CAPF AC 2023 Q14: Science & Technology – Chemistry
- (a) Ti
- (b) Fe
- (c) Pt
- (d) Zn
Answer: (c) Pt
Explanation
Noble metals and oxygen
How to think: metals at the bottom of the reactivity series resist attack by air.
Platinum is a noble metal like gold.
It does not combine with oxygen directly, even when heated strongly.
| Metal | With oxygen |
|---|---|
| Ti (titanium) | Forms titanium dioxide on heating; a thin oxide film protects it in air |
| Fe (iron) | Rusts in moist air; burns as filings to form Fe₃O₄ |
| Zn (zinc) | Burns to form white zinc oxide |
| Pt (platinum) | Does not react directly |
Uses that follow from this: platinum is used for jewellery, laboratory crucibles and electrodes.
It is also the catalyst in catalytic converters of vehicles.
Like gold: platinum dissolves only in aqua regia.
Answer: (c) Pt.
Why each option is right or wrong
- (a) Incorrect. Titanium reacts with oxygen on heating to give titanium dioxide.
- (b) Incorrect. Iron combines with oxygen readily, as rusting and burning show.
- (c) CORRECT. Platinum is a noble metal and does not react with oxygen directly.
- (d) Incorrect. Zinc burns in air to form zinc oxide.
Key point
Noble metals: gold, platinum (also silver to a large extent) do not react with oxygen even at high temperature. Reactivity series bottom: Cu, Hg, Ag, Au, Pt. Both gold and platinum dissolve in aqua regia.
Remember
Gold and platinum do not react with oxygen.
Common mistakes
- Marking titanium because it does not corrode; that is due to its oxide film.
- Not knowing the symbol Pt stands for platinum.
- Thinking zinc is unreactive because it is used to galvanise iron.
Exam tip
Educated guess: think of which metal is used for jewellery and stays bright for ever. Among the four, only platinum is a precious metal.
Sources
- NCERT Class X Science — Metals and Non-metals (what happens when metals are burnt in air)
- NCERT Class XII Chemistry — The d- and f-Block Elements