Answer: when there is heat of fusion Hfus we can't use q=mcΔt because there Is a change of state from solid to liquid with out a corresponding change in temperature (hidden heat).
Therefore whenever there is a change from one state to another there is no change in temperature
So instead of using q=mcΔt
We use q=m*Hfus
What is specific Latent heat of fusion?
Specific Latent heat of fusion of a substance is the quantity of heat required to change a kilogram of a substance from the solid state to The liquid state at melting point.
The S. I unit is in joules per kilogram (j/kg)