Hard and Soft Water

Hard vs Soft Water

Rainwater is close to pure, carrying only traces of dissolved atmospheric gases. But as it flows over rock and soil, water's strong solvent power lets it dissolve a variety of salts. When it picks up calcium and magnesium salts — as hydrogencarbonates, chlorides and sulphates — it becomes hard water, which famously fails to lather with soap. Water free of these dissolved calcium/magnesium salts is called soft water, and it lathers easily.

The soap problem: hard water reacts with the sodium stearate in ordinary soap to precipitate insoluble calcium/magnesium stearate — the familiar "scum":

$2C_{17}H_{35}COONa(aq) + M^{2+}(aq) \rightarrow (C_{17}H_{35}COO)_2M(aq)\downarrow + 2Na^+(aq) \quad (M = Ca, Mg)$ …