Fundamental properties of perfect solutions

A solution is described as perfect if equation (20.1) is valid for all values of the mole fractions. If in (20.1) we put Xi = l, we see that h T P) simply the chemical potential of pure component i at the same temperature and pressure i.e, p). The term RT In Xi thus 

The Gibbs free energy per mole of a binary mixture is given by 

Thus the increase of g which results from mixing the components 1 and 2, or the Gibbs free energy of mixing is 

This simple form for the free energy of mixing is a characteristic of perfect solutions. The enthalpy of mixing corresponding to (20.11) is zero since (c/. 4.32) 

Thus the mixing of two components to form a perfect solution takes place at constant enthalpy. This means that if the components are mixed at constant T and p, no absorption or evolution of heat occurs. For since p is constant, the first law gives, cf. (2.15), 

---