Question

The temporary hardness of water due to bicarbonate can be removed by adding:
(A) $CaC{{O}_{3}}$
(B) $Ca{{(OH)}_{2}}$
(C) $CaC{{l}_{2}}$
(D) $HCl$

Answer 1

Hint: Hardness of water is due to the presence of salts like carbonates, chlorides, sulphates, etc of calcium ($C{{a}^{2+}}$) and magnesium ($M{{g}^{2+}}$) ions. Hardness of water can be of two types, i.e. temporary hardness and permanent hardness. Temporary hardness of water is due to the presence of soluble carbonates of calcium and magnesium.

Complete answer:
Water which does not form foam with soap is hard water. The two different types of hardness of water are as follows:
> Temporary hardness: Temporary hardness of water is primarily due to the presence of bicarbonate salts of $C{{a}^{2+}}$ and $M{{g}^{2+}}$ ions, which are soluble in water, i.e. $Ca{{(HC{{O}_{3}})}_{2}}$ and $Mg{{(HC{{O}_{3}})}_{2}}$. This can be removed by heating or treating the hard water with slaked lime.
Permanent hardness: Permanent hardness of water is due to the presence of salts such as chlorides ($C{{l}^{-}}$) and sulphates ($SO_{4}^{2-}$) of $C{{a}^{2+}}$ and $M{{g}^{2+}}$ ions. It cannot be removed by boiling water.
Let us now discuss how temporary hardness of water can be removed.
Metal bicarbonates are called chemically metal hydrogen carbonates. When calcium hydroxide also known as slaked lime $Ca{{(OH)}_{2}}$ is added in temporary hard water, calcium carbonate $CaC{{O}_{3}}$ and magnesium carbonate $MgC{{O}_{3}}$ are formed which are insoluble in water and thus, precipitate out.
The hardness of water is removed and the water becomes soft. This process of softening of water by using slaked lime $Ca{{(OH)}_{2}}$ is popularly known as Clark’s process. The chemical reaction involved is given below:

\[Mg{{(HC{{O}_{3}})}_{2}}(aq)+Ca{{(OH)}_{2}}(aq)\to MgC{{O}_{3}}(s)+CaC{{O}_{3}}(s)+{{H}_{2}}O(l)\]

Similar reaction can be written for $Ca{{(HC{{O}_{3}})}_{2}}$.

\[Ca{{(HC{{O}_{3}})}_{2}}(aq)+Ca{{(OH)}_{2}}(aq)\to 2CaC{{O}_{3}}(s)+2{{H}_{2}}O(l)\]

Temporary hardness can also be removed by boiling the hard water. On boiling, $Ca{{(HC{{O}_{3}})}_{2}}$ and $Mg{{(HC{{O}_{3}})}_{2}}$ present in water give insoluble $CaC{{O}_{3}}$ and $MgC{{O}_{3}}$, respectively.

The correct answer to the question is option (B).

Note: Point to be noted here is that bicarbonates of calcium and magnesium ions are soluble in water whereas their carbonates are insoluble. Their bicarbonate salts can be converted to carbonates.