Water hardness and methods of softening water
Hard water contains many Ca²⁺ and Mg²⁺ ions, soft water very few. Temporary (carbonate) hardness is caused by Ca(HCO₃)₂ and Mg(HCO₃)₂, permanent hardness by the sulfates and chlorides of Ca and Mg (CaSO₄, MgCl₂); together they give the total hardness. Hard water does harm: scale forms in boilers and pipes, more fuel is used, soap does not lather (Ca²⁺ and Mg²⁺ make insoluble salts), and meat and vegetables cook poorly. Temporary hardness disappears on boiling: Ca(HCO₃)₂ → CaCO₃↓ + H₂O + CO₂↑. Lime water or alkali also removes it: Ca(HCO₃)₂ + Ca(OH)₂ → 2CaCO₃↓ + 2H₂O. Soda removes both kinds: Ca(HCO₃)₂ + Na₂CO₃ → CaCO₃↓ + 2NaHCO₃ and CaSO₄ + Na₂CO₃ → CaCO₃↓ + Na₂SO₄ (permanent hardness does not go away on boiling). Industry uses ion exchange: in water passed through a cation exchanger Ca²⁺ and Mg²⁺ are exchanged for Na⁺: Na₂R + Ca²⁺ → CaR + 2Na⁺. Distillation gives the purest water.
Kettle scale observation: ask adults about, or look at a picture of, the white stony layer at the bottom of a pot of boiled water (CaCO₃, MgCO₃). Removing scale with acid is done by adults only. In class fill in the table “Which method for which hardness?”