The specific latent heat of fusion is the energy required to change 1 kg of a substance from solid to liquid at its melting point, without any change in temperature.
The formula
Q = m Lf
where Q is the energy in joules (J), m is the mass in kilograms (kg), and Lf is the specific latent heat of fusion in joules per kilogram (J/kg).
Why temperature stays constant
During melting, the energy supplied does not increase the kinetic energy of the particles (which would raise the temperature). Instead, it is used to break the bonds between particles in the solid structure. The particles gain potential energy as they move from fixed positions in a solid to the more disordered arrangement in a liquid.
The temperature remains at the melting point until all the solid has melted. Only then does the temperature start to rise again.
On a heating curve
A heating curve (temperature vs time, with constant heating) shows a flat section at the melting point. The length of this flat section depends on the mass and the specific latent heat of fusion. A large latent heat means the flat section lasts longer.
Example
The specific latent heat of fusion of ice is 334,000 J/kg.
To melt 0.5 kg of ice at 0 degrees Celsius: Q = 0.5 x 334,000 = 167,000 J = 167 kJ
This energy changes the ice to water at 0 degrees. Additional energy (calculated using specific heat capacity) is needed to raise the water temperature above 0 degrees.
Fusion vs vaporisation
The specific latent heat of vaporisation (boiling) is always larger than the specific latent heat of fusion (melting) for the same substance. Boiling requires more energy because the particles must be completely separated from each other, not just rearranged.
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