The amount of energy needed to melt a substance at its melting point is called the heat of fusion and for water the value is 6.01 kJ/mol. The amount of energy needed to vaporize a substance at its boiling point is called the heat of vaporization and for water the value is 40.7 kJ/mol. Use the relative distances in the states of matter and the concepts intermolecular forces to explain the difference.