the compression takes place at Tc itself, then a surface separating two phases does not appear and the volumes at each end of the horizontal part of the isotherm have merged to a singlepoint, the critical point of the gas. The temperature, pressure, and molar volumeat the critical point are called the critical temperature, Tc, critical pressure, pc, andcritical molar volume, Vc, of the substance. Collectively, pc, Vc, and Tc are the critical constants of a substance.
EX: “The temperature below which the continuous increase of pressure on a gas ultimately brings about liquefaction and above which no liquefaction can take place no matter what so ever pressure be applied”.
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