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Triple point of water

Gas–liquid–solid triple point

See also: Properties of water § Triple point

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 273.16±0.0001 K⁴ and a vapour pressure of 611.657 pascals (6.11657 mbar; 0.00603659 atm).⁵⁶

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.⁷

High-pressure phases

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 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.

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. 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.

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).⁹

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

External links

• Media related to Triple point at Wikimedia Commons

References

1. IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (1994) "Triple point". doi:10.1351/goldbook.T06502. /wiki/International_Union_of_Pure_and_Applied_Chemistry ↩

2. James Thomson (1871) "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 he designates as the triple point ; and he shows how this point belongs to the three important curves, as being their intersection." https://www.nature.com/articles/004288a0 ↩

3. Definition of the kelvin at BIPM. http://www1.bipm.org/en/si/base_units/ ↩

4. "SI Brochure: The International System of Units (SI) – 9th edition". BIPM. Retrieved 21 February 2022. https://www.bipm.org/documents/20126/41483022/SI-Brochure-9-EN.pdf/2d2b50bf-f2b4-9661-f402-5f9d66e4b507 ↩

5. 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. https://www.nist.gov/srd/upload/jpcrd477.pdf ↩

6. 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.131.1539M. doi:10.1256/qj.04.94. S2CID 122365938. https://zenodo.org/record/1236243 ↩

7. Carr, Michael H. (2007). The Surface of Mars. Cambridge University Press. p. 5. ISBN 978-0-521-87201-0. 978-0-521-87201-0 ↩

8. 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.131.1539M. doi:10.1256/qj.04.94. S2CID 122365938. https://zenodo.org/record/1236243 ↩

9. Cengel, Yunus A.; Turner, Robert H. (2004). Fundamentals of thermal-fluid sciences. Boston: McGraw-Hill. p. 78. ISBN 0-07-297675-6. 0-07-297675-6 ↩

10. See Butane (data page) /wiki/Butane_(data_page) ↩

11. See Chloroform (data page) /wiki/Chloroform_(data_page) ↩

12. "Trichloromethane". NIST Chemistry WebBook, SRD 69. NIST (National Institute of Science and Technology). Retrieved 11 May 2024. https://webbook.nist.gov/cgi/cbook.cgi?ID=C67663&Mask=4#Thermo-Phase ↩

13. See Ethanol (data page) /wiki/Ethanol_(data_page) ↩

14. See Formic acid (data page) /wiki/Formic_acid_(data_page) ↩

15. 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..27.1217D. doi:10.1063/1.556028. /wiki/Journal_of_Physical_and_Chemical_Reference_Data ↩

16. Hoffer, J. K.; Gardner, W. R.; Waterfield, C. G.; Phillips, N. E. (April 1976). "Thermodynamic properties of 4He. 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. S2CID 120473493. /wiki/Journal_of_Low_Temperature_Physics ↩

17. Hoffer, J. K.; Gardner, W. R.; Waterfield, C. G.; Phillips, N. E. (April 1976). "Thermodynamic properties of 4He. 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. S2CID 120473493. /wiki/Journal_of_Low_Temperature_Physics ↩

18. Hoffer, J. K.; Gardner, W. R.; Waterfield, C. G.; Phillips, N. E. (April 1976). "Thermodynamic properties of 4He. 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. S2CID 120473493. /wiki/Journal_of_Low_Temperature_Physics ↩

19. See Hexafluoroethane (data page) /wiki/Hexafluoroethane_(data_page) ↩

20. "Protium | isotope | Britannica". 27 January 2024. https://www.britannica.com/science/protium-isotope ↩

21. Walas, S. M. (1990). Chemical Process Equipment – Selection and Design. Amsterdam: Elsevier. p. 639. ISBN 0-7506-7510-1. 0-7506-7510-1 ↩

22. See Isobutane (data page) /wiki/Isobutane_(data_page) ↩

23. "Silane-Gas Encyclopedia". Gas Encyclopedia. Air Liquide. https://encyclopedia.airliquide.com/silane ↩

24. 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. https://www.nist.gov/srd/upload/jpcrd477.pdf ↩

25. 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.131.1539M. doi:10.1256/qj.04.94. S2CID 122365938. https://zenodo.org/record/1236243 ↩