Triple Point Definition and Example (Chemistry)

Learn what the triple point means in chemistry

A phase diagram indicates the temperature and pressure of the triple point.
A phase diagram indicates the temperature and pressure of the triple point. Matthieumarechal
by Anne Marie Helmenstine, Ph.D.
Dr. Helmenstine holds a Ph.D. in biomedical sciences and is a science writer, educator, and consultant. She has taught science courses at the high school, college, and graduate levels.
Updated August 06, 2019

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.

Example

The triple point for water is at 0.01 degree 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.