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

[rlamont <rlamont@POSTOFFICE.PROVIDENCE.EDU>]

Be careful here. It's not the temperature of the water vapor that
is germane here. In a sense, (and I know this is simplifying
probably a little too much) it is the temperature of the liquid
water that matters. A cold glass of water on a humid day will
form condensation. This is not really because of the air
temperature - it is because droplets that may form on the glass
evaporate slowly because the liquid water is cool. But the humid
air is bombarding the drop with a tremendous number of vapor
molecules causing more condensation than evaporation. If the air
becomes warmer, but it moisture content does not change, the
growth rate of the droplets will not change significantly. The
air may set the temperature of the vapor, but that temperature is
only a minor player here.

To really think about this matter simply and clearly, one really
has to assume that the liquid water and water vapor are at the
same temperature. Otherwise, one will not have the exact
conditions for the Clausius-Clapeyron equation to hold.

Bob at PC


> -----Original Message-----
> From: Forum for Physics Educators [mailto:PHYS-
> L@list1.ucc.nau.edu] On Behalf Of Folkerts, Timothy J
> Sent: Friday, April 08, 2005 3:55 PM
> To: PHYS-L@LISTS.NAU.EDU
> Subject: Re: Dalton's law of partial pressures
>
> 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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