Question

# Permanent hardness of water may be removed by the addition of:A. Sodium carbonateB. AlumC. Potassium permanganateD. Lime

Hint: Solve this question by keeping in mind the causes of permanent hardness of water and the reaction it needs to undergo to remove to hardness causing elements.

Sodium Carbonate - $N{{a}_{2}}C{{O}_{3}}$, is the disodium salt of carbonic acid and has an alkalinizing property. When dissolved in water, sodium carbonate forms carbonic acid and sodium hydroxide. The reaction is as follows –
$N{{a}_{2}}C{{O}_{3}}+{{H}_{2}}O\to N{{a}_{2}}O+{{H}_{2}}C{{O}_{3}}\to 2NaOH+{{H}_{2}}O+C{{O}_{2}}$
$C{{a}^{2+}}\left( aq \right)\text{ }+\text{ }C{{O}_{3}}^{2}\left( aq \right)\text{ }\to \text{ }CaC{{O}_{3}}\left( s \right)$