When a substance melts or boils, the energy supplied changes the energy stored (the internal energy) but not the temperature. The energy needed for a substance to change state is called latent heat. The particles gain potential energy as they move apart, but their average kinetic energy stays the same, so the temperature stays constant.
The specific latent heat of a substance is the amount of energy needed to change the state of one kilogram of the substance with no change in temperature. The specific latent heat of fusion is for a change from solid to liquid. The specific latent heat of vaporisation is for a change from liquid to vapour.
The equation is: energy for a change of state = mass × specific latent heat, or E = m L. Energy is in joules (J), mass in kilograms (kg) and specific latent heat in J/kg. For example, melting 0.20 kg of ice with a specific latent heat of fusion of 330000 J/kg needs 0.20 × 330000 = 66000 J.
Heating and cooling graphs show this clearly. On a graph of temperature against time, the sloping sections show the temperature changing. The flat sections show a change of state, because the temperature stays the same while the substance melts, boils, freezes or condenses.
Do not mix up the two ideas. Specific heat capacity is about a temperature change. Specific latent heat is about a change of state with no temperature change. In the practical to measure the latent heat of fusion of water, an electric heater melts ice in a funnel. You collect the water for a set time, compare with a funnel without a heater to allow for melting from the room, and use the mass of extra water melted in E = m L.
