CRITICAL AND BOILING POINTS

When a liquid is heated in a sealed vessel, boiling does not occur. Instead, the temperature, vapor pressure, and density of the vapor rise continuously. At the same time, density of the liquid also decreases as a result of its expansion. There comes a stage at which density of the vapor is equal to that of remaining liquid and the surface between two phases disappears. The temperature at which the surface disappears is the critical temperature, Tc, and the corresponding vapor pressure is the critical pressure, P. At and above this temperature, a single uniform phase fills the container and an interface no longer exists. That is, above the critical temperature the liquid phase of the substance does not exist. 

For a reduced temperature (7 j = T/Tc) in the range 0.9-1.2, the reduced fluid density (p/ =p/Pc) can increase fi om gas-like values of 0.1 to liquid-like values of 2.5 as the reduced pressure (P/j = P/P is increased to values greater than 1.0. But as is increased to 1.55, the supercritical fluid becomes more expanded and reduced pressures greater than ten are needed to obtain liquid-like densities. By operating in the critical region, the pressure and temperature can be used to regulate density, which in turn regulates the solvent power of a supercritical fluid. 

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