Q.Why do soaps not work in hard water ?

Soap is the sodium or potassium salt of a long-chain fatty acid. Its cleansing action depends on it staying dissolved/dispersed in water to form micelles.

Hard water contains dissolved calcium ($Ca^{2+}$) and magnesium ($Mg^{2+}$) ions (typically as bicarbonates, sulphates or chlorides). When soap is added to hard water, these ions react with the soap anion to form insoluble calcium and magnesium salts of the fatty acid:

$2C_{17}H_{35}COO^-Na^+ + Ca^{2+} → (C_{17}H_{35}COO)_2Ca↓ + 2Na^+$

This insoluble precipitate is commonly seen as scum (a sticky curd-like deposit) — it:

• Wastes soap, since the soap is consumed forming the precipitate instead of being available to dissolve grease/dirt, and …

Recall the composition of hard water ($Ca^{2+}$/$Mg^{2+}$) and how soap anions react with these ions to form insoluble salts (scum). Conne …

• Saying soap is simply 'insoluble' in hard water rather than explaining the precipitation reaction with $Ca^{2+}$/$Mg^{2+}$. …