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I wonder if wet and dry air have the same density, I think they must have the same because both are "air" however, if we set both with the same pressure and temperature to try to standardize the other variables, is it true? that? I think so, but even if I try to set more variables, I still think that the densities must be the same. I would appreciate your opinion too much.

OscarR
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  • "because both are air"... what do you mean by that? Even if we considered humid air as an ideal gas, the density of ideal gases differ depending on the particle mass. Hydrogen and helium are less dense than air, although they both have a molar volume of 22.4 liters at standard conditions. Water has a molar mass of 18 g, while the molar mass of nitrogen is 28g and that of oxygen is 24g. So I would expect humid air to be less dense, but due to hydrogen bonds between water molecules, density might be increased again because it contributes a "non-ideal gas" part to the considerations. – oliver Jun 09 '21 at 22:52
  • @oliver The molar mass of $\mathrm O_2$ is ~32 g. – PM 2Ring Jun 10 '21 at 08:04
  • @PM2Ring: Of course you're right. At the time I wrote my comment, I guess it made perfect sense to me that 2 x 16 = 24... Or maybe my hyperoptic eyes weren't able to distinguish between C and O when looking at the periodic table. :-D – oliver Jun 10 '21 at 13:09
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    @oliver Well, $\mathrm{ {}^{12}O}$ does exist, but with a half-life of 6.3E-21 s, it's pretty hard to make molecules from it. ;) – PM 2Ring Jun 10 '21 at 13:16

2 Answers2

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Avogadro's law states that "equal volumes of all gases, at the same temperature and pressure, have the same number of molecules." As oliver points out, water molecules weigh less than O2 or N2 molecules.

Keith McClary
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Dry air is denser than wet air- enough so to affect takeoff roll distances for aircraft.

niels nielsen
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  • And the fact that wet air is less dense then dry air has implications regarding convective heat transfer from the surface of the earth to high altitudes, where water vapor can radiate heat to outer space, condense, and fall as rain. – David White Jun 09 '21 at 23:45
  • @DavidWhite I noticed that some items (eg. transparent plastic jars) took unreasonably long to dry in the dishwasher rack, in the usual upside down orientation. They dry faster right side up. – Keith McClary Jun 10 '21 at 00:30
  • @KeithMcClary: I have noticed that too. But I have always been thinking (and still do), that this is because the hot vapor rises up and gets caught in the upside down items. I assume that you open the dishwasher as soon as it has finished, so it is far from thermal equilibrium. – oliver Jun 10 '21 at 13:19
  • @oliver Yes, but there are still drops of water in them hours after they have cooled and most other items seem dry. I did a rough experiment, turning one of two over (shortly after finished), and it seemed to dry faster. (1 kg peanut butter jars.)There could be a Question in this. – Keith McClary Jun 10 '21 at 15:25