Triple point
The single pressure and temperature at which solid, liquid, and gas of a substance coexist in equilibrium.
Definition
The triple point is the unique combination of temperature and pressure at which all three phases of a substance — solid, liquid, and vapour — coexist in stable equilibrium. On a phase diagram it is the single point where the sublimation, vaporisation, and melting curves meet. For water it lies at 0.01 °C and 611.657 Pa.
Because it is a single, sharply defined, reproducible state of a pure substance, the triple point of water served to define the kelvin from 1954 until 2019: the kelvin was exactly 1/273.16 of the thermodynamic temperature of the triple point of water. The 2019 SI redefinition shifted the definition to a fixed Boltzmann constant, but the triple point remains a key practical fixed point in thermometry.
This entry is a stub; the triple point is treated in more depth alongside the critical point and the Clausius–Clapeyron relation later in the branch.
History
Implicit in Gibbs's phase rule (1870s); adopted as the defining fixed point of the kelvin in 1954 and used until the 2019 SI redefinition.