Although this is taking more time than it's worth, my curiosity has
gotten the better part of me. I have been looking at a lot of general
chemistry texts, a lot of physical chemistry texts, science
dictionaries and science encyclopedias, and I have been talking to the
chemists here. My conclusion is that there just isn't any agreed upon
difference between vapor and gas. Although a few sources seem to
differentiate between the two, the criteria for differentiation are not
the same from one source to another.
For example, some sources might claim a substance that is liquid or
solid at ordinary temperatures should be called a vapor when it has
evaporated; reserving gas for substances that are in the gas phase at
ordinary temperatures.
Another source says a vapor is an "air dispersion" of a substance that
is normally solid or liquid. This definition would include fog and
clouds because "dispersion" is the most common word I could find to
describe what a substance is when it exists as single molecules plus
clusters of molecules plus macroscopic droplets all dispersed in air.
But the overwhelming evidence, both printed and by personal testimony
from chemists is that the words vapor and gas are used interchangeably.
I can cite published examples in very popular physical chemistry texts
if anyone wishes to check them out.
A widely published graph entitled "Compressibility Factor for Various
Gases as a Function of Reduced Pressure" includes data for water,
methane, ethane, propane, butane, isopentane, heptane, nitrogen, and
carbon dioxide. Hence these substances are all being referred to as
gases. That makes sense because the compressibility factor is a
measure of how well (or poorly) a substance follows ideal gas behavior
at various combinations of temperature and pressure. This is
especially interesting because in this regard all these substances
follow the same curves. For example, nitrogen (which some insist is a
gas) and gas-phase water (which some insist is a vapor) plot right on
top of each other when compressibility factor (PV/RT) is plotted as a
function of reduced pressure (P/critical-pressure). This proves that
water behaves just as much like an ideal gas as nitrogen does if we
compare the two equally near or equally far from their respective
critical points. In fact, all substances mentioned above behave this
way. So in terms of ideal-gas behavior, all gasses (and/or vapors)
behave about the same when examined under similar circumstances, where
similar circumstances means with respect to their critical points.
Some might say, "Yes, that's my point... near the critical point the
substance does not behave like an ideal gas... hence it should be
called a vapor." But then there would be some temperatures and
pressures at which we would have to call any substance (including
helium) a vapor... and other temperatures and pressures at which we
would have to call any substance (including water) a gas. Since the
average scientist doesn't have all those conditions memorized for even
common substances, we had better allow ourselves to use vapor and gas
interchangeably or else we're always going to be sticking our feet in
our mouths. Besides, who knows at what degree of deviation from
ideal-gas behavior a substance switch from being a gas to being a
vapor?
I conclude that the people who spend their research lives studying such
things do not make a distinction between vapor and gas. I also
conclude that the best name we should use for water that exists in the
"fog phase" is the word "dispersion."
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