I am both a chemist and a physicist, having degrees in both, and my
Ph.D. is chemical physics. I am not saying this to establish myself as
an expert. Rather, I want to establish that I have always had one foot
in the chemistry department and one foot in the physics department.
That means I am thoroughly familiar with the way both disciplines
define and use words, including situations that strike me as good usage
and situations that strike me as poor usage.
Of course in the field of chemistry, the people who work with
gases/vapors are the physical chemists. It is clear to me that they
use these words interchangeably for the most part; yet they sometimes
make distinctions.
It is very common to find wording like the following in a physical
chemistry textbook: "The heat of vaporization is the heat required to
change a substance from the liquid phase to the gaseous phase." (By the
way, some of the newer books are beginning to avoid the use of "heat"
and alway use enthalpy, internal-energy, free-energy, or entropy
depending upon which one they're referring to.) At any rate, in every
pchem book I pulled from my shelf, I could find pages with gases, gas
phase, vapor, vapor phase all on the same page and used
interchangeably.
On the other hand, when describing pressure, it seems common to refer
to "gas pressure" as the pressure of the entire gaseous phase whereas
"vapor pressure" typically means the "partial pressure" of just one
component of the whole gas phase.
I think John Cooper's response (under a different subject heading) is
on the right track. We have to look to usage (which ultimately defines
the terms) and we sometimes have to look to history to find why the
usage evolved the way it did.
I think it is likely that the old definition of water gas (which is
still listed in dictionaries, the Chemical Dictionary, and the Merck
Index) forced people to use water vapor when they wanted to refer to
water in the gaseous phase. That is, the phrase "water gas" was
already in widespread use to mean something different.
I think the distinction between condensable and non-condensable
substances made sense when it was believed that there were some
"permanent gases." But we can't make that distinction today.
I think the confusion over water in the gas phase as opposed to water
droplets in clouds or fog is a confusion likely to remain for a long
time. We just need to live with it and explain it to our students
whenever the opportunity arises. But I don't think anyone has single
definitive words for this yet.
Michael D. Edmiston, Ph.D. Phone/voice-mail: 419-358-3270
Professor of Chemistry & Physics FAX: 419-358-3323
Chairman, Science Department E-Mail edmiston@bluffton.edu
Bluffton College
280 West College Avenue
Bluffton, OH 45817