The energy needed to change the state of a substance, with no change in temperature, depends on its mass and its specific latent heat. The equation is:
thermal energy for a change in state (J) = mass (kg) × specific latent heat (J/kg)
Use the specific latent heat of fusion for melting or freezing, and the specific latent heat of vaporisation for boiling or condensing. The mass must be in kilograms. During the change the temperature stays constant, because the energy changes the potential energy of the particles.
Example: water has a specific latent heat of fusion of 334000 J/kg. To melt 0.5 kg of ice at 0 °C, energy = 0.5 × 334000 = 167000 J. Water has a specific latent heat of vaporisation of 2260000 J/kg. To boil 0.1 kg of water at 100 °C, energy = 0.1 × 2260000 = 226000 J. The latent heat of vaporisation is much larger because the particles must be separated completely.