Phase Transitions

We have seen a generic phase diagram earlier. It showed that under certain conditions (along the boundary lines), one could have the substance existing simultaneously under more than one state (for example, liquid and gas). That is, the system along the lines is not homogeneous. Away from that line of course the state of the system is homogeneous and is called a phase of the system. When one crosses a line, by tuning the parameters, the system undergoes a phase transition, often accompanied by an absorption or release of heat into the surroundings (e.g. consider melting of ice or the boiling of water). In phase transitions the entropies of the respective phases before and after are often different: Such transitions are called first-order phase transitions, while those for which the entropy is continuous across the boundary are called second-order. For first-order phase transitions, one defines the latent heat $L$ as that which is exchanged with the surroundings,

$$L= T \Delta S.$$ (4.27)