Introduction Many metal ions form compounds that are insoluble in water.
Common precipitates are carbonates, hydroxides, sulfates, and sulfides.
Ions that we consider spectator ions when discussing acid-base equilibria will form insoluble salts. An insoluble salt in contact with water maintains an equilibrium with the ions. In simple cases where there are no common ions or competing equilibria, the ion concentrations depend only on the equilibrium constant for the particular precipitate. When we talk about solubility equilibria we always write the equilibrium with the solid on the left. The equilibrium constant expression for an insoluble salt is written following the same rules as for any other equilibrium .