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Question:
I am puzzled. More humid air is lighter because of the hydrogen molecules. Water absorbs more heat, but why would humid air absorb more heat at the same temperature if it has less mass? This does not seem consistent with the kinetic theory of gases.


Replies:
Your puzzlement is understandable, because the answer has a lot of qualifications. First, humid air is less dense (mass per unit volume) than an equal volume of dry air because you are exchanging heavier oxygen and nitrogen with lighter mass water (not hydrogen) to make up the same total volume. To a good approximation at atmospheric conditions, the property of air is the sum of its components. But in the case of a gas, e.g. air, you have the possibility of measuring the heat capacity of air at constant pressure or at constant volume. For ideal gases this difference is a constant consistent with the kinetic theory of gases. The amount of water vapor present in air is limited by the vapor pressure of liquid water. At 25 C. the vapor pressure of water is about 25 mm of Hg only about 0.03 atm, so the effect of water is limited to about 3% of the total pressure. While the contribution to the heat capacity of the total amount of air is different, for all but the most precise measurements the air does not "know" whether the molecules are nitrogen, oxygen, or water vapor.

Vince Calder


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