ABSTRACT
Since the standard state for all solids has been chosen to be the pure solid, this has meant that one has so far only considered cases when the solid activities are unity. However, given that many elements are present in natural systems, natural mineral solids are rarely highly pure. One needs to understand why non-pure solids form and how they behave. A side benefit will be that the general thermodynamics of solid solutions applies equally well to solutions of liquids in one another, e.g., trichloroethylene (TCE) dissolved in a mostly water phase, and vice versa, TCE dissolved mix of other chlorinated solvents, etc. When a chemical (liquid or solid) is pure, it will exert the full thermodynamic activity that corresponds to its mol fraction of 1.0. The solubility of water in heptane is very low, as is the solubility of heptane in water.
