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Triple point
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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|ā38.8|°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''', 288ā298 (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=ā23|u=Jā K<sup>ā1</sup>}}, and the triple point of water became an experimentally measured constant.
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