Reactions

Letters to the editor

I enjoyed very much the article on water-from-air technologies (C&EN, Oct. 15, page 26), but perhaps it might have discussed the inherent energy differences between liquid water from fog and liquid water from water vapor. Of course liquid from vapor involves the heat of vaporization (condensation), which warms the condensing medium and must be removed and, hopefully, put to use elsewhere in the system. Liquid water from fog is different. Here, condensation has already occurred, and the inherent energy difference is the change in surface area times the surface tension, far less than the heat of vaporization (condensation).

For those with time on their hands, it is fun to calculate, on the basis of change in surface area times surface tension, the energy required to reduce a liter of water to molecular-sized water bits. This is somewhat absurd, as there is no real surface tension or surface area of a small handful of water molecules, but one still ends up with something surprisingly close to the heat of vaporization (which is as it should be).

Jim Birkett
Nobleboro, Maine