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What information can be read from the regions of a pure substance’s phase diagram?

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Diagram showing four physical states of matter: solid with tightly packed molecules, liquid with loosely packed molecules, gas with widely spaced molecules, and plasma with free electrons and nuclei, each labeled with descriptions and an arrow indicating increasing energy. — phase States of matter.

In a phase diagram for a pure substance, the regions (or areas) separated by boundary curves represent the conditions of pressure and temperature under which only one specific phase—solid, liquid, or gas—is stable.12

Key characteristics of these regions include:

Stability: Within any given region, the substance exists as a single phase. Small changes in temperature or pressure within these areas will not alter the phase of the substance.1
Degrees of Freedom: According to the phase rule, these regions are considered divariant. This means that both pressure and temperature can be varied independently while the substance remains in the same phase.2
Boundaries: The regions are delineated by curves where two phases coexist in equilibrium. At the triple point, three phases coexist, while the critical point marks the condition where the distinction between liquid and gas phases disappears.12