UPSC Prelims 2003 Q22: Science & Technology – Physics
Consider the following statements:
1. Steam at 100 °C and boiling water at 100 °C contain the same amount of heat.
2. Latent heat of fusion of ice is equal to the latent heat of vaporization of water.
3. In an air - conditioner, heat is extracted from the room air at the evaporator coils and is rejected
out at the condenser coils.
Which of these statements is/are correct?
- (a) 1 and 2
- (b) 2 and 3
- (c) Only 2
- (d) Only 3
Answer: (d) Only 3
Explanation
Heat, latent heat and air conditioners
| Statement | Verdict |
|---|---|
| 1. Steam at 100 C and boiling water at 100 C contain the same heat | False: steam holds the extra latent heat of vaporisation |
| 2. Latent heat of fusion of ice equals latent heat of vaporisation of water | False: about 336 kJ/kg versus 2,260 kJ/kg |
| 3. An air conditioner extracts heat at the evaporator and rejects it at the condenser | True |
Only statement 3 is correct.
Answer: (d) Only 3.
Why each option is right or wrong
- (a) Incorrect. Statements 1 and 2 are false.
- (b) Incorrect. Statement 2 is false.
- (c) Incorrect. Statement 2 is false.
- (d) CORRECT. Only statement 3 is correct.
Key point
Latent heat: fusion 80 cal/g, vaporisation 540 cal/g.
Remember
Steam at 100 C holds more heat than water at 100 C.
Common mistakes
- Believing steam and water at 100 C have equal heat.
- Equating the two latent heats of water.
- Swapping the evaporator and condenser roles.
Exam tip
Steam burns are worse than boiling water because of latent heat.
Sources
- NCERT Class XI Physics — Thermal Properties of Matter (change of state and latent heat)
- NCERT Class IX Science — Matter in Our Surroundings (latent heat)