Triple Point Definition and Example (Chemistry)

In chemistry and physics, the triple point is the temperature and pressure at which solid, liquid, and vapor phases of a particular substance coexist in equilibrium. It is a specific case of thermodynamic phase equilibrium. The term "triple point" was coined by James Thomson in 1873.

Examples: The triple point for water is at 0.01° Celsius at 4.56 mm Hg. The triple point of water is a fixed quantity, used to define other triple point values and the kelvin unit of temperature.

Note the triple point may include more than one solid phase if a specific substance has polymorphs.