Gibbs phase rule
Saturation vapour pressures of organic compounds

Any pure substance (e.g. water, acetone, etc.) can exist in any of the 3 main physical states of matter (solid, liquid, vapor). There are, however, particular values of temperature and pressure for which the pure body can be two-phase (presence of 2 phases in equilibrium), or even three-phase (presence of three phases in equilibrium).

The number of phases in which a pure body can simultaneously be found is governed by Gibbs' phase rule. This rule states that the variance var of a system in thermodynamic equilibrium containing a pure body is :

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