Heat change, ΔQ=mCΔT

Where,

ΔT=tempeature change

M=mass

C=heat capacity

Since the temperature is constant,

ΔQ=0,

The heat supplied might have been supplied to the body was used in the breaking the bond of the molecules, changing the state of the solid.

Thus, the temperature of the solid remains constant. In the period of heat may have been supplied to the body and heat may have been extracted from the body.