Question

The temporary hardness of water is due to _________ salts of calcium and magnesium.
A. carbonates and hydrogen carbonates
B. bicarbonates only
C. chlorides and bicarbonates
D. chlorides and sulphates

Answer 1

Hint: Hard water contains minerals due to presence of salts. The temporary hard water contains insoluble salts of calcium $\left( \text{Ca} \right)$ and magnesium $\left( \text{Mg} \right)$. These salts give calcium and magnesium cations $\left( \text{C}{{\text{a}}^{+2}}\text{,M}{{\text{g}}^{+2}} \right)$, which makes the water hard.

Complete step by step answer:
Let us first discuss hard water first.
Hard water is a type of water in which mineral content is high. Hard water is formed when water percolates via the deposits of limestone, gypsum or chalk. It is made up of calcium and magnesium bicarbonates and sulphates.

There are two types of hardness in water, one is temporary hardness and other is permanent hardness.
(1) Temporary hardness: Temporary hardness is caused by the presence of dissolved bicarbonate salts like calcium bicarbonate and magnesium bicarbonate. These salts give calcium and magnesium cations $\left( \text{C}{{\text{a}}^{+2}}\text{,M}{{\text{g}}^{+2}} \right)$ and the bicarbonate and carbonate anions $\left( \text{CO}_{3}^{2-},\text{HCO}_{3}^{-} \right)$. The presence of these metal cations makes the water hard.
(2) Permanent hardness: It is usually caused by the presence of calcium chloride $\left( \text{CaC}{{\text{l}}_{2}} \right)$ and calcium sulphate $\left( \text{CaS}{{\text{O}}_{4}} \right)$ and magnesium chloride $\left( \text{MgC}{{\text{l}}_{2}} \right)$ and magnesium sulphate $\left( \text{MgS}{{\text{O}}_{4}} \right)$ in the water, which do not gets precipitated, as the temperature increases.
The temporary hardness of water is due to bicarbonates only salts of calcium and magnesium.
So, the correct answer is “Option B”.

Additional Information:
There are many ways by which temporary hardness can be removed-
(1) Boiling: After filtration, the insoluble carbonates are obtained by boiling. The reaction is
$\text{Mg}{{\left( \text{HC}{{\text{O}}_{3}} \right)}_{2}}\xrightarrow{\vartriangle }\text{MgC}{{\text{O}}_{3}}\downarrow +{{H }_{2}}\text{O}+\text{C}{{\text{O}}_{2}}$ and $\text{Ca}{{\left( \text{HC}{{\text{O}}_{3}} \right)}_{2}}\xrightarrow{\vartriangle }\text{CaC}{{\text{O}}_{3}}\downarrow +{{H }_{2}}\text{O}+\text{C}{{\text{O}}_{2}}$.
(2) Clark’s process: Dissolved carbon dioxide $\left( \text{C}{{\text{O}}_{2}} \right)$ is changed into bicarbonates $\left( \text{HCO}_{3}^{-} \right)$ and then carbonates $\left( \text{CO}_{3}^{2-} \right)$. As a result, solubility products of salt are increased which causes calcium carbonate to precipitate. Similarly, magnesium can be precipitated as magnesium hydroxide in a double displacement reaction.

The reactions involved are:
$\text{Ca}{{\left( \text{HC}{{\text{O}}_{3}} \right)}_{2}}\left( \text{aq}\text{.} \right)+\text{Ca}{{\left( \text{OH} \right)}_{2}}\left( \text{aq}\text{.} \right)\to 2\text{CaC}{{\text{O}}_{3}}\left( \text{s} \right)+2{{\text{H}}_{2}}\text{O}\left( \text{l} \right)$ and $\text{Mg}{{\left( \text{HC}{{\text{O}}_{3}} \right)}_{2}}\left( \text{aq}\text{.} \right)+\text{Ca}{{\left( \text{OH} \right)}_{2}}\left( \text{aq}\text{.} \right)\to \text{CaC}{{\text{O}}_{3}}\left( \text{s} \right)+\text{MgC}{{\text{O}}_{3}}\left( \text{s} \right)+2{{\text{H}}_{2}}\text{O}\left( \text{l} \right)$.

Note: This Clark’s process is used to remove temporary hardness only. In the process, both the calcium and magnesium in the water and the calcium added with the lime are precipitated. But while removing permanent hardness, washing soda is added to chlorides and sulphates which get precipitated as magnesium and calcium carbonates. The reactions are
$\text{CaC}{{\text{l}}_{2}}+\text{NaC}{{\text{O}}_{3}}\to \text{CaC}{{\text{O}}_{3}}+2\text{NaCl}$ and $\text{MgS}{{\text{O}}_{4}}+\text{N}{{\text{a}}_{2}}\text{C}{{\text{O}}_{3}}\to \text{MgC}{{\text{O}}_{3}}+\text{N}{{\text{a}}_{2}}\text{S}{{\text{O}}_{4}}$.