4.3.2.3a - Specific Latent Heat and State-Change Energy
When a substance changes state, energy is transferred even though the temperature can stay the same. That energy is called latent heat because it changes the internal energy of the substance without showing up as a temperature rise. In this lesson you will learn what specific latent heat means, how fusion and vaporisation differ, and how to use E = mL in GCSE calculations.
Energy without a temperature change
Heating a substance usually increases its temperature. During a change of state, the pattern is different: the energy transferred changes the internal energy of the substance, but the temperature does not change while the state change is happening.
For example, ice at its melting point can be supplied with energy and remain at the melting point until it has melted. The energy is not "lost"; it is being used to change the arrangement of the particles so the substance changes state.
Latent Heat
Latent heat is the energy needed for a substance to change state.
The key exam idea is the word state. Latent heat is about melting, freezing, boiling, evaporating or condensing. It is not the same as simply warming a substance up.
One common mistake is to write that temperature always increases when energy is supplied. That is only true when the substance is not in the middle of a change of state.
Specific latent heat
Different substances need different amounts of energy to change state. The mass also matters: changing the state of 2 kg needs twice as much energy as changing the state of 1 kg, if the substance and state change are the same.
Specific Latent Heat
The specific latent heat of a substance is the amount of energy required to change the state of 1 kg of the substance with no change in temperature.
The unit for specific latent heat is J/kg. This means joules of energy per kilogram of substance.
Notice the word specific. Latent heat can mean the total energy transferred in one change of state. Specific latent heat means the energy per kilogram, so it lets you scale the calculation for any mass.
Fusion and vaporisation
AQA uses two named types of specific latent heat in this section.
Specific latent heat of fusion is for the change of state from solid to liquid. Melting is the forward change, and freezing is the reverse change.
Specific latent heat of vaporisation is for the change of state from liquid to vapour. Boiling and evaporation produce vapour; condensation is the reverse change.
The same idea applies in reverse. If a substance freezes or condenses, energy is transferred away from the substance. The size of the energy change can still be found using E = mL, but the direction of energy transfer is opposite.
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