Hard water is water that has high mineral content like magnesium, calcium, sodium and so on. Hard water is formed when water percolates through deposits of limestone and chalk which are largely made up of calcium and magnesium carbonates. There are two types of hard water, they are:
Temporary hardness of water is the hardness caused by the presence of dissolved calcium hydrogentrioxocarbonate(IV), Ca(HCO3)2, which decomposes on heating. Temporary hardness of water can be easily removed by boiling.
Permanent hardness of water is the hardness of water caused by the presence of calcium and magnesium ions or calcium and magnesium sulphates presence in the form of soluble tetraoxosulphates (VI), Calcium carbonate and Chlorides. Permanent hardness can be removed by adding sodium carbonate, by distillation and by adding caustic soda.