Question

Temporary hardness of water is removed in Clark’s process by adding:
(a) caustic soda
(b) calgon
(c) borax
(d) lime

Answer 1

Hint: Hard water is very high in its content of the calcium and magnesium minerals such as calcium bicarbonate, magnesium bicarbonate, calcium chloride, magnesium chloride, calcium sulphate and magnesium sulphate.

Complete step-by-step solution:
The amount of dissolved calcium and magnesium in water leads to water hardness. When soap is used with hard water, it reacts with the calcium (which is relatively high in hard water) to form "soap scum". On heating hard water, such as in a home water heater, solid deposits of calcium carbonate are observed. Hard water is not a health risk rather hard drinking water generally contributes a small amount toward total calcium and magnesium human dietary needs but it poses critical problems in industrial settings.
“Hardness is caused by compounds of calcium and magnesium, and by a variety of other metals. General guidelines for classification of waters are: 0 to 60 mg/L (milligrams per litre) as calcium carbonate is classified as soft; 61 to 120 mg/L as moderately hard; 121 to 180 mg/L as hard; and more than 180 mg/L as very hard”.
Water reacts with carbon dioxide to form very weak carbonic acid which is an even better solvent than water. As this water moves through soil, rock, limestone, chalk and gypsum, it dissolves very small amounts of minerals and holds them in solution. Out of all the minerals calcium and magnesium are the most common minerals that make water ‘hard’.
$Ca{ CO }_{ 3 }(s)+C{ O }_{ 2 }(aq)+{ H }_{ 2 }O(l)\rightleftharpoons { Ca }^{ 2+ }(aq)+2HC{ O }_{ 3 }^{ - }(aq)$
There two types of hardness:
-Temporary Hardness: It is caused by dissolved bicarbonate minerals such as calcium bicarbonate and magnesium bicarbonate. The temporary hardness can be reduced by boiling the water or by using the lime softening method. On boiling the bicarbonate mineral decomposes to carbonate and precipitates out of the solution.
$Ca{ (HCO }_{ 3 }{ ) }_{ 2 }(aq)\xrightarrow { \triangle } CaC{ O }_{ 3 }(s)+C{ O }_{ 2 }(g)+{ H }_{ 2 }O(l)$
$Mg{ (HCO }_{ 3 }{ ) }_{ 2 }(aq)\xrightarrow { \triangle } MgC{ O }_{ 3 }(s)+C{ O }_{ 2 }(g)+{ H }_{ 2 }O(l)$
In lime softening method i.e. Clark’s process, lime water (aqueous solution of calcium hydroxide) is added to the water. This leads to the formation of insoluble calcium carbonate which precipitates out. The reactions are given below:
$ \begin{matrix} Ca(HC{ O }_{ 3 }{ ) }_{ 2 }(aq) \\ Calcium\quad bicarbonate \end{matrix}+\begin{matrix} Ca{ (OH) }_{ 2 }(aq) \\ Calcium\quad hydroxide \end{matrix}\rightarrow \begin{matrix} { 2CaCO }_{ 3 }(s) \\ Calcium\quad carbonate \end{matrix}+\begin{matrix} 2{ H }_{ 2 }O(l) \\ Water \end{matrix} $
$ \begin{matrix} Mg(HC{ O }_{ 3 }{ ) }_{ 2 }(aq) \\ Magnesium\quad bicarbonate \end{matrix}+\begin{matrix} Ca{ (OH) }_{ 2 }(aq) \\ Calcium\quad hydroxide \end{matrix}\rightarrow \begin{matrix} { CaCO }_{ 3 }(s) \\ Calcium\quad carbonate \end{matrix}+\begin{matrix} Mg{ (CO) }_{ 3 }(s) \\ Magnesium\quad carbonate \end{matrix}\begin{matrix} 2{ H }_{ 2 }O(l) \\ Water \end{matrix} $
-Permanent Hardness: It is caused by the sulphate and chloride compounds of magnesium and calcium which do not decompose on heating the water. In order to remove permanent hardness, a water softener or ion-exchange column has to be used.
Therefore the correct answer is (d) lime.

Note: The lime softening method uses the Le-Chatelier’s principle. When Calcium hydroxide (lime) solution is added to hard water, it reacts with the dissolved carbon dioxide or the carbonic acid due to which the equilibrium is shifted in the backward direction and more of calcium carbonates and magnesium carbonates are produced that precipitate out of the solution.