Thermodynamics, 5th Edition

(2.5.10)

Since all x_i are nonnegative and less than unity the entropy of mixing is indeed positive; this rationalizes the experimental fact that the mixing of components to form an ideal solution is always spontaneous: coffee and cream may readily be mixed, while the reverse process is much harder to achieve. Also, in view of conditions (1) and (2), $\Delta \tilde{E}=\Delta {\tilde{C}}_P=\Delta {\tilde{C}}_V=0$.

The chosen unit for concentration is the mole fraction. The use of other variables, such as molarity or molality is also common, but leads to more elaborate relations that will not ...