1. Standard conditions. Standard conditions

Depending on temperature and pressure conditions, a pure substance can exist in different phases. During heating or cooling, a pure substance is frequently transformed from one phase to another. The main phase transitions are described in the diagram below.

The study of a phase transition highlights the existence of change-of-state quantities, which represent the variations in the thermodynamic properties of a pure substance as it undergoes a transformation from one phase to another. For example, the enthalpy of fusion is the variation in enthalpy observed during the transition from the solid to the liquid state, at a given temperature and pressure. This thermodynamic...