Critical temperature

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In physical chemistry, thermodynamics, chemistry and condensed matter physics, a critical point, also called a critical state, specifies the conditions (temperature, pressure and sometimes composition) at which a phase boundary ceases to exist. There are multiple types of critical points such as vapor-liquid critical points and liquid-liquid critical points.

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Pure substances: vapor-liquid critical point

The term "critical point" is sometimes used to denote specifically the vapor-liquid critical point of a material. The vapor-liquid critical point denotes the conditions above which distinct liquid and gas phases do not exist.

As shown in the pure species phase diagram to the right, this is the point at which the phase boundary between liquid and gas terminates. In water, the critical point occurs at around 647 K (374 °C or 705 °F) and 22.064 MPa (3200 PSIA or 218 atm)[1].

As the critical temperature is approached, the properties of the gas and liquid phases approach one another, resulting in only one phase at the critical point: a homogeneous supercritical fluid. The heat of vaporization is zero at and beyond this critical point, so there is no distinction between the two phases. Above the critical temperature a liquid cannot be formed by an increase in pressure, but with enough pressure a solid may be formed. The critical pressure is the vapor pressure at the critical temperature. On the diagram showing the thermodynamic properties for a given substance, the point at which critical temperature and critical pressure meet is called the critical point of the substance. The critical molar volume is the volume of one mole of material at the critical temperature and pressure.

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