Isotherms of Real Gases and Critical Constants

This behavior occurs until a certain high temperature is reached denoted and called the critical temperature. At that temperature, the constant pressure plateau shrinks into a single point (point C) called the critical point The molar volume at that point is called critical molar volume and the pressure is the critical pressure P. A gas cannot be condensed to a liquid at temperatures above and there is no clear distinction between the liquid and gaseous phases because the two states cannot coexist with a sharp boundary between them. Experimentally, if a certain amount of gas and liquid is placed inside a pressurized container with transparent quartz windows and kept below T, two layers will be observed, separated by a sharp boundary. As the tube is warmed, the boundary becomes less distinct because the densities, and therefore the refractive indices, of the liquid and gas approach a common value. When the T is reached, the boundary becomes invisible and the iridescent aspect exhibited by the fluid is called critical opalescence. Hence the following definitions can be drawn for the critical constants of a real gas. 

The critical temperature of a gas is the absolute temperature, denoted T, above which the liquid phase cannot be formed no matter how great a pressure is applied to the system. 

The critical pressure of a gas denoted is the vapor pressure of the liquid at the critical point. Hence, below the critical temperature any substance at a pressure above its vapor pressure will be liquid. 

The critical density is the density of the fluid at the critical temperature and pressure. 

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