Question

1. Why does evaporation cause water vapor to be enriched in light isotopes (H₂¹⁶O) in comparison to liquid water?

   	a.	H216O is condensed more readily
   	b.	H216O is lighter than H218O
   	c.	H216O bonds to minerals in water less readily
   	d.	H216O undergoes radioactive decay

Answer 1

1) Why does evaporation cause water vapor to be enriched in light isotopes (H216O) in comparison to liquid water?

• H216O is lighter than H218O

18O is two neutrons heavier than 16O and causes the water molecule in which it occurs to be heavier by that amount. 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 surface, the first water vapor formed during evaporation of liquid water is enriched in H216O, and the residual liquid is enriched in H218O. When water vapor condenses into liquid, H218O preferentially enters the liquid, while H216O is concentrated in the remaining vapor. As an air mass moves from a warm region to a cold region, water vapor condenses and is removed as precipitation. The precipitation removes H218O, leaving progressively more H216O-rich water vapor. This distillation process causes precipitation to have lower 18O/16O as the temperature decreases. Additional factors can affect the efficiency of the distillation, such as the direct precipitation of ice crystals, rather than liquid water, at low temperatures.