Question

Permanent hardness is due to presence of soluble salts of Mg and Ca in the form of chlorides and sulphates in ${H_2}O$. It can be removed by (A) Boiling (B) Clark’s method(C) Treatment with $N{a_2}C{O_3}$(D) All of these

Hint: The hardness of water is due to the presence of soluble bicarbonates, chlorides and sulphates of calcium and magnesium water which does not give lather with soap in hard water.

Types of hardness of water : The hardness of water can be classified into two types
a) Temporary hardness
b) Permanent hardness
a) Temporary hardness – It is due to the presence of magnetism and calcium bicarbonates. There are two methods for removal of temporary hardness.
(i) By Boiling – Ca and Mg bicarbonates are converted into insoluble carbonates which are removed by filtration.
For example – When we boil water then we see that there is a deposition at the bottom of the vessel. The deposition is actually insoluble carbonates of Ca and Mg. Now filter the water of the cooling.
(ii) By Clark Method – Calcium hydroxide is used in this process. It removes the hardness of water by converting bicarbonates and carbonates.
For example – It is an old method. Calcium hydroxide (Lime water) is added to water and remains it undisturbed for some hours then carbonates of Ca and Mg settle down, which are removed by filtration.
b) Permanent hardness – It is due to the presence of sulphate and chloride salt of calcium and magnesium.
Permanent hardness can be removed by using $N{a_2}C{O_3}$. The salt of Ca and Mg convert into their carbonates, which are insoluble in water.

$CaC{l_2} + N{a_2}C{O_3} \to CaC{O_3} \downarrow + 2NaCl$

$MgC{l_2} + N{a_2}C{O_3} \to MgC{O_3} \downarrow + 2NaCl$

So, from the above discussion, we can say that permanent hardness can’t be removed by the boiling clark method. Only $N{a_2}C{O_3}$ can remove permanent hardness.

Here, the correct answer is (C).

Note: Sometimes students get confused and think that all methods are used to remove hardness of water. But different methods are used to remove both types of hardness. Temporary hardness can be removed by simple methods whereas permanent hardness cannot.