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.