Phase Transitions and Criticality

Abstract

The water, when heated at constant pressure, boils at a well defined temperature, turning into steam. To each value of the pressure imposed on the water, there corresponds a boiling temperature. The water-steam transition temperature increases with pressure and, in a temperature-pressure diagram, it is represented by a line with a positive slope. On the transition line the liquid and vapor coexist in any proportion. However, the liquid and the vapor present well defined and distinct densities that depend only on the transition temperature. As we increase the temperature, along the coexistence line, the difference between the densities of the liquid and vapor becomes smaller and vanishes at a point characterized by well defined temperature and pressure. At this point, called critical point, the liquid and vapor becomes identical and the line of coexistence ends. Beyond this point there is no distinction between liquid and vapor.