Latent heat is the energy absorbed or released during a change of state without a change in temperature. “Latent” means hidden: the energy changes the arrangement of particles rather than their kinetic energy.
Two types
- Latent heat of fusion: energy absorbed when a solid melts (or released when a liquid freezes).
- Latent heat of vaporisation: energy absorbed when a liquid boils or evaporates (or released when a gas condenses).
The latent heat of vaporisation is always larger than the latent heat of fusion for the same substance, because separating particles from a liquid to a gas requires overcoming stronger attractive forces than separating them from a solid to a liquid.
Why temperature stays constant
During melting or boiling, all the energy supplied goes into breaking the bonds between particles. The kinetic energy of the particles does not increase, so the temperature does not change.
Heating curve interpretation
On a temperature-time graph for a substance being heated at a constant rate:
- A flat section at the melting point shows the latent heat of fusion being absorbed.
- A flat section at the boiling point shows the latent heat of vaporisation being absorbed.
- The longer flat section (at the boiling point) reflects the larger latent heat of vaporisation.
Evaporation and cooling
When a liquid evaporates, the fastest (most energetic) particles escape from the surface. The remaining particles have a lower average kinetic energy, so the temperature of the liquid drops. This is why sweating cools the body.
Common errors and how to correct them
Confusing specific heat capacity and latent heat. Specific heat capacity relates to a temperature change within the same state. Latent heat relates to a change of state at constant temperature. They apply to different sections of a heating curve.
Saying “heat is absorbed and released at the same time during melting.” During melting, energy is absorbed to break bonds. During freezing, energy is released as bonds form. These are opposite processes.
How to apply this in an exam
When describing a heating curve, state explicitly: “the temperature remains constant because energy is being used to break the bonds between particles, not to increase kinetic energy.” Name the type of latent heat (fusion for melting, vaporisation for boiling).
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