There are two types of hardness , one is called temporary hardness and another one is permanent hardness. Temporary hardness is caused by calcium or magnesium hydrogen carbonate. Temporary hardness can be removed simply by boiling the water or chemical treatment. 

* Ca(HCO3)2+Ca(OH)2 - 2CaCO3+2H2O

* Mg(HCO3)2+2Ca(OH)2 - 2MgCO3+2H2O

* MgCO3+Ca(OH)2-Mg(OH)2+CaCO3

Permanent hardness is caused by calcium or magnesium sulfhate. It can be removed by chemical treatment.

 *  CaSo4+Na2CO3 -  CaCO3

+Na2SO4

*  MgSO4+Na2SO3 - MgCO3+Na2SO4

* MgCO3+Ca(OH)2 -Mg(OH)2+CaCO3.