Exercise. Explain in detail, using thermodynamic principles, why the mixing of pure chemicals to form a homogenous mixture is a so-called spontaneous process, while the separation of that mixture into its pure (or nearly pure) species is not.
Solution. The entropy of mixing is always positive as in a mixture the number of configurations achievable by a system is larger than that of the corresponding pure species. In situations when there is no enthalpic effect associated with mixing (as is the case of mixing ideal gases) then this entropic effect leads to a negative free energy of mixing, which means that mixing is a spontaneous process.