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What happens when temperature and pressure reach a boundary line on a 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.

When the temperature and pressure of a substance reach a boundary line on a phase diagram, the system enters a state of thermodynamic equilibrium where two phases coexist simultaneously.12

Specifically, these boundary lines represent the conditions under which a substance can transition between states, such as solid and liquid, liquid and vapour, or solid and vapour. At any point along these curves, the two phases are in balance; for instance, along a liquid-vapour boundary, the substance exists as both liquid and vapour at the same time.1

These equilibrium conditions are governed by the phase rule, which dictates the degrees of freedom available to the system. While the areas between these lines represent conditions where only one phase can exist, the lines themselves define the precise limits where phase changes occur. This relationship continues until reaching the critical point, where the distinction between phases—such as liquid and vapour—vanishes, or the triple point, where all three phases exist in equilibrium together.21