Many metal ions form compounds that are insoluble in water. We call them insoluble salts (duhhh) or precipitates. 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. For example:
Ba(IO3)2 (s) 
Ba2+(aq) + 2 IO3-(aq)
The equilibrium constant expression for an insoluble salt is written following the same rules as for any other equilibrium. The equilibrium constant is called the solubility product, Ksp. The Ksp expression for the above equilibrium is:
Ksp = [Ba2+][IO3-]2
Ksp Values for Some PrecipitatesFormulaNameKspAgClsilver chloride1.8x10-10Al(OH)3aluminum hydroxide2x10-32BaCO3barium carbonate5x10-9Ba(IO3)2barium iodate1.6x10-9BaSO4barium sulfate1.3x10-10Fe(OH)2iron(II) hydroxide8x10-16Fe(OH)3iron(III) hydroxide4x10-38FeSiron sulfide6x10-18PbCrO4lead chromate1.8x10-14Pb(OH)2lead hydroxide2.5x10-16PbSlead sulfide7x10-28PbSO4lead sulfate1.6x10-8
The equilibria of insoluble salts is described in more detail in the document on solubility.
