Solubility Product of Calcium Carbonate

Calcium carbonate is the scale in kettles, boilers, cooling towers and shower heads, and K_sp = 3.36 × 10⁻⁹ for calcite at 25 °C is what predicts it. Set [Ca²⁺] = [CO₃²⁻] = s and the saturation concentration is 5.8 × 10⁻⁵ mol/L, about 5.8 mg/L as CaCO₃ in pure water — but real water is nothing like pure, and the bicarbonate equilibrium, ionic strength and complexation all shift the practical figure upward by an order of magnitude or more. Aragonite, the same compound in a different crystal form, is slightly more soluble at 6.0 × 10⁻⁹, and it is the polymorph that marine shells and corals build with.

Two features make this constant unusually consequential. First, solubility falls as temperature rises, so the compound deposits on the hottest surface in a system — precisely the heat-exchanger wall. Second, the carbonate concentration in the ion product depends steeply on pH through carbonic acid's second dissociation, so a one-unit pH rise increases the driving force tenfold. Both effects are wrapped up in the Langelier saturation index, which converts a water analysis into a single scaling-or-corroding verdict, and the same equilibrium is the mechanism behind ocean acidification dissolving shells as CO₂ rises.