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.