1. the energy required to fuse 1 mole of two molecules together
2. the energy required to convert, at its melting point, 1 mole of a substance in the liquid state into the solid state
3. the energy required to convert, at its melting point, 1 mole of a substance in the solid state into the liquid state

18. The heat of fusion is:

3. the energy required to convert, at its melting point, 1 mole of a substance in the solid state into the liquid state