Question

Water softening by Clark’s process uses:
A. $\text{CaHC}{{\text{O}}_{3}}$
B. $\text{NaHC}{{\text{O}}_{3}}$
C. $\text{N}{{\text{a}}_{2}}\text{C}{{\text{O}}_{3}}$
D. $\text{Ca}{{\left( \text{OH} \right)}_{2}}$

Answer 1

Hint: Clark's process is also known as Lime softening used for water softening which uses the addition of lime water to remove hardness by precipitation. The temporary hardness of water is caused by bicarbonates of calcium $\text{Ca}{{\left( \text{HC}{{\text{O}}_{3}} \right)}_{2}}$ and magnesium $\text{Mg}{{\left( \text{HC}{{\text{O}}_{3}} \right)}_{2}}$.

Complete answer:
Let us discuss the reagent in Clark’s process:
Clark’s process is used for the large-scale removal of temporary hardness of water. Lime softening can also be used to remove iron, manganese and arsenic from water.
This process involves addition of a limited amount of calcium hydroxide $\left[ \text{Ca}{{\left( \text{OH} \right)}_{2}} \right]$.
Let us now discuss the Clark’s process and its functioning:
Limewater is added to water as a result, the pH is raised and the equilibrium of carbonate ions in the water is shifted.
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, the solubility product of salt is 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 $\text{Mg}{{\left( \text{HC}{{\text{O}}_{3}} \right)}_{2}}\left( \text{aq}\text{.} \right)+\text{Ca}{{\left( \text{OH} \right)}_{2}}\left( \text{aq}\text{.} \right)\to \text{CaC}{{\text{O}}_{3}}\left( \text{s} \right)+\text{MgC}{{\text{O}}_{3}}\left( \text{s} \right)+2{{\text{H}}_{2}}\text{O}\left( \text{l} \right)$.
This process produces large volumes of calcium carbonate in a finely-divided white precipitate which also contain organic matter flocculated out of water.

Therefore, water softening by Clark’s process uses $\left[ \text{Ca}{{\left( \text{OH} \right)}_{2}} \right]$ which is option ‘d’.

Additional Information:
The process is effective at removing microorganisms and dissolved organic matter by flocculation. Lime softening can be applied to the concentrate of membrane processes.

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, ion exchange softening is used where sodium is exchanged for calcium and magnesium ions.
The reaction is $\text{Ca}{{\left( \text{HC}{{\text{O}}_{3}} \right)}_{2}}+\text{N}{{\text{a}}_{2}}\text{X}\to \text{CaX}+\text{2NaHC}{{\text{O}}_{3}}$ and $\text{Mg}{{\left( \text{HC}{{\text{O}}_{3}} \right)}_{2}}+\text{N}{{\text{a}}_{2}}\text{X}\to \text{MgX}+\text{2NaHC}{{\text{O}}_{3}}$.