Hard water is usually defined as water which contains a high concentration of calcium and magnesium ions.

Hard water is formed when water percolates through deposits of calcium and magnesium-containing rocks such as limestone, chalk and dolomite. When water percolates thru such rocks , it dissolves the minerals thus producing hard water which enters the water table.

• when water passes through limestone , it dissolves the Calcium and hence the hardness in water shall be because of Calcium.
• similarly when water passes through Dolomite , the Dolomite introduces Magnesium to the water.

Hardness in water is caused by divalent cations dissolved in water like Ca2+ and Mg2+ .
These cations have a tendency to combine with anions to form stable compounds known as salts.

There are two types of water hardness, temporary and permanent.

Temporary Hardness 

Temporary Hardness is due to the bicarbonate ion, HCO3-, being present in the water as Calcium bicarbonate – Ca(HCO3)2 and Magnesium bicarbonate – Ca(HCO3)2.

Both calcium bicarbonate and magnesium bicarbonate decompose when heated. The original insoluble carbonate is reformed. This happens when water is boiled. The precipitation reactions are as follows:

Ca(HCO3)2→CO2+H2O+CaCO3

Mg(HCO3)2→CO2+H2O+MgCO3

As you can se boiling the water causes the precipitation of solid calcium carbonate or solid magnesium carbonate. This removes the calcium ions or magnesium ions from the water, and so removes the hardness. Therefore, hardness due to bicarbonates is said to be temporary.

Permanent Hardness 

Permanent hardness is hardness (mineral content) that cannot be removed by boiling. When this is the case, it is usually caused by the presence of sulphates and chlorides of Magnesium and Calcium ( MgSo4 , Mgcl2 and Caso4 and Cacl2 ) in the water, which do not precipitate out as the temperature increases.

Ions causing permanent hardness of water can be removed using a water softener, or ion exchange column.
Total Permanent Hardness = Calcium Hardness + Magnesium Hardness .

Measurement of Hardness of Water
Following estimates are universally recognised for checking the hardness of water.

Seawater is considered to be very hard due to various dissolved salts. Typically seawater’s hardness is in the range of 6630 ppm. In contrast, freshwater has hardness in the range of 15 – 375 ppm.