Lime Softening:

The process is used for the removal of calcium (temporary) hardness. Lime is added to precipitate calcium carbonate, which has solubility of about 25 mg/l at 20⁰C.

Ca(HCO₃)₂ + Ca(OH)₂   →  2CaCO₃ + 2H₂O (ppt)

Lime-soda Softening

It is used for the removal of all forms of calcium hardness. Soda ash is added and the non-carbonate hardness is converted to the sparingly soluble calcium carbonate. In addition to the reaction above, following reactions take place:

CaSO₄ + NaCO₃        →        CaCO₃ + Na₂SO₄

CaCl₂ + Na₂CO₃        →        CaCO₃ + 2NaCl