CAPF AC 2021 Q27: Science & Technology – Chemistry
- (a) Na
- (b) Cu
- (c) Fe
- (d) Au
Answer: (d) Au
Explanation
The metal that reacts with nothing ordinary
How to think: use the reactivity series.
Metals at the top react violently.
Metals at the bottom hardly react at all.
Gold sits at the very bottom.
| Metal | With oxygen | With water | With dilute acids |
|---|---|---|---|
| Na (sodium) | Burns quickly | Violent, even when cold | Violent |
| Fe (iron) | Rusts, burns as filings | With steam | Gives hydrogen |
| Cu (copper) | Forms black copper oxide on heating | No | No |
| Au (gold) | No | No | No |
Why copper is not the answer: copper does not react with water or dilute acids, but it does react with oxygen on heating.
Extra fact: gold dissolves only in aqua regia, a 3 : 1 mixture of hydrochloric and nitric acids.
Answer: (d) Au.
Why each option is right or wrong
- (a) Incorrect. Sodium is one of the most reactive metals and reacts with all three.
- (b) Incorrect. Copper reacts with oxygen on heating to form black copper oxide.
- (c) Incorrect. Iron rusts in moist air and reacts with steam and dilute acids.
- (d) CORRECT. Gold is a noble metal and does not react with oxygen, water or acids.
Key point
Reactivity series (high to low): K, Na, Ca, Mg, Al, Zn, Fe, Pb, [H], Cu, Hg, Ag, Au. Gold and platinum are noble metals; gold dissolves only in aqua regia.
Remember
Gold is at the bottom of the reactivity series.
Common mistakes
- Marking copper because it does not react with water or dilute acids.
- Forgetting that copper turns black when heated in air (copper oxide).
- Not knowing the symbol Au stands for gold (from the Latin aurum).
Exam tip
Educated guess: think of jewellery. Gold ornaments stay shiny for centuries because gold reacts with nothing around us.
Sources
- NCERT Class X Science — Metals and Non-metals (chemical properties of metals)
- NCERT Class VIII Science — Materials: Metals and Non-Metals