Re: Terminology Confusion

[James Mclean <jmclean@CHEM.UCSD.EDU>]

Gordon Smith wrote:
> If I have a closed container, with gas at some pressure and temperature,
> and a pool of water at the bottom, I eventually reach an equilibrium state
> wherein the vapor leaving the water is the same as the vapor entering the
> water.
>
> My confusion: Is the gas above the water saturated with water vapor?
Yes, not because your setup defines saturation, but because physics tells
us that under these conditions the result satisfies the definition.

The definition is that the gas is saturated when it "contains" as much
water vapor as it can under equilibrium conditions.  This maximum pressure
(or density) of water vapor is determined solely by the chosen
temperature.  If some liquid (or solid, as appropriate) water at the same
temperature is available, then the water vapor pressure will always rise to
it's maximum value.  If the vapor pressure is higher than this maximum,
then some liquid (or solid) will condense out.  (This is ignoring surface
tension effects.)

As has been pointed out, this terminology might be a bit confusing in some
situations, since at a given temperature the maximum pressure of water
vapor does not depend on the density, nature, or even existence of the gas
(assuming ideal gases).  However, in cases where it is natural to think of
the gas as controlling the system temperature, and thereby controlling the
maximum water vapor pressure, it seems that the terminology is appropriate.

--
--James McLean
jmclean@chem.ucsd.edu
post doc, UC San Diego, Chemistry
moving this fall to SUNY Geneseo