Triple Point Definition and Example (Chemistry)

Learn what the triple point means in chemistry

Water in ice, vapor and liquid form
Triple point of water.

 

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By
Anne Marie Helmenstine, Ph.D.
Chemistry Expert
  • Ph.D., Biomedical Sciences, University of Tennessee at Knoxville
  • B.A., Physics and Mathematics, Hastings College
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.
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Anne Marie Helmenstine, Ph.D.
Updated November 18, 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.

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Helmenstine, Anne Marie, Ph.D. "Triple Point Definition and Example (Chemistry)." ThoughtCo, Feb. 16, 2021, thoughtco.com/definition-of-triple-point-604674. Helmenstine, Anne Marie, Ph.D. (2021, February 16). Triple Point Definition and Example (Chemistry). Retrieved from https://www.thoughtco.com/definition-of-triple-point-604674 Helmenstine, Anne Marie, Ph.D. "Triple Point Definition and Example (Chemistry)." ThoughtCo. https://www.thoughtco.com/definition-of-triple-point-604674 (accessed April 10, 2021).