UPSC Prelims 2002 Q150: Science & Technology – Physics
Assertion (A): An iron ball floats on mercury but gets immersed in water.
Reason (R): The specific gravity of iron is more than that of mercury.
- (a) Both A and R are individually true and R is the correct explanation of A
- (b) Both A and R are individually true but R is not a correct explanation of A
- (c) A is true but R is false
- (d) A is false but R is true
Answer: (c) A is true but R is false
Explanation
Assertion-Reason: an iron ball in mercury and in water
| Statement | Verdict |
|---|---|
| Assertion: an iron ball floats on mercury but sinks in water | True — iron (7.9) is lighter than mercury (13.6) but heavier than water (1) |
| Reason: the specific gravity of iron is more than that of mercury | False — it is less than mercury's |
| Floating rule | A body floats when its density is less than that of the liquid |
| Result | A is true but R is false |
Iron has a specific gravity of about 7.9, mercury about 13.6 and water 1. Iron floats on mercury because it is less dense, and sinks in water because it is denser. The reason says the opposite for mercury, so it is false.
Answer: (c) A is true but R is false.
Why each option is right or wrong
- (a) Incorrect. R is false, since iron is less dense than mercury.
- (b) Incorrect. R is false, so both cannot be individually true.
- (c) CORRECT. A is true; R is false because iron is less dense than mercury.
- (d) Incorrect. A is true; iron floats on mercury.
Key point
Specific gravity: mercury 13.6, lead 11.3, iron 7.9, water 1; a body floats if its density is below that of the fluid.
Remember
Iron floats on mercury because its specific gravity is smaller.
Common mistakes
- Thinking heavier substances always sink.
- Mixing the mass of the ball with its density.
- Reading R as the cause of floating.
Exam tip
Compare 7.9 with 13.6 and with 1 before choosing.
Sources
- NCERT Class IX Science — Gravitation (buoyancy and Archimedes' principle)
- NCERT Class XI Physics — Mechanical Properties of Fluids