Question

Hardness of water is mainly due to the dissolved salts of ______ and ______.
(A) Calcium and magnesium
(B) Calcium and sodium
(C) Sodium and magnesium
(D) Calcium and potassium

Answer 1

Hint: We know that water having high mineral content is known as hard water. Hard water generally contains carbonates and bicarbonates. Hard water cannot be used for cleaning purposes as it does not form foam.

Complete step by step solution:
We know that 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.
The water having high concentrations of carbonates, bicarbonates, sulphates and chlorides of calcium and magnesium dissolved in it is known as hard water. The hard water does not allow cleaning action.
There are two types of hardness: temporary hardness and permanent hardness.
The temporary hardness of water is due to the carbonates and bicarbonates of calcium and magnesium. The temporary hardness of water can be removed by boiling the water. This is because the carbonates and bicarbonates precipitate out as the temperature increases.
The permanent hardness of water is due to the sulphates and chlorides of calcium and magnesium. The permanent hardness of water cannot be removed by boiling. This is because the sulphates and chlorides do not precipitate out as the temperature increases.
Thus, hardness of water is mainly due to the dissolved salts of calcium and magnesium.\

Thus, the correct option is (A) calcium and magnesium.

Note: Hard water does not allow the cleaning action. When synthetic detergents are used, the synthetic detergents dissolve in hard water and form foam. Thus, cleaning by hard water is possible when synthetic detergents are used.