NOTICE: Citizendium is still being set up on its newer server, treat as a beta for now; please see here for more.
Citizendium - a community developing a quality comprehensive compendium of knowledge, online and free. Click here to join and contribute—free
CZ thanks our previous donors. Donate here. Treasurer's Financial Report -- Thanks to our content contributors. --

Talk:Ideal gas law/Draft

From Citizendium, the Citizens' Compendium
< Talk:Ideal gas law
Revision as of 09:31, 6 January 2009 by Paul Wormer (Talk | contribs) (Some further streamlining: new section)

Jump to: navigation, search
This article has a Citable Version.
Main Article
Related Articles  [?]
Bibliography  [?]
External Links  [?]
Citable Version  [?]
Video [?]
Tutorials [?]
To learn how to fill out this checklist, please see CZ:The Article Checklist. To update this checklist edit the metadata template.
 Definition Relates pressure, volume and temperature for hypothetical gases of atoms or molecules with negligible intermolecular forces. [d] [e]

Some comments

  1. IUPAC prescribes lowercase p for pressure.
  2. Is it necessary to give all forms? In other words can't we expect the reader to know some elementary algebra? (When I went to highschool it was first grade material, student's age around 12 years)
  3. Molar gas constant R in SI units: 8.314 472 J mol−1K−1 see: Don't you also want to give: R = NA kB ?
  4. The word "constant" in roman: \mathrm{constant}. Same for mol, K, and atm.
  5. The name of the law varies. I learned in school "law of Boyle and Gay-Lussac".
  6. You give these worked-out problems, that is OK for a textbook, but for an encyclopedia?

Hope these comments are useful. --Paul Wormer 05:19, 4 October 2007 (CDT)

ideal gas and ideal gas law

Maybe we should merge the two into one article? ideal gas doesn't exist yet, so no real merging is necessary. What do you think? Yuval Langer 06:46, 4 October 2007 (CDT)

Gas laws developed in 1660s

We need to change this back, because only 1 law was started in 1660s. The rest were developed later. David E. Volk 02:14, 2 November 2007 (CDT)


This article needs lots of work on explanations, particularly on what the formulas mean what why they're used, and more elaboration on some concepts. I think it needs some narrative work. --Robert W King 14:58, 8 January 2008 (CST)

Please explain

David Volk: Would you please explain why this equation (at fixed temperature and pressure) has a period in it? Also please explain Vm . - Milton Beychok 12:52, 4 February 2008 (CST)

Special cases

For the various special-cases, rather than writing "= constant" or giving the constant a name such as , how about using the proportional-to symbol? IE:

  • Amonton's law: (constant V, n)
  • Boyle's law: (constant T, n)
  • (constant V, T)
  • Charles's law: (constant p, n)
  • Avagadro's law: (constant p, T)

Downside: people might not know the meaning of the "" symbol. How about in English for the benefit of the notationally phobic?

Amonton's law: pressure is proportional to temperature (constant quantity and volume).
Boyle's law: pressure is inversely proportional to volume (constant quantity and temperature).

Warren Schudy 20:23, 5 February 2008 (CST)

Explaining edits made to Intro section.

The wording was re-arranged and reformatted so that the discussion of real gas deviation from ideal gas behavior is now in a separate paragraph.

Revised the wording "... one of the real gas equations, such as the van der Waals equation must be used" to state that there are many equations of state (EOS) that can be used for real gases, of which the van der Waals equation is the simplest. The words "must be used" made it sound as if the use of the van der Waals equation was somehow mandatory.

The ideal gas equation parameters were changed to bold font to make them more obvious. The words "absolute temperature in degrees Kelvin" were changed to "absolute temperature" because the equation applies for other absolute temperature scales as well ... such as degrees Rankine that is still used by many USA engineers. The word "pressure" was changed to "absolute pressure" because the ideal gas equation requires absolute pressure (as differentiated from gauge pressure).

Added a reference to document the quoted numerical value of R. Also added a table of various values of R in other units (using the same table as in the molar gas constant article, for consistency).

To the best of my knowledge, I did not significantly change any of the content ... just made it more readable (I hope) and a wee bit more correct. - Milton Beychok 00:43, 6 May 2008 (CDT)

Nice changes Milton

Milton, thanks for the changes. I still don't what to do with the "Special cases" subsection. I have repeatedly considered deleting the whole thing, writing it out in paragraph form rather than equation form, or rewriting the equations as suggested above with the proportional symbol. Any thoughts on this section? David E. Volk 10:08, 16 May 2008 (CDT)

Personally, I would delete that entire section as you have previously considered. It really doesn't add anything substantive to the article. Regards, Milton Beychok 10:45, 16 May 2008 (CDT)
How about moving it to an /Advanced subpage? J. Noel Chiappa 11:53, 16 May 2008 (CDT)

David, how about this?

David, I wanted to create a definition for the Definition subpage, but could not think of one that was consistent with the current lead-in section. So I thought, why not change the lead-in? This is what I came up with:

"The ideal gas law applies to hypothetical gases consisting of atoms or molecules with no intermolecular forces that are in constant random motion undergoing perfectly elastic collisions with the gas container walls and each other.
The ideal gas law is useful for calculating temperatures, volumes, pressures or number of moles ..... (and the rest of the current lead-in section)."

The definition for the Definition subpage could then be: "A gas law that applies to hypothetical gases consisting of atoms or molecules with no intermolecular forces that are in constant random motion undergoing perfectly elastic collisions with the gas container walls and each other."

What do you think? Milton Beychok 11:50, 16 May 2008 (CDT)

Ooooh, I like that! Although in the def, to keep it short, I'd substitute 'particles', or something like that, for "atoms or molecules"; you'd have to substitute 'interactive' for "intermolecular" (and how do you have "intermolecular forces" if you have a gas which isn't molecules, but I digress :-). But given the nature of the theory, which treats the entities as featureless particles, I think some wording like that would be OK. They can get the full story in the article... J. Noel Chiappa 12:20, 16 May 2008 (CDT)

My definition

I went with this definition (below), which shortens the ideas of others above. Note I left the name out, because I think that is the rule for definitions.

Relates pressure, volume and temperature for hypothetical gases of inelastic particles with negligible intermolecular forces. David E. Volk 12:35, 16 May 2008 (CDT)

Very good, I like that better than my suggested definition. But I still think the article might be improved by the intro section having a first paragraph that states it is about hypothetical gases such as I proposed above:
"The ideal gas law applies to hypothetical gases consisting of atoms or molecules with no intermolecular forces that are in constant random motion undergoing perfectly elastic collisions with the gas container walls and each other."
Perhaps "particles" could replace "atoms or molecules" and perhaps "interactive" could replace "intermolecular". Milton Beychok 12:59, 16 May 2008 (CDT)

Inelastic particles?

In my understanding an inelastic particle is a particle with internal structure that can take up energy. A tennis ball that can be squeezed in, or a molecule that can vibrate. An elastic collision transfers only momentum (translational kinetic energy). So, in my view most ideal gases consist of elastic particles. In any case an ideal gas of "mono-atomic molecules" (term used by T.L. Hill, an outstanding thermodynamicist) consists of elastic particles.

For diatomic and polyatomic molecules the term is debatable, because collisions can excite rotations. Although rotations are seen as external motions, one could argue that a collision exciting a rotation is inelastic and hence a polyatomic molecule could be seen as an inelastic particle even if it is so rigid that its vibrations are not excited. Personally, I would call a polyatomic molecule only inelastic if its vibrations (or electrons) were excited, but as I say, this is debatable. --Paul Wormer 05:10, 17 June 2008 (CDT)

Towards approval

Following a request by Milton, I had a quick look at the cluster and don't think it is ready for approval yet. In brief:

  1. I share Paul's concerns with respect to the (in)elasticity
  2. The sections "Background" and "Special cases" largely overlap, while the article as a whole appears incomplete to me without some derivation from first principles (which could go into an Advanced subpage) and without some specific hints to non-specialists on why this law is important
  3. The Bibliography should contain some of the classic papers
  4. There should be some useful websites (e.g. with applets) to add to the External links

Hope this helps. --Daniel Mietchen 01:25, 4 January 2009 (UTC)

The typography needs some improvement, too. Don't use = in a sentence and don't use bold letters like: p = absolute pressure (and what is absolute pressure?) I would write p is the pressure of the gas contained in a vessel of volume V, or something similar. I could do this editing, but then I cannot approve the article. --Paul Wormer 11:17, 4 January 2009 (UTC)
Hi, Paul. CZ:Approval process states "Group approval. If there are three editors, all of whom are expert in the topic of an article, and all of which have been at work on an article as authors, then any one of them may approve of an article with the concurrence of the other two." In other words, you may have worked on the article but you can still approve it if you are one of three editors joining to approve the article. That is why I invited Daniel Mietchen, David Volk and you to join in the approval. In that way, everyone can contribute in improving the article and yet join in the approval. So, Daniel and Paul, please make whatever changes you think are appropriate.
As for the meaning of absolute pressure, see Pressure#Absolute pressure versus gauge pressure. In my opinion, it is always important to make that distinction. Milton Beychok 16:59, 4 January 2009 (UTC)
Point taken about absolute pressure. Tomorrow I will probably do some editing on the article. About inelastic scattering: I believe that the present text is incorrect, only the translational partition function depends on volume, and hence the ideal gas law is not influenced by inelastic rotational and/or vibrational excitation by scattering, (I will enter this tomorrow).
It occurred to me that the ideal gas law has a built-in contradiction: the molecules don't interact and have no size, so from the point of view of scattering theory they cannot exchange energy. Yet there is thermal equilibrium. I checked this in some stat mech books and some of them talk about collisions with the wall and others say that there are just enough collisons to thermalize the gas, but that nevertheless the interactions can be neglected. --Paul Wormer 17:16, 4 January 2009 (UTC)

bold letters

Milt I saw you made some changes but let p, etc. bold. These are scalar (not vector) quantities, so I would write p, etc. --Paul Wormer 17:20, 4 January 2009 (UTC)

Paul, feel free to change them. I just wanted them to stand out ... no other reason. Milton Beychok 17:58, 4 January 2009 (UTC)


I did a fairly major rewrite:

  1. Removed reference to collisions and sizes of molecules.
  2. Added a stat mech derivation that shows that neither sizes nor collisions play a role.
  3. Removed section about "when ideal gas law fails". In intro already reference to other equations of state; that seemed sufficient to me.
  4. Commented out last section, because it is repetitive, but it is still there, if somebody disagrees it can easily be restored.

--Paul Wormer 11:31, 5 January 2009 (UTC)

Minor edits to Paul's rewrite

Paul, as per your post on my Talk page, I have gone over your rewrite. Most of my edits were very minor rewords. I also removed periods amd commas from the end of equations. In my opinion, they are distracting and they really are not needed.

I mus admit that the derivation is over my head. However, I added one more equation at the end of the derivation in order to finsh the derivation. Rather than make the reader have to mentally substitute N = nNA and NAkB = R into your last equation, I did it for the reader. Milton Beychok 22:41, 5 January 2009 (UTC)

Ah, the punctuation of mathematical equations, the great divide between chemists and physicists. You know that the Journal of Physical Chemistry (an ACS journal) does not punctuate and that the Journal of Chemical Physics (an APS journal) does punctuate. Apparently chemical engineers are on the chemistry side of the divide :-) . --Paul Wormer 08:27, 6 January 2009 (UTC)

Some further streamlining

I removed an historical sentence from the intro (and changed the section title "Background" to "Historical background"). I entered into the intro the fact that a gas consisting of polyatomic molecules may be ideal, in contrast to what our influential competitors have to say about it. And I inserted a few more steps into the stat mech derivation to help the reader, following the example set by Milton. --Paul Wormer 09:31, 6 January 2009 (UTC)