Term: triple point
https://doi.org/10.1351/goldbook.T06502

Definition:
The point in a one-component system at which the temperature and pressure of three phases are in equilibrium. If there are p possible phases, there are p ! ( p- 3 ) ! 3 ! triple points. Example: In the sulfur system four possible triple points (one metastable) exist for the four phases comprising rhombic S (solid), monoclinic S (solid), S (liquid) and S (vapour).

Related Terms:
1) metastable (http://doi.org/10.1351/goldbook.M03872).

Source: PAC, 1994, 66, 577. 'Definitions of terms relating to phase transitions of the solid state (IUPAC Recommendations 1994)' on page 593 (https://doi.org/10.1351/pac199466030577)

Citation: 'triple point' in IUPAC Compendium of Chemical Terminology, 5th ed. International Union of Pure and Applied Chemistry; 2025. Online version 5.0.0, 2025. https://doi.org/10.1351/goldbook.T06502

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