Explicit expressions for the deviations from IG, SI, and DI behavior

This section is devoted to illustrating explicitly the three fundamentally different types of ideal mixtures. The first and simplest case is that of the ideal-gas (IG) mixtures, which, as in the case of an ideal gas, are characterized by the complete absence (or neglect) of all intermolecular forces. This case is of least importance in the study of solution chemistry. 

The second case, referred to as symmetric ideal (SI) solutions, occurs whenever the various components are similar to each other. There are no 

Any mixture of two components can be viewed as deviating from one of the ideal reference cases. This can be written symbolically as 

There are several ways of reporting experimental data on the deviations from ideal behavior. The most common ones are either the activity coefficients of each component, or the total excess Gibbs energy of the system. If the vapor above the liquid -mixture can be assumed to be an ideal gas, then it is also convenient to plot Pa/Pa as a function of xA where PA and PA are the partial pressure and the vapor pressure of A, respectively. 

We now consider two particular examples of a system which, on the one hand, are not trivial, since interactions between particles are taken into account, yet are sufficiently simple that all three activity coefficients can be written in an explicit form. 

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