Question

Water softening by Clark’s process uses:
A.Calcium bicarbonate
B.Sodium hydroxide
C.Potash alum
D.Calcium hydroxide

Answer 1

Hint:The simple definition of water hardness is the amount of dissolved calcium and magnesium within the water. Hard water is high in dissolved minerals, largely calcium and magnesium. There are two kinds of water hardness, temporary and permanent. Permanent hardness is because of calcium and magnesium nitrates, sulphates, and chlorides etc. This hardness of hardness can't be eliminated by boiling.

Complete step by step answer:
We will learn about Clark’s method for water softening. Clark’s method is used to remove temporary hardness from water.
Temporary hardness is a type of water hardness caused by the presence of dissolved bicarbonate minerals (calcium bicarbonate and magnesium bicarbonate).
Boiling promotes the formation of carbonate from the bicarbonate and precipitates calcium carbonate out of solution, leaving water that is softer upon cooling.
In this method some amount of calcium hydroxide is added to the hard water, as a result of that some insoluble carbonates are found, which are separated through the method of filtration.
$\begin{align}
  & (NaP{{O}_{3}})\,or\,N{{a}_{2}}(N{{a}_{4}}{{P}_{6}}{{O}_{18}})\,\to \,2N{{a}^{2+}}\,+\,N{{a}_{4}}{{P}_{6}}{{O}_{18}}^{2-} \\
 & C{{a}^{2+}}\,+\,N{{a}_{4}}{{P}_{6}}{{O}_{18}}^{-2}\,\to \,2N{{a}^{+}}\,+\,CaN{{a}_{2}}{{P}_{6}}{{O}_{18}}^{-2} \\
\end{align}$
The slaked lime and the calcium hydroxide is added with the hard water (Temporary). Then the insoluble calcium carbonate is precipitated out by filtration and there is no longer hardness produced.
In this way the temporary hardness of water is removed.
Slaked lime used in the above process is equivalent to Calcium hydroxide so the answer is calcium hydroxide.

So, the correct option is D.

Additional information:
 In Clagon's method, sodium-hexa-meta-phosphate $(NaPO_3)_6$ referred to as Calgon is used. The hardness in water is removed by the adsorption of $Ca++$ and $Mg++$ ions. Ion exchange Resin Method: in this method, the permanent hardness of water is removed by using resins.

Note:
While solving this question it should be noted that Clark’s method and Clagon’s method are different. The former is used for removing temporary hardness of water whereas the latter is used to remove permanent hardness of water.