In science, the set of conditions under which a liquid and its vapour become identical. The conditions are the critical temperature, the critical pressure, and the critical density. If a closed vessel is filled with a pure substance, partly liquid and partly vapour, and the average density equals the critical density, the critical conditions can be achieved. As the temperature is raised, the vapour pressure increases, and the gas phase becomes denser while the liquid expands and becomes less dense. At the critical point, the densities of liquid and vapour become equal, eliminating the boundary between the two.
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