Solubility Product (Ksp)
Definition
The equilibrium constant for a solid dissolving: a tiny Ksp means the salt barely dissolves at all.
Symbol: ·
Technical definition
The solubility product (Ksp) is the equilibrium constant for the dissolution of a sparingly soluble ionic solid, for example AgCl(s) forming Ag+ + Cl-, with Ksp = [Ag+][Cl-]. Smaller Ksp means lower solubility, and comparing the ion product Q to Ksp predicts precipitation.
Example
AgCl has Ksp = 1.8 x 10^-10, so little dissolves that silver chloride is considered insoluble, yet that tiny solubility is exactly calculable.
Common confusion
Ksp is not the solubility itself. For AgCl the solubility s equals the square root of Ksp, but for CaF2 the solubility is the cube root of Ksp/4: the stoichiometry matters.