The specific latent heat of vaporization of a substance is the quantity of heat needed to change unit mass from
- (a)liquid to vapour with a change of temperature
- (b)liquid to vapour without a change of temperature
- (c)vapour to liquid without a change of temperature
- (d)vapour to liquid with a change of temperature
Correct — B, liquid to vapour without a change of temperature. Specific latent heat of vaporization is the heat needed to convert unit mass of a substance from the liquid state to the vapour state at its boiling point, and this happens without any change of temperature. During boiling the absorbed heat goes entirely into breaking the intermolecular forces and raising the molecules' potential energy, not into raising the temperature, so the thermometer stays fixed at the boiling point until all the liquid has vaporized.
- (a)liquid to vapour with a change of temperature — The direction (liquid → vapour) is right, but a phase change happens at constant temperature; latent heat by definition involves no change of temperature.
- (c)vapour to liquid without a change of temperature — This is condensation (vapour → liquid), in which latent heat is released rather than absorbed — the reverse of vaporization.
- (d)vapour to liquid with a change of temperature — Wrong on both counts — vaporization is liquid → vapour, and it occurs at constant temperature, not with a temperature change.
Latent heat is the heat absorbed or released when a substance changes state at constant temperature. The specific latent heat of vaporization is the heat per unit mass to turn a liquid into a vapour at its boiling point; the energy overcomes intermolecular attractions instead of raising the temperature.
This is a definition question. The distractors either flip the direction (condensation instead of vaporization) or add a temperature change. Remember the two anchors: vaporization is liquid → vapour, and 'latent' means the temperature stays constant.
- Latent heat is exchanged during a change of state at constant temperature.
- The specific latent heat of vaporization of water is about 2 260 kJ/kg.
- The heat goes into potential energy (breaking bonds), so the temperature does not rise.
- The reverse process, vapour → liquid, is condensation and releases the same latent heat.
- Assuming the temperature keeps rising while a liquid boils.
- Mixing up vaporization (liquid → vapour) with condensation (vapour → liquid).
A definition item — the trap options flip the direction or add a temperature change; recall that latent heat means a change of state at constant temperature.
UPSC_2003_GS1_Q222003Consider 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
Same concept — latent heat of vaporization. That UPSC item hinges on steam at 100 °C carrying extra latent heat over boiling water at 100 °C; this one defines that latent heat as the constant-temperature liquid-to-vapour change.
- practice — not a real PYQ
During the boiling of water at 100 °C, the heat supplied is used mainly to
- (a)raise the temperature further
- (b)break intermolecular bonds and change the state
- (c)increase the kinetic energy only
- (d)lower the pressure
Answer(b) break intermolecular bonds and change the state — latent heat causes no temperature rise.
- practice — not a real PYQ
The heat needed to change unit mass of a solid to liquid at its melting point without a change of temperature is the
- (a)latent heat of vaporization
- (b)specific heat capacity
- (c)latent heat of fusion
- (d)calorific value
Answer(c) latent heat of fusion.