Re: The importance of being pedant

[William Beaty <billb@ESKIMO.COM>]

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?


> 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.


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.


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