Ideal Solution - Formal Definition

Formal Definition

Different related definitions of an ideal solution have been proposed. The simplest definition is that an ideal solution is a solution for which each component (i) obeys Raoult's law for all compositions. Here is the vapor pressure of component i above the solution, is its mole fraction and is the vapor pressure of the pure substance i at the same temperature.

This definition depends on vapor pressures which are a directly measurable property, at least for volatile components. The thermodynamic properties may then be obtained from the chemical potential μ (or partial molar Gibbs energy g) of each component, which is assumed to be given by the ideal gas formula

.

The reference pressure may be taken as = 1 bar, or as the pressure of the mix to ease operations.

On substituting the value of from Raoult's law,

.

This equation for the chemical potential can be used as an alternate definition for an ideal solution.

However, the vapor above the solution may not actually behave as a mixture of ideal gases. Some authors therefore define an ideal solution as one for which each component obeys the fugacity analogue of Raoult's law ,

Here is the fugacity of component in solution and is the fugacity of as a pure substance. Since the fugacity is defined by the equation

this definition leads to ideal values of the chemical potential and other thermodynamic properties even when the component vapors above the solution are not ideal gases. An equivalent statement uses thermodynamic activity instead of fugacity.