8 × 10 kPa Ethanol 0150 ! 150 K ( − 123 ° C ) 6993429990000099999 ♠ 4.3 × 10 kPa Ethylene 0104.0 ! 104.0 K ( − 169.2 ° C ) 6999120000000000000 ♠ 0.12 kPa ( 0.0012 atm ) Formic acid 0281.40 ! 281.40 K ( 8.25 ° C ) 7000220000000000000 ♠ 2.2 kPa ( 0.022 atm ) Helium - 4 ( lambda point ) 0002.18 ! 2.1768 K ( − 270.9732 ° C ) 7000500000000000000 ♠ 5.048 kPa ( 0.04982 atm ) Helium - 4 ( hcp − bcc − He - II ) 0001.46 ! 1.463 K ( − 271.687 ° C ) 7003263800000000000 ♠ 26.036 kPa ( 0.25696 atm ) Helium - 4 ( bcc − He - I − He - II ) 0001.77 ! 1.762 K ( − 271.388 ° C ) 7003300000000000000 ♠ 29.725 kPa ( 0.29336 atm ) Helium - 4 ( hcp − bcc − He - I ) 0001.77 ! 1.772 K ( − 271.378 ° C ) 7003304000000000000 ♠ 30.016 kPa ( 0.29623 atm ) Hexafluoroethane 0173.08 ! 173.08 K ( − 100.07 ° C ) 7001266000000000000 ♠ 26.60 kPa ( 0.2625 atm ) Hydrogen 0013.84 ! 13.84 K ( − 259.31 ° C ) 7000704000000000000 ♠ 7.04 kPa ( 0.0695 atm ) Hydrogen chloride 0158.96 ! 158.96 K ( − 114.19 ° C ) 7001139000000000000 ♠ 13.9 kPa ( 0.137 atm ) Iodine 0386.65 ! 386.65 K ( 113.50 ° C ) 7001120700000000000 ♠ 12.07 kPa ( 0.1191 atm ) Isobutane 0113.55 ! 113.55 K ( − 159.60 ° C ) 6995194810000000000 ♠ 1.9481 × 10 kPa Krypton 0115.76 ! 115.76 K ( − 157.39 ° C ) 7001741200000000000 ♠ 74.12 kPa ( 0.7315 atm ) Mercury 0234.2 ! 234.2 K ( − 39.0 ° C ) 6993165009999999999 ♠ 1.65 × 10 kPa Methane 0090.68 ! 90.68 K ( − 182.47 ° C ) 7001117000000000000 ♠ 11.7 kPa ( 0.115 atm ) Neon 0024.57 ! 24.57 K ( − 248.58 ° C ) 7001432000000000000 ♠ 43.2 kPa ( 0.426 atm ) Nitric oxide 0109.50 ! 109.50 K ( − 163.65 ° C ) 7001219200000000000 ♠ 21.92 kPa ( 0.2163 atm ) Nitrogen 0063.18 ! 63.18 K ( − 209.97 ° C ) 7001126000000000000 ♠ 12.6 kPa ( 0.124 atm ) Nitrous oxide 0182.34 ! 182.34 K ( − 90.81 ° C ) 7001878500000000000 ♠ 87.85 kPa ( 0.8670 atm ) Oxygen 0054.36 ! 54.36 K ( − 218.79 ° C ) 6999152000000000000 ♠ 0.152 kPa ( 0.00150 atm ) Palladium 1825 ! 1,825 K ( 1,552 ° C ) 6997350000000000000 ♠ 3.5 × 10 kPa Platinum 2045 ! 2,045 K ( 1,772 ° C ) 6996200000000000000 ♠ 2.0 × 10 kPa Radon 202 ! 202 K ( − 71 ° C ) 7001700000000000000 ♠ 70 kPa ( 0.69 atm ) ( mono ) Silane 0088.48 ! 88.48 K ( − 184.67 ° C ) 7001117000000000000 ♠ 0.00019644 bar ( 0.00019387 atm ) Sulfur dioxide 0197.69 ! 197.69 K ( − 75.46 ° C ) 7000167000000000000 ♠ 1.67 kPa ( 0.0165 atm ) Titanium 1941 ! 1,941 K ( 1,668 ° C ) 6997530000000000000 ♠ 5.3 × 10 kPa Uranium hexafluoride 0337.17 ! 337.17 K ( 64.02 ° C ) 7002151690000000000 ♠ 151.7 kPa ( 1.497 atm ) Water 0273.16 ! 273.16 K ( 0.01 ° C ) 6999611657000000000 ♠ 0.611657 kPa ( 0.00603659 atm ) Xenon 0161.3 ! 161.3 K ( − 111.8 ° C ) 7001815000000000000 ♠ 81.5 kPa ( 0.804 atm ) Zinc 0692.65 ! 692.65 K ( 419.50 ° C ) 6998650000000000000 ♠ 0.065 kPa ( 0.00064 atm ) * Note : for comparison, typical atmospheric pressure is 101.325 kPa ( 1 atm ). See also Critical point ( thermodynamics ) Gibbs ' phase rule References ^ Jump up to : IUPAC, Compendium of Chemical Terminology, 2nd ed. ( the `` Gold Book '' ) ( 1997 ). Online corrected version : ( 1994 ) `` Triple point ''. Jump up ^ Definition of the kelvin at BIPM Jump up ^ James Thomson ( 1873 ) `` A quantitative investigation of certain relations between the gaseous, the liquid, and the solid states of water - substance, '' Proceedings of the Royal Society, 22 : 27 -- 36. From a footnote on page 28 : ``... the three curves would meet or cross each other in one point, which I have called the triple point. '' ^ Jump up to : 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. ^ Jump up to : Murphy, D.M. ( 2005 ). `` Review of the vapour pressures of ice and supercooled water for atmospheric applications ''. Quarterly Journal of the Royal Meteorological Society. 131 ( 608 ) : 1539 -- 1565. Bibcode : 2005QJRMS. 131.1539 M. doi : 10.1256 / qj. 04.94. Jump up ^ Carr, Michael H. ( 2007 ). The Surface of Mars. Cambridge University Press. p. 5. ISBN 0 - 521 - 87201 - 4. Jump up ^ Cengel, Yunus A. ; Turner, Robert H. ( 2004 ). Fundamentals of thermal - fluid sciences. Boston : McGraw - Hill. p. 78. ISBN 0 - 07 - 297675 - 6. Jump up ^ See Butane ( data page ) Jump up ^ See Chloroform ( data page ) Jump up ^ See Ethanol ( data page ) Jump up ^ See Formic acid ( data page ) Jump up ^ Donnelly, Russell J. ; Barenghi, Carlo F. ( 1998 ). `` The Observed Properties of Liquid Helium at the Saturated Vapor Pressure ''. Journal of Physical and Chemical Reference Data. 27 ( 6 ) : 1217 -- 1274. Bibcode : 1998JPCRD... 27.1217 D. doi : 10.1063 / 1.556028. ^ Jump up to : Hoffer, J.K. ; Gardner, W.R. ; Waterfield, C.G. ; Phillips, N.E. ( April 1976 ). `` Thermodynamic properties of He. II. The bcc phase and the P-T and VT phase diagrams below 2 K ''. Journal of Low Temperature Physics. 23 ( 1 ) : 63 -- 102. Bibcode : 1976JLTP... 23... 63H. doi : 10.1007 / BF00117245. Jump up ^ See Hexafluoroethane ( data page ) Jump up ^ Walas, S.M. ( 1990 ). Chemical Process Equipment -- Selection and Design. Amsterdam : Elsevier. p. 639. ISBN 0 - 7506 - 7510 - 1. Jump up ^ See Isobutane ( data page ) Jump up ^ `` Silane - Gas Encyclopedia ''. Gas Encyclopedia. Air Liquide. Retrieved 13 October 2017. States of matter ( list ) State Solid Liquid Gas / Vapor Plasma Low energy Bose -- Einstein condensate Fermionic condensate Degenerate matter Quantum Hall Rydberg matter Strange matter Superfluid Supersolid Photonic matter High energy QCD matter Lattice QCD Quark -- gluon plasma Supercritical fluid Other states Colloid Glass Liquid crystal Quantum spin liquid Magnetically ordered Antiferromagnet Ferrimagnet Ferromagnet String - net liquid Superglass Transitions Boiling Boiling point Condensation Critical line Critical point Crystallization Deposition Evaporation Flash evaporation Freezing Chemical ionization Ionization Lambda point Melting Melting point Recombination Regelation Saturated fluid Sublimation Supercooling Triple point Vaporization Vitrification Quantities Enthalpy of fusion Enthalpy of sublimation Enthalpy of vaporization Latent heat Latent internal energy Trouton 's ratio Volatility Concepts Binodal Compressed fluid Cooling curve Equation of state Leidenfrost effect Macroscopic quantum phenomena Mpemba effect Order and disorder ( physics ) Spinodal Superconductivity Superheated vapor Superheating Thermo - dielectric effect Retrieved from `` https://en.wikipedia.org/w/index.php?title=Triple_point&oldid=805088256 '' Categories : Chemical properties Phase transitions Thermodynamics Threshold temperatures Gases 1873 introductions Talk Contents About Wikipedia Aragonés Asturianu বাংলা Bân - lâm - gú Български Bosanski Brezhoneg Català Čeština Dansk Deutsch Eesti Español Esperanto فارسی Français Galego 한국어 Հայերեն हिन्दी Hrvatski Bahasa Indonesia Interlingua Íslenska Italiano עברית Қазақша Kiswahili Latina Latviešu Magyar Македонски മലയാളം Монгол Nederlands 日本 語 Norsk Norsk nynorsk Occitan پنجابی Piemontèis Plattdüütsch Polski Português Română Русский Scots සිංහල Simple English Slovenčina Slovenščina Српски / srpski Srpskohrvatski / српскохрватски Suomi Svenska தமிழ் Українська اردو Tiếng Việt 粵語 中文 Edit links This page was last edited on 13 October 2017, at 00 : 06. 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Question: the temperature and pressure at which a liquid and a gas are in equilibrium
Answer: triple point