Question

The reaction involved in the removal of temporary hardness of water is:

• A. $\text{Ca}({\text{HCO}}_3$)${}_2$ $\stackrel{\text{Heat}}{\to }$ ${\text{CaCO}}_3+{\text{H}}_2\text{O}+{\text{CO}}_2\uparrow$
• B. ${\text{CaCl}}_2+{\text{Na}}_2{\text{CO}}_3$ $\to$ ${\text{CaCO}}_3(\downarrow )+2\text{NaCl}$
• C. ${\text{MgSO}}_4+$ ${\text{Na}}_2{\text{Al}}_2{\text{Si}}_2{\text{O}}_8.{\text{xH}}_2\text{O}$ $\to$ ${\text{MgAl}}_2{\text{Si}}_2{\text{O}}_8.{\text{xH}}_2\text{O}+{\text{Na}}_2{\text{SO}}_4$
• D. ${\text{2CaCl}}_2+({\text{NaPO}}_3$)${}_6$ $\to$ ${\text{Na}}_2[{\text{Ca}}_2{\text{PO}}_3$)${}_6$] + $\text{4NaCl}$

Answer 1

The correct option is A

$\text{Ca}({\text{HCO}}_3$)${}_2$ $\stackrel{\text{Heat}}{\to }$ ${\text{CaCO}}_3+{\text{H}}_2\text{O}+{\text{CO}}_2\uparrow$

Temporary hardness of water is caused due to the dissolved bicarbonate of calcium and magnesium. It can be removed by the process of boiling. When the hard water containing calcium bicarbonate is boiled, then this bicarbonate salt decomposes into insoluble calcium carbonate alongwith evolution of carbon dioxide gas and water. The reaction involved is:
$\text{Ca}({\text{HCO}}_3$)${}_2$ $\stackrel{\text{Heat}}{\to }$ ${\text{CaCO}}_3+{\text{H}}_2\text{O}+{\text{CO}}_2\uparrow$

This insoluble carbonate is later filtered out.