# Critical pressure

The critical pressure is the pressure at which liquefaction of a gas occurs when it reaches the critical temperature; in other words it can be considered as the limit value of the saturated vapor pressure of a liquid at its critical temperature (above this point there are no distinct gas and liquid phases). As one approaches the critical temperature of a substance, it is known that the properties of the gas and liquid phases become the same, resulting in a single phase. At pressures above the critical pressure, the transformation of the liquid into gas occurs without passing through the vapor phase, therefore in a practically instantaneous manner and preserving the continuity of the physical characteristics.

The ratio between pressure and critical pressure is called reduced pressure, and it is a dimensionless quantity. The reduced pressure is used by the corresponding states theorem to calculate the compressibility factor of a gas or gas mixture.

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