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.

)benzene and toluene form nearly ideal solutions. consider an equimolar solution of benzene and toluene. at 20 °c the vapour pressures of pure benzene and toluene are 9.9 kpa and 2.9 kpa, respectively. the solution is boiled by reducing the external pressure below the vapour pressure. calculate (i) the pressure when boiling begins, (ii) the composition of each component in the vapour, and (iii) the vapour pressure when only a few drops of liquid remain. assume that the rate of vaporization is low enough for the temperature to remain constant at 20 °c.