Specific Latent Heat
Specific Latent Heat
A heater can transfer energy while the temperature stays constant. In that interval, the substance is changing state.
Energy changes the state, not the temperature
The property for a state change is .
Definition
2 marksWhat is meant by specific latent heat?
Model answer: The thermal energy needed per unit mass to change the state of a substance at constant temperature.
- A complete answer states three ideas: energy, per unit mass and no temperature change during the change of state (9702/41/M/J/25 Q3(a)).
- During melting or vaporisation, the supplied energy increases the separation of particles. Their average kinetic energy and the temperature do not increase. During freezing or condensation, energy is transferred away and particle separation decreases.
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| Name | Change of state | Symbol |
|---|---|---|
| Specific latent heat of fusion | solid → liquid (fusion); liquid → solid (freezing) | |
| Specific latent heat of vaporisation | liquid → gas (vaporisation); gas → liquid (condensation) |
is the change from solid to liquid. is the change from liquid to gas. The reverse changes are freezing and condensation.
Symbols
- = energy transferred during the state change (J)
- = mass that changes state (kg)
- = specific latent heat (J kg⁻¹)
Why vaporisation needs more energy
The specific latent heat of vaporisation is usually larger than the specific latent heat of fusion.
During vaporisation, particles become much farther apart than during fusion. Their potential energy therefore increases by a larger amount. The temperature and average kinetic energy do not increase during the state change (9702/41/M/J/25 Q3(b)).
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Use mass in units that match L
Worked example
Vaporise a small mass of water
Find the energy needed to vaporise 2.00 g of water at its boiling point. The specific latent heat of vaporisation is given below.
- Convert the mass to kilograms.
- Substitute the values.
Common mistake