When a substance is added to a solution it can either dissolve completely or form a precipitate. If it dissolves completely, it forms a saturated solution that contains the maximum concentration of solute ions possible in the solvent (its solubility). If there is more solute than can be dissolved, it separates from the solution by crystallization, which decreases the concentration of the solute and gives the solution its characteristic appearance. In the case of solids, the separation process can be accelerated by adding an acid or by adding water.
This article discusses the properties of some common ionic salts and looks at the conditions under which they can be soluble or insoluble. In addition, it describes the solubility rules that help chemists predict whether a salt will or will not dissolve in a given solvent. If two of these rules seem to contradict one another, the rule that comes first takes precedence.
For example, if sodium bromide, NaBr, is added to a solution of lead(II) nitrate, Pb(NO3)2, the result will be the formation of a white precipitate of PbBr2. This is because Rule #1 states that salts of alkali metal cations tend to be soluble and Rule #2 states that carbonates are insoluble.