When a substance melts or boils, energy is supplied but the temperature does not change. The energy needed depends on the mass of the substance and its specific latent heat. This is written as an equation:
thermal energy for a change of state = mass × specific latent heat
In symbols, Q = m × L. The energy is in joules (J), the mass is in kilograms (kg) and the specific latent heat is in J/kg. Notice there is no temperature change in this equation, because the temperature stays constant while the state changes.
There are two values of L for a substance. The specific latent heat of fusion applies to melting and freezing. The specific latent heat of vaporisation applies to boiling and condensing. For ice, L for fusion is 334000 J/kg and ice melts at 0 °C. For water, L for vaporisation is 2260000 J/kg.
Worked examples: melting 0.5 kg of ice needs 0.5 × 334000 = 167000 J. Boiling away 0.2 kg of water needs 0.2 × 2260000 = 452000 J. Rearranged, L = Q ÷ m, so 668000 J melting 2 kg of ice gives 334000 J/kg.