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.
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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Updated on 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.

This is a phase diagram, which includes the critical point and triple point.
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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 July 20, 2024).