Definition of the ideal solution model

For an ideal solution, the Gibbs free energy G defined as  

In all calculations involving the ideal solution model, we assume that we know the molar Gibbs free energy of each of the pure species as a function of temperature and pressure. Mathematically, this means that know the form of the functions (T, p). Physically, this means that we know everything about the thermodynamics of the pure species. 

Once we know the Gibbs free energy of a system as a function of temperature, pressure, and composition, we know everything about its thermodynamics. For example, the total volume of the system Y can be derived from the Gibbs free energy  

The total volume is equal to the sum of the volumes of the pure components. Therefore, there is no change of volume on mixing for an ideal solution. 

In an ideal solution, we see that the chemical potential of a species depends on its mole fraction and not directly on the composition of the other components in the system. Also, we see that mixing causes the chemical potential of each component to decrease. 

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