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  4. Which one of the following is not a method to remove permanent hardness of water?

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Q. Which one of the following is not a method to remove permanent hardness of water?

TS EAMCET 2015

Solution:

Clark's method is not used to remove permanent hardness of water. This method is used to remove temporary hardness of water.

$Ca \left( HCO _{3}\right)_{2}+ Ca \left( OH _{2}) \longrightarrow 2 CaCO _{3} \downarrow+2 H _{2} O \right. $

$Mg \left( HCO _{3}\right)_{2}+2 Ca ( OH )_2\longrightarrow $

$2 CaCO _{3} \downarrow+ Mg \left( OH) _{2} \downarrow+2 H _{2} O \right.$

If excess of lime is added, water will again become hard because of adsorption of $CO _{2}$ from the atmosphere.

$Ca \left( OH _{2})+2 CO _{2} \longrightarrow Ca \left( HCO _{3}\right)_{2}\right.$