The hardness of the temporary and the permanent hardness is removed through many methods. This process of converting hard water to soft water is called softening of water. The technique for removing temporary hardness is as follows:
(i) Boiling – During boiling, the soluble Mg(HCO3)2 is converted into insoluble Mg(OH)2 and Ca(HCO3)2 is changed to insoluble CaCO3.
Mg (HCO3)2 Mg (OH)2 + 2CO2
(ii) Clark’s method – In this method calculated amount of lime is added to hard water. It precipitates out calcium carbonate and magnesium hydroxide which can be filtered off.
Ca (HCO3)2 + Ca (OH)2 2CaCO3 + 2H2O
Mg (HCO3)2 + 2Ca (OH)2 2CaCO3 + Mg (OH)2 + 2H2O
The technique for removing permanent hardness is:
(i) By washing soda method – Washing soda reacts with soluble calcium and magnesium chlorides and sulphates in hard water to form insoluble carbonates.
MCl2 + Na2CO3 MCO3 + 2NaCl
MSO4 + Na2CO3 MCO3 + Na2SO4 (M = Mg, Ca)
(ii) Calgon’s method – Sodiumhexametaphosphate (Na6P6O18), commercially called calgon, when added to hard water, the following reaction takes place:
Na6P6O18 2Na+ + Na4P6O182-
M2+ + Na4P6O182- [Na2MP6O18]2- + 2Na+
The complex anion keeps the Mg2+ and Ca2+.
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