Question

Permanent hardness in water cannot be cured by:
A) Boiling
B) Ion exchange method
C) Treatment with washing soda
D) Calgon’s method

Answer 1

Hint: The answer here is based on the fact that the permanent hardness of water cannot be removed by simple physical methods and this fact leads you to the required answer.

Complete step by step answer:
We have studied the reasons for water getting hardened and also the methods used for the removal of hardness of water and making it soft water.
Now, let us see the elements responsible for the hardness of water.
- Hard water basically has high mineral content in it and this is formed because of the percolation of water through the deposits of limestone, gypsum or chalk which are highly made of magnesium and also calcium carbonates, bicarbonates and even sulphates.
- Thus, due to the presence of mainly calcium and magnesium salts in the form of sulphates, this causes hardness in water.
- This hardness are removed by several techniques where ion exchange method can also be employed which exchange the cations that is $C{{a}^{2+}}$ and $M{{g}^{2+}}$ by addition of carbonates where calcium deposits as calcium carbonate and magnesium a magnesium hydroxide.
- Washing soda when dissolved in hard water and the ions react with this to precipitate as insoluble salts and leaves water as soft.
- Calgon’s method involves treatment of calgon (sodium hexametaphosphate) to form anionic complex that keeps the $C{{a}^{2+}}$ and $M{{g}^{2+}}$ in solution.
- The physical methods like boiling, filtration do not remove hardness. Boling only evaporates the eater and does not remove hardness.

Thus, the correct answer is option A) Boiling.

Note: Note that to remove the salts that cause water to get hardness a method called permutit can also be used which works on the principle same as that of ion exchange where ions causing hardness percolates through the lumps in permutit and later exchanges with this sodium permutit into calcium and magnesium permutit.