Pure water is called life. Water is polluted every day due to bathing, washing etc. and it becomes hard for various reasons. We can transform hard water into the mild water in various ways.
Hardness of Water
Rivers, canals, bills, ponds, sea, and tube well etc. are the source of water. Various kind of mineral salts is dissolved in this water. If calcium or magnesium chloride, sulfate, carbonate or bicarbonate etc. salts are dissolved in the water then it is not easy to get foam of soap. Hence, this kind of water is called hard water.
However, water can be also hard if it contains iron or manganese salts dissolved in it. Now the question is ‘why does not this kind of water produce foam of soap?’
We know soap is a sodium or potassium salt of a higher organic acid. For example, sodium stearate (C17H35COONa) is the sodium salt of stearic acid. When we use this soap in hard water, so long calcium or magnesium salt remains in this water, it does not produce any foam, only the soap gets decayed.
Salts of magnesium and other salts will also undergo the same kind of reaction. If hard water is used in water pipes or boilers of industries, the mineral salts present in that water gathers on the pipe as a thick layer and thus the water flow is obstructed.
In case of a boiler, there is wastage of heat and the boiler may even burst. The property of water that causes the soap not to clean the dirt’s properly is called hardness of water.
Classification of the hardness of water
It is of two kinds:
(i) Temporary Hardness
The hardness of water caused by bicarbonate (HCO3–) salts of calcium, magnesium, iron etc. in water is called temporary hardness and this kind of water is known as temporary hard water.
How to remove temporary hardness of water?
When heat is applied to this kind of water it produces insoluble carbonate salt. It gathers at the bottom of the container as a precipitate. The water can be easily separated by using a filter. Thus, the temporary hardness of water is removed and the water turns into mild water. 
(ii) Permanent Hardness
The hardness of water caused by chloride or sulfate salts of calcium, magnesium, iron etc. in water is called permanent hardness and this kind of water is known as permanent hard water. Mere heating cannot remove this kind of hardness. Usually, the water of closed water body is mild water. Rainwater is also mild and it does not contain much amount of salts of magnesium, calcium etc. metals.
How to remove permanent hardness of water?
Various measures or reactions are used to remove this kind of hardness from water. Addition of sodium carbonate to permanent hard water reacts with the calcium ion and magnesium ion and produce precipitates of calcium carbonate and magnesium carbonate. Thus the calcium ion and magnesium ion is removed from the water and it becomes mild.
Source- NCTB Books
