Remove the dissolved Ca2+ and Mg2+ ions to make hard water soft.
There are two types of hardness – temporary and permanent.
Temporary hardness is caused by the hydrogencarbonate ion, HCO3-, in Ca(HCO3)2.
Permanent hardness is caused by dissolved calcium sulfate (among other things).
1) Temporary hardness is removed by boiling. When heated, the calcium hydrogencarbonate decomposes to form calcium carbonate, which is insoluble. This solid is the ‘limescale’ on your kettle.
E.g. Calcium Hydrogencarbonate à Calcium Carbonate + Water…