Question

Permanent hardness of water cannot be removed by:
A. Washing soda
B. Permutit method
C. Ion exchange method
D. Boiling

Answer 1

The permanent hardness of water contains soluble salts of magnesium and calcium which are present in the form of chlorides and sulphides in water which cannot be easily removed.

Complete step by step answer:
-The hardness of water is due to the presence of soluble bicarbonates, chlorides and sulphates of calcium and magnesium. The hard water does not give leather with soap.
The hardness of water is usually two types:
(1) Temporary hardness
(2) Permanent hardness
-The temporary hardness of water is due to magnesium carbonate and calcium carbonate in water. The temporary hardness can be removed by boiling of water. When we boil the water the soluble magnesium hydrogen carbonate and calcium hydrogen carbonate forms magnesium hydroxide and calcium hydroxide which is insoluble in water so it can be removed by filtration.
-The permanent hardness of the water is due to the presence of soluble salts of magnesium and calcium which are present in the form of chloride and sulphides in water. The permanent hardness cannot be removed by boiling.
-The permanent hardness of the water can be removed by Gan’s Permutit method, Ion exchange resin method, and washing soda method.
-Thus, the permanent hardness of water cannot be removed by boiling water.
Therefore, the correct option is D.

Note:
The temporary hardness can also be removed by Clark’s method where calcium hydroxide is used as Clark’s reagent. The permanent hardness can also be removed by Calgon’s process where sodium-hexa-meta-phosphate is known as Calgon reagent.