For an ideal gas, pressure times volume is a constant if the temperature and number of moles are constant. The plot of pressure versus volume results in a hyperbola called a pV isotherm. For different constant temperatures, the set of pV diagrams is plotted. Non-ideal gases behave like an ideal gas at higher temperatures. If the temperature of a non-ideal gas reduces to critical temperature, the pV curve shows a region with zero slopes. Suppose the temperature is lowered further, the region having zero slope increases. These regions represent a change in volume without any change in pressure. For isotherm T2, the region A-B on the isotherm represents the liquification phase. The gas is in a vapor state until point A and in a liquid phase beyond point B. For isotherms with temperatures lower than T2, liquification starts at lower pressures and higher volumes. For isotherms above the critical temperatures, no liquification takes place. The critical temperature is a property of the gas under study for a given critical pressure.