During fusion, the absorbed heat energy does not increase the substance’s overall temperature. Instead, this energy works to overcome the strong intermolecular attractions holding the solid together. The particles break out of their fixed positions in the rigid solid lattice.
The solid and liquid phases coexist at a single temperature until melting finishes. Only after the solid completely melts will additional heat raise the liquid temperature. The heat required to melt one mole of a substance is a specific value.
Chemists identify this specific energy requirement as the molar enthalpy of fusion. This important value is represented by the symbol ΔHfus in thermodynamic chemical equations. Every pure chemical substance possesses its own unique molar enthalpy of fusion value.
Many beginning chemistry students incorrectly confuse this physical phase change with nuclear fusion. Nuclear fusion involves atomic nuclei combining to form entirely new chemical elements. Chemical fusion simply involves changing the physical state from solid to liquid.
The chemical identity of the substance does not change during the melting process. Chemical fusion is the exact reverse physical process of freezing or liquid solidification. Both fusion and freezing occur at the exact same temperature for a pure substance. The only real difference is the physical direction of the overall heat energy flow.