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How does pressure affect the state of matter of a substance?

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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.

Pressure plays a fundamental role in determining the phase of a substance. By altering the pressure and temperature, a substance can be forced to transition between solid, liquid, and gaseous states, as illustrated by a phase diagram.

In general, increasing pressure favors denser states of matter. For example, gases can be compressed into liquids or solids at sufficiently high pressures. Under extreme pressure, materials often undergo structural transformations to achieve more efficient, closely packed atomic arrangements. At pressures exceeding $10^{6}$ bars, it is believed that most substances transition into a metallic state as electron orbitals overlap and electrons become delocalized.12

The relationship between pressure and phase transitions is governed by the Clapeyron equation, which relates the slope of phase-boundary lines to changes in latent heat and volume. While most substances become denser when freezing, some, like water, exhibit unique behavior where the solid is less dense than the liquid, causing the melting point to decrease as pressure increases.3