Question

What is the cause of temporary hardness? How it is removed by Clark’s process and why the reagent used in it must be calculated?

Answer 1

First we must know that the hardness is generally caused due to the presence of a high amount of minerals especially calcium and magnesium salts.

Complete step by step answer: Let’s start with discussing the hardness, hardness of the water can be defined as the water with high amounts of minerals in it especially the calcium and magnesium salts. When heated in a vessel these salts get deposited on the walls reducing the heating effect.
Temporary hardness is mainly caused by the presence of calcium hydrogen carbonate $\left( {Ca{{\left( {HC{O_3}} \right)}_2}} \right)$ and Magnesium hydrogen carbonate $\left( {Mg{{\left( {HC{O_3}} \right)}_2}} \right)$.
Now, in second part of the question we have been asked about the Clark’s process, In Clark’s Process, $Ca{\left( {OH} \right)_2}$ i.e. slaked lime is added in the hard water which precipitates $Ca{\left( {HC{O_3}} \right)_2}$ by forming insoluble salt. The reaction is given below
$Ca{(HC{O_3})_2}{\text{ }} + {\text{ }}Ca{(OH)_2}{\text{ }} \to{\text{ }}2CaC{O_3}{\text{ }} + {\text{ }}2{H_2}O$
$CaC{O_3}$ is insoluble in water and hence precipitates outs. Like this the temporary hardness is removed using Clark’s process.
Now, why do we have to calculate the amount of reagent added? Well because slake lime i.e. $Ca{\left( {OH} \right)_2}$ in itself is a source of $C{a^{2 + }}$ ions which means it can cause hardness of water. So, the reagent used should be calculated while adding and care must be taken.

Note: It is important to remove the hardness of the water as it might harm the boiler by forming scales of minerals inside the boiler which results in the reduced efficiency of the boiler. Also due to the hardness of water, scum formation happens in soap resulting in reduced cleaning effect of soaps.