CBSE Class 9 Answered

This method depends upon the use of zealot or base exchange complexes. The zeolites are hydrated silicates or sodium and alluminium with a general formula…(Na₂O)_x(AlO₃)_y(SiO₂)_z(H₂O)_n

When zeolites or base exchange complexes are brought in contact with hard water following reaction occur.

For temporary Hardness,

Ca(HCO₃)₂+Na₂O.Z -------------à CaO.Z + Na₂CO₃ + H₂O

Mg(HCO₃)₂+Na₂O.Z -------------à MgO.Z + Na₂CO₃ + H₂O

For Permanent Hardness,
            CaSO₄ + Na₂O.Z ---------------à CaO.Z + NaSO₄

            MgSO₄ + Na₂O.Z -------------à MgO.Z + NaSO₄

In where Z is an abbreviation for the Al₂O₃SiO₂H₂O part of Zeolite. The soft water obtained from this base exchange process is of 0-20 hardness or levelness. After a long time the whole of the sodium in base exchange substance is replaced by calcium or magnesium, it is said to be exhausted because it will not soften any hard water more.

Then it has to be generated.

Regeneration: CaO.Z+2NaCl -----------à Na₂O.Z+CaCl₂

The CaCl₂ and residual NaCl are washed away and the regenerated Na₂O.Z can be used to soften the hard water again.