CAPF AC 2020 Q11: Science & Technology – Chemistry

CAPF AC 2020, Paper I (General Ability and Intelligence) · Science & Technology

Which one of the following methods can be used to remove permanent hardness of water?

Answer: (b) Ion-exchange

Explanation

Removing hardness of water

How to think: note the word permanent.

Boiling and lime work only on temporary hardness.

Cross them out first.

Hard water has dissolved salts of calcium and magnesium.

It does not lather easily with soap.

Type of hardness	Caused by	Removed by
Temporary	Bicarbonates of calcium and magnesium	Boiling; Clark's method (lime)
Permanent	Chlorides and sulphates of calcium and magnesium	Washing soda; Calgon; ion-exchange; synthetic resins

The options checked:

Option	Verdict
(a) Boiling	Incorrect; removes only temporary hardness
(b) Ion-exchange	Correct; sodium ions replace calcium and magnesium ions
(c) Lime treatment	Incorrect; Clark's method removes temporary hardness
(d) Bubbling sulphur dioxide gas	Incorrect; it does not remove calcium or magnesium ions

In the ion-exchange (zeolite or permutit) method, hard water passes through sodium aluminium silicate.

The zeolite takes up calcium and magnesium ions and gives out sodium ions.

Answer: (b) Ion-exchange.

(a) Incorrect. Boiling breaks down bicarbonates, so it removes only temporary hardness.
(b) CORRECT. Ion-exchange swaps calcium and magnesium ions for sodium ions and removes permanent hardness.
(c) Incorrect. Lime treatment (Clark's method) removes temporary hardness only.
(d) Incorrect. Sulphur dioxide gas does not take calcium or magnesium ions out of water.

Temporary hardness (bicarbonates): boiling, Clark's method. Permanent hardness (chlorides, sulphates): washing soda, Calgon, ion-exchange.

Permanent hardness: ion-exchange, washing soda, Calgon; not boiling.

Thinking boiling removes every kind of hardness.
Mixing up the salts: bicarbonates are temporary, chlorides and sulphates are permanent.
Placing Clark's method under permanent hardness; it uses lime for temporary hardness.

Educated guess: boiling and lime are for temporary hardness, so they go. Sulphur dioxide has nothing to do with hardness. Ion-exchange is left.