Question

Hardness of water can be detected with:
A.${\text{N}}{{\text{a}}_{\text{2}}}{\text{C}}{{\text{O}}_{\text{3}}}$ solution
B.Soap water
C.Dil. ${\text{HCl}}$
D.Acidified ${\text{KMn}}{{\text{O}}_{\text{4}}}$ solution

Answer 1

The water having high mineral content is known as hard water. Hard water cannot be used for cleaning purposes as it does not form foam.

Complete step by step answer:
The water having high concentrations of calcium and magnesium dissolved in it is known as hard water. Hardness of water is caused due to the percolation of water through ores like limestone, gypsum, chalk which are made up of calcium and magnesium carbonates, bicarbonates and sulphates.
Soaps are sodium and potassium salts of long chain fatty acids and help in cleaning action.
When we dissolve soap in hard water, the calcium and magnesium ions in hard water react with the salts of fatty acid and calcium and magnesium salts of fatty acids are formed.
The calcium and magnesium salts of fatty acids are not soluble in water and form a white layer on the surface of water. This is known as scum. The formation of foam is not possible due to the scum formed.
Thus, hardness of water can be detected with soap water.

Thus, the correct option is option (B).

Note:
The scum formed does not allow the formation of foam. Thus, hard water does not allow the cleaning action. When synthetic detergents are used instead of soap, the synthetic detergents dissolve in hard water and form foam. Thus, cleaning by hard water is possible when synthetic detergents are used. Synthetic detergents have ammonium salts of long chain fatty acids. Soaps form scum in hard water but detergents do not.