Triple Point

CORE DEFINITION

In thermodynamics, the triple point is the set of temperature and pressure values at which the three phases (gas, liquid, and solid) of a substance coexist in thermodynamic equilibrium. For example, the solid-liquid-gas triple point of water is 0.01 °C (273.16 K) and 611.73 Pa (about six-thousandths of standard atmospheric pressure, 101.325 kPa). The triple point of mercury is −38.8344 °C and 0.2 MPa. The solid-liquid-gas triple point of water was once used to define the thermodynamic temperature scale in the International System of Units (SI). Under this definition, the triple point temperature of water (273.16 K, 0.01 °C) was not a measured value but a true value. However, after the 2019 redefinition of the SI, the triple point of water became a measured value. Triple points of other substances are also used to define the International Temperature Scale, such as hydrogen (13.8033 K). If a substance has multiple solid and liquid phases, triple points form between adjacent phases on the phase diagram. Many solids (such as iron and ice) have different solid phases at different temperatures and pressures, so they have multiple triple points.