[Phys-L] Re: Dalton's law of partial pressures

["Folkerts, Timothy J" <FolkertsT@BARTONCCC.EDU>]

John D said
> To my ears, "how much does it hold" refers to capacity,
> i.e. how much it _could_ contain, as opposed to how
> much it presently contains.

I can see that interpretation.   "Holds" could mean "could hold" = "could contain", or "holds" could mean "actually holds" = "actually contains". We can agree to be more precise here.  But I think he bigger semantics question deals with "air".

Then Bob L said:
> The other gases have nothing to do with the evaporation
> and condensation of the water. Therefore, the N2 and O2 do not
> "hold" moisture. They are irrelevant to the process.

The saturation partial pressure of water vapor depends on the temperature of the water vapor in a region.

And the temperature of the water vapor depends on the temperature of the "air" in a region.  So the air does have "something" to do with it.  If you cool the air, you cool the water vapor.

Bob did a thought experiment changing the pressure of the air while holding the temp constant.  Let me do a thought experiment were I hold the pressure constant and change the temperature (a much more typical situation for "air").  If I close all the doors to my kitchen, put a pot of water on my stove, and let it boil, I will eventually reach an equilibrium where the water boiling from the pot equals the water condensing on the walls.  There will be a certain amount of water "in the air".

If I turn up my furnace by 10 degrees and try again, there will be more water "in the air" when it reaches equilibrium.

I certainly can't accurately say that "the N2 & O2 molecules somehow latch onto the water molecules and help suspend them".  There are no "hydrated air molecules" that are formed
I think I may say "The presence of warm N2 & O2 in the space allows more H2O molecules to float around the space than if there were cold N2 & O2 molecules in the room".

So a region with warm air can contain (and typically does contain) more water than a region with cold air.  Do I really need to say "the space in my kitchen that is generally occupied by air is holding more water" in order to not mislead fellow scientists or students?  How incorrect is it to say that "the warm air in my kitchen is holding more water"?  Or "warm air help increase the water-carrying capacity of the room"?  (Those aren't just a rhetorical question.  I would like to hear some opinions.)

Tim F


P.S.  When Allen Shepard hit a golf ball on the moon, once it left the club, was it "in the air"? ;-)
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