Triple point
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In physics, the triple point of a substance is the temperature and pressure at which three phases (gas, liquid, and solid) of that substance may coexist in thermodynamic equilibrium.
Phase-diag.png
For example, the triple point temperature of mercury is -38.8344 °C at a pressure of 0.2 mPa.
The triple point of water is used to define the kelvin, the unit of thermodynamic temperature in the International System of Units. The number given for the temperature of the triple point of water is an exact definition rather than a measured quantity.
See also equations of state.
Triple point of water
The single combination of pressure and temperature at which water, ice, and water vapour can coexist in a stable equilibrium occurs at exactly 273.16 kelvins (0.01 °C) and a pressure of 611.73 pascals (ca. 6 millibars). At that point, it is possible to change all of the substance to ice, water, or steam by making infinitesimally small changes in pressure and temperature. (Note that the pressure referred to here is the vapor pressure of the substance, not the total pressure of the entire system.)
Water has an unusual and complex phase diagram, although this does not affect general comments about the triple point. At high temperatures, increasing pressure results in first liquid, and then solid water (above around <math>10^9<math> Pascal a crystalline form of ice which is denser than water forms). At lower temperatures the liquid state ceases to appear with compression causing the state to pass directly from gas to solid. It is however sometimes possible to melt ice by increasing pressure under specific conditions.
At a constant pressure higher than the triple point, heating ice necessarily passes from ice to liquid then to steam. In pressures below the triple point, such as in outer space where the pressure is low, liquid water cannot exist; ice skips the liquid stage and becomes steam on heating, in a process known as sublimation.
External links and references
- Definition of the kelvin (http://www1.bipm.org/en/si/base_units/) at BIPM.
- The Phase Diagram of Water (http://www.lsbu.ac.uk/water/phase.html).
See also
de:Tripelpunkt es:Punto triple fr:Point triple it:Punto triplo nl:Tripelpunt ja:三重点 pl:Punkt potrójny pt:Ponto triplo sl:Trojna točka sk:Trojný bod fi:Kolmoispiste sv:Trippelpunkt zh:三相点