Re: The importance of being pedant

[Leigh Palmer <palmer@SFU.CA>]

At 10:42 AM -0700 7/22/99, William Beaty wrote:

> On Wed, 21 Jul 1999, Leigh Palmer wrote:
>
> > A gas is incapable of condensing; that is why one distinguishes
> > a vapor from a gas.
>
> I expect you mean that, at a particular temperature, we can force a
> "vapor" to condense by compressing it, while a "gas" will not condense no
> matter how high the pressure?

Not quite. When I speak of a gas, I mean a substance which behaves
similarly to an ideal gas. Near condensation to the liquid phase no
substance behaves very much like an ideal gas. The same is somewhat
less true of gases (or vapors) which condense to the solid phase*.
Ultimately a vapor behaves more nearly like a gas the greater the
separation of its molecules in the vapor phase.

To see what I mean, look at the isotherms on a PV plot for a pure
substance. (An example is Figure 2-1 in Zemansky & Dittman.) While
these figures are usually distorted somewhat by the publishers'
art departments, it should be clear that as one goes to higher
temperatures these isotherms approach more nearly the rectangular
hyperbolic shape of the ideal gas isotherm. Isotherms near and
below the critical temperature are clearly not hyperbolae.

> > Often the region of a simple phase diagram
> > for temperatures greater than the critical temperature is called
> > the "gas phase", but the boundary between this and the vapor
> > phase below that temperature is evidently artificial. I don't
> > know who thought up this particular convention, but it lacks
> > physical meaning in my view, and I prefer the old fashioned way.
> > When one speaks of a gas it should be in reference to a
> > substance in vapor phase and far from saturation. In my opinion
> > distinguishing a gas from a vapor by its temperature will surely
> > lead to misconceptions.
>
> I wonder how many *educators* believe that gases are fundamentally
> different than vapors, and that vapors always contain suspended droplets.
> When misconceptions appear in textbooks, they usually don't stay limited
> to students' minds alone.

I would be surprised (no, astonished) to find that any teacher of
physics holds this misconception. I sincerely doubt that any author
of a textbook in which it appears (like Zemansky) is doing anything
but propagating a convention that, to him, appears harmless because
he says to himself "I understand there's no phase transition; why
would anyone make that mistake?"

> Unless I'm totally crazy, a vapor *is* a gas, correct?  There is no
> distinct phase of matter called "vapor."  For example, at 1atm pressure at
> below 100C temperature, H2O-gas is a "vapor", but if we raise the
> temperature above 100C, H2O-gas becomes a "gas."  Yet there is no change
> in the material itself.

As I said before, it is conventional to draw a distinction between
vapor and gas at the critical isotherm, though the chemist and
physicist may disagree about which is which. Zemansky & Dittman
actually are quite specific about this matter, supplementing that
misleading diagram with text:

 "... Above the critical temperature only the *gas phase* exists.
 The isotherm at the critical temperature is called the *critical
 isotherm*..." [asterisks denote italics]

On the accompanying figure the title "Vapor phase" is printed in
the appropriate space below the critical isotherm.

There's nothing magic about ~100 degrees C except that water vapor
and liquid coexist in equilibrium at a pressure of one atmosphere
at that temperature. At a temperature of 110 degrees C and pressure
of 1 atmosphere water vapor is distinctly nonideal, and I doubt it
would be termed a gas by anyone. "Steam" (dry steam, if you must)
and "water vapor" are preferable to the terms "gas" and "water gas"
for water in this state.

Leigh

*Another peeve of mine is the use of the word "sublimate" - with
its Freudian linguistic baggage - rather than the perfectly correct,
shorter, and more euphonious "sublime".