15.3 Soft and Hard Water

Soft water is that water which produces good lather with soap.

Hard water is that water which does not produce lather with soap. Causes of hardness in water. The rain water while coming down absorbs carbon dioxide from the atmosphere. The water mixed with carbon dioxide, when passes through the beds of the soil, converts insoluble carbonates of calcium and magnesium into soluble bicarbonates. It may also dissolve chlorides and sulphates of calcium and magnesium. These salts make the water hard.

Thus, rain water dissolves many salts of divalent cations like $\mathrm{M}{\mathrm{g}}^{2+}$$\mathrm{Mg^{2 + }}$ $\mathrm{C}{\mathrm{a}}^{2+}$$\mathrm{Ca^{2 + }}$ , and anions like ${\mathrm{C}\mathrm{l}}^{-}$$\mathrm{Cl}^{- }$ ${\mathrm{S}\mathrm{O}}_4^{2-}$$\mathrm{SO}_{4}^{2 - }$ ${\mathrm{H}\mathrm{C}\mathrm{O}}_3^{-}$$\mathrm{HCO}_{3}^{- }$ and ${\mathrm{C}\mathrm{O}}_3^{2-}$$\mathrm{CO}_{3}^{2 - }$ for example, gypsum $(\mathrm{C}\mathrm{a}\mathrm{S}{\mathrm{O}}_4\cdot 2{\mathrm{H}}_2\mathrm{O})$$(\mathrm{CaSO_{4}\cdot 2H_{2}O})$ and limestone $(\mathrm{C}\mathrm{a}\mathrm{C}{\mathrm{O}}_3)$$(\mathrm{CaCO_{3}})$ . These salts make the water hard. Gypsum is sparingly soluble in water, while limestone is insoluble in water. However, in the presence of carbon dioxide small quantity of limestone is soluble in water according to the above chemical reaction.

15.3.1 Types of Hardness of Water

Hardness is of two types:

(i) Temporary hardness is because of presence of bicarbonates of calcium and magnesium. (ii) Permanent hardness is because of presence of sulphates and chlorides of calcium and magnesium.

15.3.2 Method of Removing Hardness

The removal of $Mg^{2 + }$ and $Ca^{2 + }$ ions which are responsible for the hardness is called water softening.

(i) Removal of temporary hardness

(a) By boiling

Temporary hardness of water is easily removed by boiling the water. On boiling, calcium bicarbonate $\mathrm{C}\mathrm{a}(\mathrm{H}\mathrm{C}{\mathrm{O}}_3{)}_2$$\mathrm{Ca(HCO_3)_2}$ decomposes to produce insoluble calcium carbonate, which precipitates out of the solution.

(b) Clark's method

A chemical method to remove temporary hardness is by the addition of slaked lime $\mathrm{C}\mathrm{a}(\mathrm{O}\mathrm{H}{)}_2$$\mathrm{Ca(OH)_2}$ . A calculated amount of lime water is added to temporary hard water. Thus, once the magnesium and calcium ions precipitate out water becomes soft.