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Q. Permanent hardness of water can be removed by adding

AFMCAFMC 2010Hydrogen

Solution:

Washing soda, $\left( Na _{2} CO _{3} \cdot 10 H _{2} O \right)$ is used to remove permanent hardness of water. It converts $Ca ^{2+}$ and $Mg ^{2+}$ salts (soluble) to carbonates (insoluble).
$ Ca \left( HCO _{3}\right)_{2}+ Na _{2} CO _{3} \rightarrow CaCO _{3} \downarrow+2 NaHCO _{3}$
$MgCl _{2}+ Na _{2} CO _{3} \rightarrow MgCO _{3} \downarrow+2 NaCl$