Questions & Answers

Permanent hardness is due to the 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. The Clark’s method
C. Treatment with $N{{a}_{2}}C{{O}_{3}}$
D. All of these.

Answer Verified Verified
Hard water is water that has dissolved salts that reduce the cleaning capacity of soaps. Think of what methods could be applied to either precipitate or coagulate the salts present that make the water hard water.

Complete step by step solution:
We know that hard water will not form lather with soaps. The reason for the temporary hardness of the water is the presence of magnesium and calcium carbonates in water. We can remove the temporary hardness by boiling the water. When boiling of water, the soluble salts of magnesium bicarbonates are converted to magnesium hydroxide which is insoluble in water. Therefore, it gets precipitated and thus we can remove it.
The permanent hardness of water is due to the presence of soluble salts of magnesium and calcium in the form of chlorides and sulphides in the water. We cannot remove this permanent hardness by simple boiling. We can treat this permanent hard water with washing soda ($N{{a}_{2}}C{{O}_{3}}$) to remove this hardness. When washing soda ($N{{a}_{2}}C{{O}_{3}}$) is reacts with the sulphide and chloride salts of magnesium and calcium, insoluble carbonates are formed. Thus, we can convert this hard water to soft water.
So, treatment with washing soda ($N{{a}_{2}}C{{O}_{3}}$) is a method to remove the permanent hardness of water. Therefore, the correct option is ‘C. treatment with $N{{a}_{2}}C{{O}_{3}}$’.

The option (b) is incorrect because Clark's method is a method to remove the temporary hardness of water not permanent hardness. Calcium hydroxide is the Clark’s reagent used in this method. It removes the temporary hardness by converting bicarbonates and carbonates. Boiling is also a method that is used to get rid of the temporary hardness of water.
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