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triple point
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 mPa.
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).〔(Definition of the kelvin ) at BIPM〕 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.〔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''."〕
==Triple point of water==
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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