CAPF AC 2023 Q10: Science & Technology – Chemistry
- (a) Passing water through microfiltration membrane
- (b) Calgon's method
- (c) Passing water through a cation exchange and an anion exchange resin bed
- (d) By boiling
Answer: (c) Passing water through a cation exchange and an anion exchange resin bed
Explanation
Making demineralised water
How to think: dissolved salts are present as ions: positive cations and negative anions.
To get water free of all salts, both kinds must be removed.
That needs two resins, one after the other.
| Step | Resin | What it does |
|---|---|---|
| 1 | Cation exchange resin (H⁺ form) | Takes up Na⁺, Ca²⁺, Mg²⁺ and gives H⁺ |
| 2 | Anion exchange resin (OH⁻ form) | Takes up Cl⁻, SO₄²⁻, HCO₃⁻ and gives OH⁻ |
| Result | H⁺ + OH⁻ → H₂O | Demineralised (deionised) water |
Why the others fail:
| Method | What it does |
|---|---|
| Microfiltration | Removes suspended particles and bacteria, not dissolved ions |
| Calgon's method | Ties up Ca²⁺ and Mg²⁺ but adds sodium ions; water is only softened |
| Boiling | Removes only temporary hardness |
Answer: (c).
Why each option is right or wrong
- (a) Incorrect. Microfiltration cannot stop dissolved salts; ions pass through.
- (b) Incorrect. Calgon softens water but leaves sodium salts dissolved in it.
- (c) CORRECT. Cation and anion exchange resins together remove all mineral salts.
- (d) Incorrect. Boiling removes only temporary hardness due to bicarbonates.
Key point
Demineralised water: cation exchange (H⁺ form) then anion exchange (OH⁻ form) resins. Temporary hardness (bicarbonates): boiling, Clark's method. Permanent hardness (chlorides, sulphates): washing soda, Calgon (sodium hexametaphosphate), ion exchange.
Remember
Cation resin + anion resin = demineralised water.
Common mistakes
- Mixing up softening (removing calcium and magnesium) with demineralising (removing all ions).
- Thinking a fine filter can hold back dissolved salts.
- Believing boiling makes water free of all minerals.
Exam tip
Educated guess: the question says free from all soluble mineral salts. Only the option that deals with both cations and anions can achieve that.
Sources
- NCERT Class XI Chemistry — Hydrogen (hard and soft water; synthetic resins method)
- J. D. Lee, Concise Inorganic Chemistry — Water and its treatment