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

[John Denker <jsd@AV8N.COM>]

On 04/08/05 12:55, Folkerts, Timothy J wrote:
> Suppose you have a 1 m^3 space filled with ~80% N2 and 20% O2 at 40C
> and 90% relative humidity. The mixture is then cooled to 5C, at which
> point the water starts to condense and falls out of the space. Which of
> the following would people be comfortable saying?
>
> a)  the 1 m^3 of space has fewer water molecules in it than it did before.
> b)  the 1 m^3 of space is holding less water than it did before.

Those are unobjectionable ... but wouldn't it be more
natural to say "contains" rather than "has" or "holds"?

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.

> c)  the 1 m^3 of atmosphere is holding less water than it did before.
> d)  the 1 m^3 of air is holding less water than it did before.

The air/atmosphere has got nothing to do with it.  Under
these conditions, oxygen, nitrogen, and water vapor are
ideal gasses to an excellent approximation.  Dalton's
law of partial pressures applies.
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