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 water the triple point temperature is exactly 273.16 K (0.01 C) and the triple point pressure is 611.73 Pa (ca. 0.006 atm).

For mercury the triple point temperature is -38.8344 °C. The triple point of water is used to define the Kelvin and the Celsius temperature scale; 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.