9.6.6 Temporary Hardness

Temporary hardness is due to the presence of magnesium and calcium hydrogen carbonates. It can be removed by:

Boiling: During boiling, the soluble Mg(HCO3)2 is converted into insoluble Mg(OH)2 and Ca(HCO3)2 is changed to insoluble CaCO3.

It is because of high solubility product of Mg(OH)2 as compared to that of MgCO3, that Mg(OH)2 is precipitated. These precipitates can be removed by filtration. Filtrate thus obtained will be soft water.

 

heating

Mg(HCO3)2  -------------→  Mg(OH)2 ↓ + 2CO2

 

heating

Ca(HCO3)2  -------------→  CaCO3 ↓ + H2O + 2CO2

 

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 ↓ + 2CO2

 

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