CAPF AC 2022 Q28: Science & Technology – Chemistry
- (a) Aluminium oxide
- (b) Calcium oxide
- (c) Sodium oxide
- (d) Potassium oxide
Answer: (a) Aluminium oxide
Explanation
Amphoteric oxides
How to think: most metal oxides are basic; they react only with acids.
A few metal oxides react with both acids and bases.
They are called amphoteric oxides.
NCERT names two: aluminium oxide and zinc oxide.
| Oxide | Nature | Reacts with |
|---|---|---|
| Aluminium oxide (Al2O3) | Amphoteric | Acids and bases |
| Calcium oxide (CaO) | Basic | Acids only |
| Sodium oxide (Na2O) | Basic | Acids only |
| Potassium oxide (K2O) | Basic | Acids only |
The two reactions:
Al2O3 + 6HCl → 2AlCl3 + 3H2O.
Al2O3 + 2NaOH → 2NaAlO2 + H2O.
The second product is sodium aluminate.
Answer: (a) Aluminium oxide.
Why each option is right or wrong
- (a) CORRECT. Aluminium oxide is amphoteric and reacts with both acids and bases.
- (b) Incorrect. Calcium oxide (quicklime) is a basic oxide.
- (c) Incorrect. Sodium oxide is strongly basic and reacts only with acids.
- (d) Incorrect. Potassium oxide is strongly basic and reacts only with acids.
Key point
Amphoteric oxides: Al2O3 and ZnO (also PbO, SnO). Metal oxides are generally basic, non-metal oxides acidic. Sodium and potassium oxides dissolve in water to give alkalis.
Remember
Amphoteric oxides: Al2O3 and ZnO.
Common mistakes
- Thinking all metal oxides are basic without exception.
- Marking calcium oxide because it is widely used as quicklime.
- Forgetting the product sodium aluminate when aluminium oxide meets a base.
Exam tip
Educated guess: sodium, potassium and calcium are very reactive metals whose oxides are plainly basic. Aluminium is the odd metal out, and amphoteric starts with A like aluminium.
Sources
- NCERT Class X Science — Metals and Non-metals (amphoteric oxides)
- NCERT Class XI Chemistry — The p-Block Elements (Group 13)