The occurrence of different phases of a one component system can be shown on a pressure temperature.
The phases present in a one line system at various temperatures can be conveniently presented on a P- versus-T diagram. An example is provided by the diagram for water for moderate pressures and temperatures. The lien labeled TC shows the pressures and temperatures of liquid and vapour exist in the equilibrium. It is a vapour-pressure curve. At temperatures higher than that of point C, the critical point, liquid vapour pressure does not occur. Therefore this liquid vapour equilibrium line finishes at C.
Consider the changes that occur as a pressure or temperature change results in the system moving in the lien TC. From point 1, for instance, the temperature can be maintained lesser to get to point 2, or the pressure can be increased to get to point 3. In either process one crosses the liquid vapour equilibrium line in the direction of consideration from the vapour to liquid. Notice, however, that if a sample phase carried from point 1 to point 2 or point 3 by a path that goes around C, no phase change will occur.
Line TB gives the temperature and pressure at such solid and vapour are in equilibrium; i.e. it is the curve for the vapour pressure of the solid line TA gives the temperatures and pressures at which ice as a function of pressure equilibrium; i.e. it represents the melting point of ice as a function of pressure. Liquid water can be cooled below its freezing point to give, as indicated by the system. It shows its existence to the fact that the rate of formulation of ice has been interfered with by the use of a very clean sample of water and a smooth container.
It is a convenient representation of all the available information about the phases of water that occurs at moderate pressures and temperatures. It shows the phase behavior of water at very large pressures. Many new phases, corresponding to ice with different crystal structures, are of common and is known as polymorphism. It is particularly remarkable that the melting point of ice VII, which exists above about 20,000 bar pressure, is over 100 degree C.
The most well-known material water that we have utilized as an illustration of P-versus-T phase diagrams is, in some ways, not at all representative. More suitable, in this regard, is one of the solid liquid equilibrium line, TA has a positive slope.