Question

Which of the following methods is used for the removal of temporary hardness of water?
(A) Treatment with washing soda
(B) Calgon method
(C) Ion-exchange method
(D) Clark’s method

Answer 1

Hint: When we talk about hard water; it means the presence of minerals in excess in water. It is categorised into permanent, and temporary hardness. To remove hardness some methods are used. In the temporary hardness carbonates are present. So, now identify the method used to remove the carbonates providing temporary hardness.

Complete step by step answer:
> Now, we have to determine the method used for the removal of temporary hardness of water
> First, we will define the temporary hardness of water. As mentioned it is caused by the presence of carbonates in water, i.e. the carbonates of magnesium, and calcium.
So, let us discuss the given options one by one.
- The first we have treatment with the washing soda. It is used to remove the permanent hardness of water.
As we know, the washing soda is also known as sodium carbonate. In the permanent hardness it reacts with the chloride, and sulphides; and there is conversion of hard water into the soft water.
- The second we have is the Calgon method, it is also used to remove the permanent hardness. The Calgon is a kind of compound also known as sodium-hexa-meta-phosphate, and adsorbs the calcium, and magnesium ions.
- Now, the third option we have is the ion-exchange method. In this method, we can see there is an exchange of calcium, and magnesium ions. It is also used to remove the permanent hardness.
- Talking about the fourth option we have is Clark’s method. As we know, Clark’s reagent is calcium hydroxide. There will be formation of carbonates by the bicarbonates.
We can write the reaction of Clark’s method, i.e.
Ca(OH)$_2$ + Ca(HCO$_3$)$_2$ $\rightarrow$ 2CaCO$_3$ $\downarrow$ + 2H$_2$O
Here, we can see the conversion calcium bicarbonate to the calcium carbonate in the presence of calcium hydroxide.
So, we can say that Clark’s method is used for the removal of temporary hardness of water.
Hence, the correct option is (D).

Note: Don’t get confused while determining the method for the removal of temporary hardness of water. Just keep in mind the permanent hardness cannot be removed by the simple methods like boiling, or the simple mixing of compounds.