a) Treatment with washing soda:
Soluble salts of calcium and magnesium turn into insoluble salts when washing soda (sodium carbonate) is mixed with it as shown below in examples.
b) Permutit process:
Permutit is the trade name for an artificially prepared sodium zeolite or sodium aluminium orthosilicate obtained by fusing together sodium carbonate, silica and alumina. Its composition corresponds to Na2Al2Si2O8 or Na2Z where Z stands for zeolite = Al2Si2O8. It possesses remarkable property of reacting with calcium and magnesium ions forming insoluble calcium or magnesium zeolite. By passing hard water through a cylinder packed with permutit all the ions responsible for the hardness of water are drawn and replaced by sodium ions.
Ca-salt + Na-permutit = Ca-permutit + Na-salt
Mg-salt + Na-permutit = Mg-permutit + Na-salt
In due course, however, the permutit is exhausted by having given up all its sodium ions. This can be regenerated by soaking in a 10% solution of common salt when the reverse change take place and calcium and magnesium ions pass into solution and sodium zeolite is formed again.
CaZ + 2NaCl = Na2Z + CaCl2
MgZ + 2Nacl = Mg2Z + MgCl2
The resulting calcium and magnesium chlorides are washed out and the regenerated permutit used over again.