The specific latent heat of a substance is the amount of energy required…

The idea

Specific latent heat is the energy needed per kilogram to change state, measured while the temperature stays put, so its unit is J/kg with no degrees in it. Two versions are named. Specific latent heat of fusion covers solid to liquid, and for water it is about 334 000 J/kg. Specific latent heat of vaporisation covers liquid to vapour, and for water it is about 2 260 000 J/kg. Vaporisation is far larger because melting only loosens the particles enough to let them slide past one another, while boiling has to separate them completely against all the remaining forces. The same energy per kilogram is released when the change runs the other way, during freezing or condensing.

What earns the marks

• The energy required to change the state of one kilogram of the substance.
• Stating that there is no change in temperature during the change of state.
• Naming fusion as the solid to liquid change and vaporisation as the liquid to vapour change.

Common mistakes and how to put them right

• Leaving out the one kilogram, so the definition could apply to any amount of substance.

• Omitting the condition that the temperature does not change.

• Swapping fusion and vaporisation, so quoting 334 000 J/kg for boiling water.

• Reading fusion as the joining of nuclei rather than as melting.

Exam tips

• Write the unit as J/kg; the absence of °C is the quickest reminder that no temperature change is involved.
• State which change of state you mean, because fusion and vaporisation have very different values for the same substance.
• If a question gives an unfamiliar substance, quote the definition in general terms rather than water's numbers.