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How does the critical point affect the difference between a liquid and its vapor?

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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.

The critical point represents the specific temperature and pressure conditions at which the distinction between a liquid and its vapor disappears.12

As a substance approaches this point along the vapor-pressure curve, the liquid expands and becomes less dense, while the vapor becomes increasingly dense. At the critical point, these densities become identical, effectively eliminating the boundary between the two phases.132

Above the critical point, the substance exists as a supercritical fluid, a state where the liquid and gas phases are no longer distinct. Because no phase-change lines are crossed when moving through this region, the transition between these states becomes smooth and continuous rather than abrupt. Consequently, the physical properties of the substance—such as density, viscosity, and heat capacity—change continuously rather than undergoing the sharp, defined shifts characteristic of a standard phase change.2