Solution:
If the excess Gibbs energy of a binary liquid system, $G^E$, is expressed as a function of the fractions of the components as $\displaystyle \frac{G^E}{RT} = A x_1 x_2$ ($A$: a constant).
\begin{parts}
\part [7] What is the range of $A$, if these two liquids form two coexisting liquid phases?
\part [8] If $A = 2.5$, what is the composition range of species $1$ leading to the observation of two coexisting liquid phases in this system?
\end{parts}
