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Q: difference between a gas and a vapor ( Answered,   1 Comment )
Subject: difference between a gas and a vapor
Category: Science > Chemistry
Asked by: asherben-ga
List Price: $100.00
Posted: 20 Jun 2002 12:44 PDT
Expires: 20 Jul 2002 12:44 PDT
Question ID: 29910
What standard textbooks state the difference between a gas and a
vapor?  I need precise citations (author, title, edition, year) and
page references.
Subject: Re: difference between a gas and a vapor
Answered By: guillermo-ga on 23 Jun 2002 15:30 PDT
Dear Asherben,

What a change of air your question was! After the first tries on the
Internet, I realized that this was more a feet question than a
fingertip one. Good chance to give my screen a break from my eyes (or
vice versa?) and go back, for a change, to the real, concrete world,
and the old and good touchable books. Spying friend’s bookshelves,
hanging out in bookstores, searching in the library, instead of
surfing websites. Well, to be honest, I couldn’t help surfing a little
bit, as you’ll see…

Let’s get to the point!

You ask for identified standard textbooks that state the difference
between a gas and a vapor. As you didn’t specify a level, I considered
a variety of books covering from junior secondary school to higher
education, around eight in total.

First thing I noticed, is that although there is a difference -- that
becomes important for certain phenomena -- its mention in textbooks is
not that frequent, or at list not remarked as an issue.

In those corresponding to junior secondary, the only reference I found
that relates vapor and gas was, surprisingly, in the opposite
direction, under the title How Heat Energy Changes Matter:

“The dew you find on the grass in the morning is caused by
condensation. The sun goes away at night. The air becomes much cooler.
This causes water vapor (a gas) to change into water.” (Bledsoe, Lucy
Jane. General Science - 2nd Edition. Globe Fearon Educational
Publisher, 1994. Pg. 167.)

This doesn’t mean that the difference doesn’t exist, neither that
they’re fooling the kids. It’s just that, up to a certain point of
knowledge, there’s no much interest in the difference, and it might
even confuse the young students in their first approaches to the

The difference between gas and vapor is, in one sense, conventional.
Considering the three basic states of matter -- solid, liquid and gas
-- we call vapor the gaseous state of a matter that mostly appears as
a liquid in nature -- or, better, in our environmental conditions--;
and a gas, as it follows, a matter which habitual presence in our
environment is gaseous.

This is a simple, rough way to express it, but not precise. For
instance, how much is “mostly” or “habitual”: isn’t it habitual enough
the water vapor in the atmosphere to call it a gas?

Check out the following quotation. It comes from my little bit of
surfing, but it used to be a book. Not a standard textbook, though,
but a very authorized old text. I really like this find. It was the
only entry that came out, searching in Google Advanced Search for the
exact phrase “vapor and gas difference” (Google results URL
The page that showed up is titled Submarine Refrigeration and
Air-Conditioning Systems - INDEX ( ), and is the index
page of the old paperback edition of one of a series of submarine
training manuals that was completed just after WW II, at the website
of the San Francisco Maritime National Park Association ( ). The index page contained the searched
phrased as the content of page 11 (of the original paperback manual),
which could be easily found surfing the site from this page, at
Submarine Refrigeration and Air-Conditioning Systems - Chapter 3 ( ).

“The terms vapor and gas both refer to matter in the physical state
that is neither solid nor liquid. There is, however, a definite
distinction between them.

“A vapor condenses very readily to the liquid state under small
changes of temperature or pressure or both, and constantly does so
under ordinary conditions in nature. It may be said to be very close
to the liquid state, although it is a vapor. A gas, on the other hand,
exists under ordinary conditions in the gaseous state. To change it to
the liquid state, special laboratory apparatus with extreme changes of
pressure and temperature is required. A gas may be said to be far
removed from the liquid state, and cannot change to it under ordinary
natural conditions.”
Submarine Refrigeration and Air-Conditioning Systems, Navpers 16163,
December 1945; at The Fleet Type Submarine Online ( ).

This quote, leaving aside its singularity and historical interest,
holds a curious relationship with the precedent. While in the school
text the easiness of condensation made no obstacle to equal vapor to
gas, in the technical manual is one of the aspects remarked to state
the difference. This shows how a slight difference becomes important
when technological or scientific complexness increases.

In that sense, the following quotation from an advanced level in
physics textbook includes another criteria, not necessarily different
and still conventional, but more precise and specific -- in one word,

“Vapors are gases which can be liquefied, so a gas above its critical
temperature cannot be referred to as a vapor. A vapor is material in
the gaseous state at or below the critical temperature.” (Breithaupt,
Jim. New Understanding Physics For Advanced Level - 4th Edition.
Nelson Thornes Publisher, 2000. Pg. 185).

Impressive, isn’t it? But, what does it mean? Well, a gas can be
liquefied by compressing it, but as long as it is at or below a
certain temperature, beyond which a gas cannot be liquefied, no matter
the pressure applied on it. This temperature is called critical
temperature. Each substance has its own critical temperature. Let’s
see a few examples:

Helium’s critical temperature is -268C. All our nature and life
conditions exist above helium’s critical temperature. Then, for any
practical purpose, we’ll always consider it as a gas, not a vapor,
except -- may be -- for very specific technological or scientific

Propane, has a critical temperature of 97C, while water’s is 374C.
Now, in our environment, both substances, when in gaseous state, will
technically be considered as vapors. The difference between them is
that at room temperature and pressure, a certain amount of water will
be liquid, while propane will remain gaseous. Still, by placing the
propane in a container and pressurizing it, we can turn it into liquid
at room temperature. On the other hand, at the same temperature we
could turn water into gas by placing it in another container and
lowering its pressure.

That’s why I said before that this statement is more precise but not
necessarily different from that of our submarine engineers friends,
who said that “a vapor condenses very readily to the liquid state
under small changes of temperature or pressure or both”.

Another criteria to state a difference between gas and vapor may be
taken from its observance of the gases laws:

“Vapors obey the three gases laws so long as they do not become
saturated. With saturated vapors S.V.P. (saturation vapor pressure)
increases with temperature but not according to the pressure law.”
(Duncan, Tom. GCSE Physics. Jim Murray Publisher, 2000. Pg. 167).

At a given temperature in a closed vessel, a vapor in dynamic
equilibrium with its liquid state --the of molecules turning into
gaseous state equals those turning into liquid state-- is called a
saturated vapor. The pressure law relates pressure, temperature and
volume. This condition in which gaseous and liquid state coexist
distorts that relationship. Notice that this condition is only
possible for those substances that can be liquefied, in other words,
substances below its critical temperature. Hence, the two criteria are

Almost with the same phrase used by Jim Breithaupt --quoted above--
E.N. Ramsden also states the difference between gas and vapor using
the critical temperature criteria:
“The gaseous state of a substance at temperatures below its critical
temperature is called a vapor.” (Ramsden, E.N.. A-Level Chemistry.
Stanley Thomas Publisher, 1994 --reprinted 1995--. Pg. 163).

Back to my Internet surfing vice, I found as a complementary knowledge
that also firefighting shows us a conceptual and a practical -- and
vital -- difference. At General Dennis J. Reimer Training and Doctrine
Digital Library ( ) there’s a page
that makes part of a firefighting training material named:
Lesson 2 - Implement a Petroleum Fire And Safety Program (

There we can read:

“The terms "gas" and "vapor" are often used to mean the same thing,
although there is a difference. A gas exists as a gas at ordinary
temperature and pressure. A vapor is a gas-like form of a substance
that is ordinarily a liquid.”

And also (above, under the title Part C - Vapors):

“Vapor characteristics. In a fire, it is the vapor that actually
“Vapors are heavier than air and collect in low areas.
“Vapors will hang low to the ground and spread over large areas.
“On hot humid days vapors are produced in greater volume.”

This page was one of the entries of the Google Advanced Search Results
for the combined criteria “books difference ‘gas and vapor’” (

As you’ve seen, we’ve found differentiation statements that,
regardless their dissimilar field of origin, depth, or main criteria,
have a basic consistence among them.

I hope to have provided you what you were looking for!

Thank you for such a challenging and instructive question!



Clarification of Answer by guillermo-ga on 24 Jun 2002 08:07 PDT
Dear Asherben,

I just want to ad some URL

Clarification of Answer by guillermo-ga on 24 Jun 2002 08:24 PDT
Dear Asherben,

I just want to add some URLs I knew and helped me to help you. You
might find them useful as well:
Chemistry Learning Center
Help Files for Gases (at Chemistry Learning Centre)

Saturation of Water Vapor (page belonging to The Department of
Geophysical Sciences of The University of Chicago website; Prof. David
Archer's directory)

the Department of Chemistry of The University of Texas at El Paso;
Prof. Dr. Michael I. Davies' directory)

Any other help you may need, please let me know.



Clarification of Answer by guillermo-ga on 24 Jun 2002 10:53 PDT
Hello again, Asherben:

Looking for the exact phrase "difference between gas and vapor" at
Google Answers Advanced Search (results page URL:
), brought back as the only result a link to a book for sale at called “Mad About Physics: Braintwisters, Paradoxes, and
Curiosities” (
). Although it looks more as a science trivia book than a standard
textbook, I had a look to the sample pages. Its structure consists of
twelve chapters in which the questions are exposed, and a second part
with all the answers. Your question appears in the first chapter (page
3) and answers to that chapter are between pages 155 to 166.
Unfortunately, I couldn’t see it because the answers pages are not
among the promotional sample pages available at Anyway, I
thought this might interest you.


Subject: Re: difference between a gas and a vapor
From: hedgie-ga on 23 Jun 2002 18:12 PDT
Apparently, different  groups use the terms differently.

In physical literature these  terms are synonyms:
 There are three phases (states of matter is the older term, term
phase is recommended).
which are gas, liquid, solid.  The  substance X in gasous state means
the same thing as
vapor of X.

In more applied area  the term vapor is prefered if substance is
liquid at the room temperature,
and atmospheric presure.
Apparently firefighters have their own language,  but some of the
statements quoted
 from their manual are dubious in any language:

?Vapor characteristics. In a fire, it is the vapor that actually
     Same gases are flammable (e.g. propan, hydrogen)
   some are not (carbon dioxide or water vapor)

 ?Vapors are heavier than air and collect in low areas. 
     Some gases are heavier then air (carbon dioxide)
     some are lighter, (e.g. hydrogen).

 Perhaps one may define vapor in a non-standard way by
 the  flammability or by density, but  one cannot do it by both
 as all four  combinations exist.

 ?On hot humid days vapors are produced in greater volume.?    
   ???  --  produced from what? 
    On humid days there is more water vapor
    in the air (air is mixture of gases) - that what humidity means.

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