Humid Air, Heat Capacity, Kinetic Theory
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.
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.
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Update: June 2012