CAPF AC 2021 Q16: Science & Technology – Chemistry
- (a) Copper oxide
- (b) Aluminium oxide
- (c) Potassium oxide
- (d) Calcium oxide
Answer: (d) Calcium oxide
Explanation
Metal oxides and sodium hydroxide
How to think: sodium hydroxide is a base.
A base reacts with acidic oxides and amphoteric oxides.
It does not react with basic oxides.
So first remove the amphoteric oxide.
| Kind of oxide | Reacts with acids | Reacts with bases such as NaOH | Examples |
|---|---|---|---|
| Basic | Yes | No | Na₂O, K₂O, CaO, MgO, CuO |
| Amphoteric | Yes | Yes | Al₂O₃, ZnO |
| Acidic | No | Yes | CO₂, SO₂ |
Aluminium oxide reacts: Al₂O₃ + 2NaOH → 2NaAlO₂ + H₂O.
The product is sodium aluminate.
So (b) is out for certain.
A loose question: the other three are all basic oxides in the NCERT classification.
Published answer keys differ: most give (d) calcium oxide, and at least one gives (a) copper oxide.
We follow (d).
| Oxide | Why it is or is not the best answer |
|---|---|
| Copper oxide | Dissolves in hot concentrated NaOH to form cuprates, so it can react |
| Potassium oxide | Reacts violently with the water of the solution to form KOH |
| Calcium oxide | Strongly basic; mixed with NaOH as soda lime without reacting with it |
Answer: (d) Calcium oxide.
Why each option is right or wrong
- (a) Incorrect. Not the keyed answer: copper oxide is basic, but it dissolves in hot concentrated NaOH; some keys accept it.
- (b) Incorrect. Aluminium oxide is amphoteric and reacts with NaOH to give sodium aluminate.
- (c) Incorrect. Not the keyed answer: potassium oxide is basic and does not react with NaOH itself, only with the water.
- (d) CORRECT. Calcium oxide is a strongly basic oxide, and a base does not react with a base.
Key point
Bases react with acidic and amphoteric oxides, not basic ones. Amphoteric oxides: Al₂O₃ and ZnO. Al₂O₃ + 2NaOH → 2NaAlO₂ (sodium aluminate) + H₂O.
Remember
Amphoteric oxides: Al₂O₃ and ZnO react with both acids and bases.
Common mistakes
- Thinking all metal oxides are basic; aluminium and zinc oxides are amphoteric.
- Marking aluminium oxide, the one option that definitely reacts with NaOH.
- Forgetting that soda lime is simply calcium oxide mixed with sodium hydroxide.
Exam tip
Educated guess: the only sure step is to remove aluminium oxide. Among three basic oxides, pick the most familiar strong base, quicklime, and remember soda lime (CaO with NaOH).
Sources
- NCERT Class X Science — Metals and Non-metals (amphoteric oxides)
- NCERT Class XI Chemistry — The s-Block Elements (oxides of alkali and alkaline earth metals)