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Why do liquid and vapor densities become equal at the critical point?

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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.

At the critical point, the densities of the liquid and vapor phases become equal because of the specific changes occurring as temperature and pressure increase.

As a substance is heated within a closed vessel, the vapor pressure rises, causing the gas phase to become denser. Simultaneously, the liquid expands and becomes less dense. When the system reaches the critical temperature and pressure, these opposing trends converge until the densities of the two phases are identical.1

This convergence eliminates the physical boundary between the liquid and gas, resulting in a single, homogeneous supercritical fluid where the distinction between the two states of matter disappears.12