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Specific Latent Heat
It takes a certain amount of energy to change the state of 1kg of water from solid to liquid. This amount of energy is called the Specific Latent Heat, lf, of water
"The amount of energy per kg (unit mass) required to change ice to water without change in temperature."
ΔQ is the heat energy added
m is the mass of the substance which changes state.
There is no temperature term involved in this equation as it all takes place at the same temperature.
Note: there are two occasions when you change state and both of these require different amounts of energy (as different things are happening to the atoms during the state changes). So there are two symbols.
lf - latent heat of fusion - solid to liquid and back.
lv - latent heat of vaporisation - liquid to gas and back.
You can actually have ls - latent heat of sublimation - solid to gas and back. But you don't often come across that.
So back to the graph.
a, c and e - Heat energy goes to change the Ek component. Specific heat capacity equations apply. (Note: specific heat capacities for ice, water and steam are different.)
b and d - Heat energy goes to change the Ep component. Specific latent heat equations apply.