 The addition of more energy is required to vaporize H218O than H216O, and H218O liberates more energy when it condenses.In addition, H216O tends to diffuse more rapidly.Because H216O requires less energy to vaporize, and is more likely to diffuse to the liquid phase, the first water vapor formed during evaporation of liquid water is enriched in H216O, and the residual liquid is enriched in H218O.