Question

# The temporary hardness of water is due to?A. $Ca(HCO_{ 3 })_{ 2 }$B. NaClC. ${ Na }_{ 2 }{ SO }_{ 4 }$D. ${ CaCl }_{ 2 }$

Hint: You can just break the given compounds into their anionic and cationic forms, then choose the one which is a conjugate base of weak acid. Now you can easily answer the question.

Hard water - Hard water has a high mineral content. It is formed when water percolates through the deposits of chalk and limestone which are made up of magnesium and calcium carbonates. It does not lather with soap, so it is not suitable for laundry purposes.
We should know that there are two types of water hardness: temporary and permanent hardness.
Temporary hardness - Temporary hardness is due to the presence of calcium hydrogen carbonate $Ca(HCO_{ 3 })_{ 2 }$(aq) and magnesium hydrogen carbonate $Mg(HCO_{ 3 })_{ 2 }$(aq).
Both calcium hydrogen carbonate and magnesium hydrogen carbonate decompose when heated. The original insoluble carbonate is reformed. This happens when water is boiled.

$Ca(HCO_{ 3 })_{ 2 }\quad \rightarrow \quad CO_{ 2 }+H_{ 2 }O+CaCO_{ 3 }\downarrow$
$Mg(HCO_{ 3 })_{ 2 }\quad \rightarrow \quad CO_{ 2 }+H_{ 2 }O+MgCO_{ 3 }\downarrow$

As you can see boiling the water causes the precipitation of solid calcium carbonate or solid magnesium carbonate. This removes the calcium ions or magnesium ions from the water, and so removes the hardness. Therefore, hardness due to hydrogen carbonates is said to be temporary.

Therefore, we can say that the correct answer is option A.

Note: Other three options given the question cause permanent hardness of water due to presence of ${ Cl }^{ - }$ , ${ Ca }^{ 2+ }$ , ${ SO }_{ 4 }^{ 2- }$ ions in their aqueous solution.