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.įor most substances the gas–liquid–solid triple point is also the minimum temperature at which the liquid 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. The smallest pressure at which liquid water can exist is equal to the triple point of water, at which gas, liquid, and solid phases can coexist. Strictly speaking, the surfaces separating the different phases should also be perfectly flat, to negate the effects of surface tension. 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. 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. The single combination of pressure and temperature at which liquid water, solid ice, and water vapor can coexist in a stable equilibrium occurs at 273.16 ☐.0001 K and a partial vapor pressure of 611.657 pascals (6.11657 mbar 0.00603659 atm). See also: Properties of water § Triple point Water boiling at 0☌ using a vacuum pump.

Triple point of water Gas–liquid–solid triple point The term 'triple point' was coined in 1873 by James Thomson, brother of Lord Kelvin. 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 ☌, or 32.018 ☏). The value of the triple point of water was fixed by definition, rather than by measurement, but that changed with the 2019 redefinition of SI base units. The triple point of water was used to define the kelvin, the base unit of thermodynamic temperature in the International System of Units (SI). None of these should be confused with the Lambda Point, which is not any kind of triple point. Instead, it has a vapor-liquid-superfluid point, a solid-liquid-superfluid point, a solid-solid-liquid point, and a solid-solid-superfluid point. 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. 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. For example, the triple point of mercury occurs at a temperature of −38.8 ☌ (−37.8 ☏) and a pressure of 0.165 m Pa. It is that temperature and pressure at which the sublimation, fusion, and vaporisation curves meet. 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. The solid green line applies to most substances the dashed green line gives the anomalous behavior of water For other uses, see Tripoint and Tripoint (disambiguation).

Liquid oxygen temperature kelvin