CBSE Class 11-science Answered

In the process called softening of water, slaked lime (Ca(OH)₂) is added to the hard water.

As calcium hydroxide is insoluble in water, calcium has to be precipitated as CaCO₃.

To remove carbon dioxide in water:

H₂CO₃ + Ca(OH)₂ → CaCO₃ + 2H₂O

To remove calcium temporary hardness:

Ca(HCO₃)₂ + Ca(OH)₂ → 2CaCO₃ + 2H₂O

To remove magnesium temporary hardness, add lime and more lime,

Stage 1

Mg(HCO₃)₂ + Ca(OH)₂ → MgCO₃ + CaCO₃ + 2H₂O

The calcium carbonate precipitates, but the magnesium carbonate does not as per its Ksp, hence further lime is added.

Stage 2

MgCO₃ + Ca(OH)₂ → Mg(OH)₂ + CaCO₃

Combining above two equations:

Mg(HCO₃)₂ + 2Ca(OH)₂ → 2CaCO₃↓ + Mg(OH)₂↓ + 2H₂O

The magnesium hydroxide and calcium carbonate precipitate.