For most of the year we have used the solubility rules to "decide" whether a precipitate will form if two electrolytes are mixed together in aqueous solution. The rules are based on observations of solutions with specific concentrations but those parameters are seldom specified.

The K_sp value can be thought of as a kind of solubility "limit" or the minimum conditions required to produce a precipitate. When the reaction quotient in a mixture (Q_sp) exceeds the K_sp then a precipitate will form. When it is less, the solution remains unsaturated.