Steve Marsden’s

Although it is possible to establish equilibrium conditions (i.e., saturated solutions) for most water-soluble solids, we are interested here primarily in those which are generally described as "insoluble".

Silver chloride, AgCl, is a good example. When solid silver chloride is placed in water the following equilibrium can be established:

AgCl(s) Ag+(aq) + Cl-(aq)

The equilibrium constant expression for this process is written in the same way as the others we have seen, bearing in mind that solids have "unit activity" and are therefore not explicitly included: