Question

Which of the following pairs of ions makes the water hard?
A. $N{a^ + }$, $SO_4^{2 - }$
B. $C{a^{2 + }}$, $HCO_3^ - $
C. $C{a^{2 + }}$, $N{O_3}$
D. $NH_4^ -$ , $Cr$

Answer 1

Hint: Depending upon the behaviour of water towards soap solution w.r.t. leather formation, it may be classified as hard water and soft water. Hard water is those which does not produce lather with soap readily.

Complete step by step answer: It has been established that the hardness of water is due to the presence of bicarbonate, chloride, sulphates of calcium, and magnesium in it.
The hardness of water can be explained as:
As we know the soaps are the sodium or potassium salt of higher fatty acid such as stearic acid, palmitic acid, oleic acid, etc. When hard water is treated with soap solution, $C{a^{2 + }}$ and $M{g^{2 + }}$ ions present in hard water react with the anions of fatty acid present in soap to form scum or crude white precipitates. As a result, hard water doesn't produce lather readily with the soap solution. The chemical reaction of soap with $C{a^{2 + }}$ and $M{g^{2 + }}$is given as:
$2{C_{17}}{H_{35}}CO{O^ - }N{a^ + } + CaC{l_2} \to {({C_{17}}{H_{35}}COO)_2}Ca \downarrow + 2NaCl$
$2{C_{17}}{H_{35}}CO{O^ - }N{a^ + } + MgS{O_4} \to {({C_{17}}{H_{35}}COO)_2}Mg \downarrow + N{a_2}S{O_4}$
However, when all the $C{a^{2 + }}$ and $M{g^{2 + }}$ ions present in hard water have been precipitated by the addition of a sufficient amount of soap, the resulting water becomes soft and readily produces lather with soap. Thus we can say hard water is not suitable for washing purposes since a lot of soap is wasted in precipitating out $C{a^{2 + }}$ and $M{g^{2 + }}$ ions present in hard water.
So, the correct answer is “Option B”.

Note: The hardness of water is two types:
-Temporary hardness: this type of hardness is due to the presence of bicarbonates of magnesium and calcium ion. It can be removed easily by simply boiling and filtering the water. Temporary hardness is also known as carbonate hardness.
-Permanent hardness: this type of hardness is due to the presence of chloride and sulphates of calcium and magnesium. It can’t be removed simply by the boiling of water.it is also known as the non-carbonate hardness of the water.