triple point

{{see also|Properties of water#Triple point}}

File:Water-triple-point-20210210.gifFollowing 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}}{{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}} and a vapour pressure of {{convert|611.657|Pa|mbar atm}}.[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.{{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 }}

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 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.{{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 }}

At high pressures, water has a complex phase diagram with 15 known 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.]]

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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. A specific isotopic composition (for 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 ITS–90 relies upon triple-point cells of hydrogen, neon, oxygen, argon, mercury, and water for delineating six of its defined temperature 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).{{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 }}

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.

• Critical point (thermodynamics)
• Gibbs' phase rule

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