the equilibrium constant for a solubility reaction. A general solubility equation could be written as: MX (s) M+ (aq) + X- (aq) where MX represents an ionic solid, M+ the positive ion, and X- the negative ion. Since pure solids have an activity of 1, they are not included in equilibrium constant expressions. Therefore, the K expression for this reaction is: Since the concentration of either ion, [M+] or [Cl-] is the amount of MX that dissolved, (in other words, MX's solubility) and the result of multiplying two numbers is called a "product", this kind of K expression is called a "solubility product", and given the symbol Ksp. Therefore the K expression would normally be written as