CAPF AC 2021 Q28: Science & Technology – Chemistry
Which one of the following metals produces hydrogen with cold water?
- (a) Silver
- (b) Sodium
- (c) Copper
- (d) Iron
Answer: (b) Sodium
Explanation
Metal that gives hydrogen with cold water
How to think: only the most reactive metals react with cold water.
These are at the top of the reactivity series: potassium, sodium and calcium.
| Metal | Reaction with water |
|---|---|
| Sodium, potassium | Cold water, violent, hydrogen catches fire |
| Calcium | Cold water, less violent |
| Magnesium | Hot water |
| Aluminium, zinc, iron | Only with steam |
| Copper, silver, gold | No reaction at all |
The reaction: 2Na + 2H2O → 2NaOH + H2 + heat.
The products are sodium hydroxide and hydrogen gas.
This is why sodium is stored under kerosene.
Answer: (b) Sodium.
Why each option is right or wrong
- (a) Incorrect. Silver does not react with water at all.
- (b) CORRECT. Sodium reacts violently with cold water and releases hydrogen.
- (c) Incorrect. Copper does not react with water, hot or cold.
- (d) Incorrect. Iron reacts only with steam, not with cold water.
Key point
Cold water: K, Na, Ca. Hot water: Mg. Steam: Al, Zn, Fe. No reaction: Cu, Ag, Au. Sodium is kept in kerosene.
Remember
Sodium + cold water → NaOH + hydrogen.
Common mistakes
- Marking iron because it rusts; rusting is slow and needs air too.
- Forgetting that iron needs steam to release hydrogen from water.
- Thinking copper or silver vessels react with the water stored in them.
Exam tip
Educated guess: silver and copper are used for coins and vessels, and iron for pipes, so they cannot react quickly with cold water. That leaves sodium.
Sources
- NCERT Class X Science — Metals and Non-metals (reaction of metals with water)
- NCERT Class VIII Science — Materials: Metals and Non-Metals