Answer. Permanent hardness can only be removed by using chemicals. Calcium (Ca2+) and magnesium (Mg2+) are removed as insoluble salts by adding washing soda (Na2CO3) or sodium zeolite.
1. By using washing soda: The addition of washing soda removes the calcium and magnesium ions as the insoluble calcium and magnesium carbonates, respectively.
Na2CO3(aq) + CaSO4(aq) CaCO3(s) + Na2SO4(aq)
Na2CO3(aq) + MgSO4(aq) MgCO3(s) + Na2SO4(aq)
Using Sodium Zeolite: Sodium zeolite is a naturally occurring resin of sodium aluminum silicate NaAl (SiO3)2, which can also be prepared artificially. It is used for softening of water at domestic as well as on industrial scale. When water is passed through resin sodium ions of the resin are exchanged with the unwanted calcium and magnesium ions of the hard water as shown in figure.
Na2 zeolite + CaSO4(aq) Ca zeolite + Na2SO4(aq)
When resin is fully used up it can be regenerated by flushing it with concentrated solution of NaCl. The reverse process takes place because of high concentration of sodium ions.
Ca zeolite + 2NaCl Na2 zeolite + CaCl2