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

Answer Verified Verified
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.

Complete answer:
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.

Additional information: Let us know something about permanent hardness also.
Permanent Hardness - When the soluble salts of magnesium and calcium are present in the form of chlorides and sulfides in water, we call it permanent hardness because this hardness cannot be removed by boiling.
We can remove this hardness by treating the water with washing soda. Insoluble carbonates are formed when washing soda reacts with the sulfide and chloride salts of magnesium and calcium and thus hard water is converted to soft water.

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.
Soft water is surface water that contains low concentrations of ions and in particular is low in ions of Calcium and Magnesium. Soft water naturally occurs where rainfall and the drainage basin of rivers are formed.