A simple experiment was devised to let students determine the solubility and solubility product, "K"[subscript sp], of calcium sulfate dihydrate in a first-level laboratory. The students experimentally work on an intriguing equilibrium law: the constancy of the product of the ion concentrations of a sparingly soluble salt. The determination of solubility is proposed either in an equimolar precipitation of CaSO[subscript 4][middle dot]2H[subscript 2]O, from which the "K"[subscript sp] is obtained, or working with an excess of one of the two reagents.