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Triple point

The triple point is the temperature and pressure at which three phases (gas, liquid, and solid) of a particular substance may coexist in thermodynamic equilibrium.

For example, the triple point temperature of water is exactly 273.16 Kelvin (0.01 C) and the triple point pressure is 611.73 pascal (ca. 0.006 atmospheres) and the triple point temperature of mercury is -38.8344 C.

The triple point of water is used to define the Kelvin and the Celsius temperature scales; the number given for the temperature of the triple point of water is an exact definition rather than a measured quantity.

Note that the pressure referred to here is the vapor pressure of the substance, not the total pressure of the entire system.

See also equations of state.