Re: Terminology Confusion

[Daniel Schroeder <DSCHROEDER@CC.WEBER.EDU>]

> > 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

What James says is almost, but not quite, true.  Let's say the gas is
air and the liquid is (mostly) water.  The density of water vapor in the
space above the liquid *does* depend on the existence and pressure of
the air, but this dependence is very weak and so is usually neglected
except under extreme (very high pressure) conditions.  It's all explained
in the wonderful book by Bohren and Albrecht, Atmospheric Thermodynamics
(Oxford U. P., 1998).  They also have a long, entertaining diatribe
against the term "saturation" and the "sponge theory" of air that it
seems to imply.  (We know that air is not a sponge because even when
the "pores" are "saturated" with water vapor, the amount of any other
volatile liquid that the air can "absorb" is essentially unaffected.)

I *highly* recommend Bohren and Albrecht's book to anyone who is
interested in thermodynamics applied to everyday situations.  (But
I don't agree with all their diatribes, especially regarding the
word "heat" and the use of differential notation.)

Dan Schroeder
Weber State University
dschroeder@cc.weber.edu