In heterogeneous equilibria, where reactants and products exist in more than one phase, the equilibrium constant expression excludes species whose concentration remains effectively constant regardless of reaction extent: pure solvents (present in vast excess, so their concentration does not meaningfully change) and pure solids (which have no meaningful concentration term since they are not dissolved in any phase). This follows from the kinetic/probabilistic derivation of the equilibrium constant, in which forward and reverse reaction rates depend on the probability of reactant or product molecules occupying the same volume; when a species' local concentration is effectively invariant (a solvent that is "everywhere" or a solid phase boundary), its probability term reduces to a constant and drops out of the rate expression, and hence out of the ratio defining K. This principle belongs to chemical equilibrium theory within general/physical chemistry, and extends the general law of mass action governing homogeneous equilibria to systems spanning multiple phases (gas, liquid, solid, aqueous).
In heterogeneous equilibria, where reactants and products exist in more than one phase, the equilibrium constant expression excludes species whose concentration remains effectively constant regardles…