The quantity of heat needed to change unit mass of a substance from liquid to vapour without changing the temperature, is called
- (a)specific heat
- (b)specific latent heat
- (c)thermal capacity
- (d)heat energy
Correct — B, specific latent heat. The heat needed to change unit mass of a substance from one state to another at constant temperature is its specific latent heat (here the specific latent heat of vaporisation, for liquid to vapour). The energy goes into breaking intermolecular bonds and doing work against pressure, not into raising the temperature — so the thermometer reading stays constant while the substance changes phase (Q = m x L).
- (a)specific heat — Specific heat is the heat needed to raise the temperature of unit mass by 1 degree (Q = m x c x delta-T). It applies while the temperature CHANGES, not during a constant-temperature phase change.
- (c)thermal capacity — Thermal (heat) capacity is the heat needed to raise the temperature of the WHOLE body by 1 degree (Q = m x c, unit J/K) — again a temperature-change quantity, and it is defined per body rather than per unit mass.
- (d)heat energy — 'Heat energy' is only a general term for thermal energy in transit; it does not specifically name the per-unit-mass, constant-temperature phase-change quantity being defined here.
During a change of state (melting or boiling) the temperature stays constant even though heat is being supplied. The heat absorbed per unit mass during this constant-temperature change is the specific latent heat L, so Q = m x L. For liquid to vapour it is the specific latent heat of vaporisation; for solid to liquid it is the specific latent heat of fusion. This is different from specific heat capacity c, which relates heat to a TEMPERATURE change (Q = m x c x delta-T).
The give-away phrase is 'without changing the temperature'. Any quantity tied to a temperature change — specific heat, thermal capacity — is ruled out, and 'heat energy' is too vague. The heat that drives a phase change at constant temperature, per unit mass, is by definition the specific latent heat.
- Specific latent heat L is the heat per unit mass to change state at constant temperature; Q = m x L (unit J/kg).
- Specific latent heat of vaporisation of water is about 2260 kJ/kg; of fusion of ice about 334 kJ/kg.
- Specific heat capacity c relates heat to a temperature change: Q = m x c x delta-T (unit J per kg per K).
- During melting or boiling the temperature stays constant while latent heat is absorbed.
- Confusing specific latent heat (constant-temperature phase change) with specific heat capacity (temperature change).
- Mixing up thermal capacity (per whole body, unit J/K) with the per-unit-mass quantities.
Thermal concepts are asked as 'heat to change state at constant temperature = ?' — the phrase 'without changing temperature' points to latent heat.
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
Same concept — latent heat. There, statement 1 is false because steam at 100 C carries the extra latent heat of vaporisation over boiling water, and statement 2 is false because the latent heat of fusion (about 334 kJ/kg) is not equal to that of vaporisation (about 2260 kJ/kg). Both this and the NDA item rest on knowing that a phase change absorbs latent heat at constant temperature.
- practice — not a real PYQ
During the boiling of water at 100 degrees C, the heat supplied is used mainly to
- (a)raise the temperature
- (b)break intermolecular bonds and change the state
- (c)increase the specific heat
- (d)decrease the pressure
Answer(b) break intermolecular bonds and change the state — this is the latent heat of vaporisation, absorbed at constant temperature.
- practice — not a real PYQ
The SI unit of specific latent heat is
- (a)J per kg per K
- (b)J/kg
- (c)J/K
- (d)J
Answer(b) J/kg — heat per unit mass (Q = m x L).