The student states that every phase transition must involve a change in energy of the molecules of the substance. The student is correct. In any phase change, there is a change in energy, for instance, phase transition from solid to liquid. The energy of the molecules in the solid is less than the energy in the liquid, this is because the molecules of the solid are compact while in the liquid, it moves freely. The energy increases as the molecules' speed increases. Similarly, the phase transition from liquid to gas experiences the same phenomenon.