Question

Methods of removing temporary hardness of water

Answer 1

Temporary hardness of water is due to the presence of bicarbonate of calcium and magnesium. it can be removed by following methods :
1 . By boiling -
The temporary hardness of water can be removed by boiling the hard water in large boilers. During boiling, the bicarbonate of calcium and magnesium decompose into insoluble carbonates and give CO2 . The insoluble carbonates can be removed by filtration.
Ca(HCO3)2 -> CaCO3 + CO2 + H2O
Mg(HCO3)2 -> MgCO3 + CO2 + H2O


2. Calcium hydroxide method (Clark's method) -
This process is used on a commercial scale. In this process, calculated amount of lime(calcium hydroxide) is added to temporary hard water. The soluble bicarbonate present in water are converted into insoluble carbonates which settle at the bottom of the tank and are removed by filtration.
•Ca(HCO3)2 + Ca(OH)2 -> 2CaCO3 + 2H2O
(soluble) (lime) (insoluble)

•Mg(HCO3)2 + Ca(OH) 2 - > MgCO3 + CaCO3 + (soluble) ( lime) ( insoluble)
2H2O

In this method, excess of lime should not be added, otherwise water will again become hard due to absorption of CO2 from the atmosphere by reacting with unused excess calcium hydroxide. Calcium bicarbonate formed will make water hard.
Ca(OH)2 + 2CO2 -> Ca(HCO3)2