Talk:Vapor pressure

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old question[edit]

I think the last mentioned temperature is not the melting point but the triple point. Is that correct?

Actually, no. When the vapor pressures of liquid and solid are equal only if the two phases (liquid and solid) are in equilibrium. If these pressures are also equal to the total pressure, then the gas phase is in equilibuim, too ( three phase -> triple point) -- Olof

And another question: to which substances does this discussion apply? Wood for instance doesn't have a vapor pressure, does it?

Sure it does. This is why I put wood in the garage for a year before putting it in the fireplace: some of its components ( water ) sublimate. However, wood is a multicomponent material, so it is better to think of vapor pressures of each of its components. -- Olof

And yet another one: what is the vapor pressure of solid water at 50 degrees Celcius? AxelBoldt

Higher than the vapor pressure of liquid water at 50 degrees Celsius. The fact that we know that it is higher doesn't imply that we are able to actually measure the value, although it could be estimated by extrapolating data from the temperature range where water is stable.
I've got to admit, though, that this page is quite confusing, and could use a better explanation of all the concepts. -- Olof

comparing and contrasting the dependence of the temperature.......[edit]

In a simple distillation, am I correct in assuming that the temperature of the liquid and vapor are equal? I am trying to compare and contrast the dependence of temperature on the distillate volume of cyclohexane and toluene. I find it easy to compare the temperature with the mixture, but would like to find some contrasting factors.

Email is Chuck

Technical accessibility[edit]

This article could use some work to make it more accessible to people who are not already familiar with the principles of thermodynamics and kinetic theory. I'm sure that schoolchildren and even adults who haven't had basic science education would find the explication of fundamental concepts here to be quite helpful. I've already drafted similar improvements for Phase (matter) (see my 10 July 2005 edits there), which are illustrative of what I have in mind. -- Beland 13:29, 10 July 2005 (UTC)

Collection of Gas over water[edit]

Perhaps a mention should be made about how Vapor Pressure affects the Collection of a Gas over water? In High School Chemistry this is the main use we have for Vapor pressure. Interpretivechaos 02:00, 30 January 2006 (UTC)

Hey you posted on my birthday. But yeah, I agree. I'll add it eventually. --M1ss1ontomars2k4 02:14, 14 May 2006 (UTC)

Vapor Pressure & Temperature[edit]

If we know the Vapor Pressure of a liquid at a given temperature, is there a way to recalculate the vapor pressure at a new temperature.

As an example, I believe toluene has a VP of around 22 at 68degrees. If it was say 80 degrees or 40 degrees can we calculate what the new vapor pressure is?


Vapour pressure relates to temperature in a non-linear way, but you can still predict the vapour pressure using the Clausius-Clapeyron_relation or the Antoine equation.
Clausius -
where P is in torr and T is in Kelvin and R is the gas constant (~8.3145 J mol-1 K-1) and ΔHvap is 38.06 kJ/mol.
I am apparently really bad at using TeX and preview is randomly breaking on it. ln ( P2 / P1) = -ΔHvap / (RT) * (1/T2 - 1/T1)
Plugging stuff in (assuming your VP listed is in.. i have no idea really atm? bar? I'm going to go with bar, and that you mean celsius)
P2 = e^([-38060 J/mol / 8.3145 J/(molK)] * [1/293.15K - 1/277.594K]) * 22 bar
P2 = 52.777
There are other ways of using this relation to do the same thing using calculus aswell.
Antoine - You just look up the numbers and plug them in really. Just make sure you have the correct units/correct formula. Sometimes it is ln, sometimes it is log, sometimes it is kelvin, sometimes it is celsius and various other variations kotepho 2006-03-19 21:03Z


Yes, we need a better column title than 'notes', but 'Boiling Point' is simply misleading. Using 'Boiling point' implies that the temperature listed in the column is the temperature at which liquid and gas coexist, as temperature is changed at constant pressure. That's not what these temperatures in this column are. Instead, this column is used to note the temperature at which the given vapor pressure is measured.

-- Olof

P-v diagram[edit]

So, the vapor pressure is the ordinate value of the horizontal lines in a P-v diagram? (Double-checking) Titoxd(?!? - cool stuff) 21:49, 25 March 2007 (UTC)

MAJOR ouch with equating vapor pressure with saturation vapor pressure. A solid or liquid can exert a vapor pressure below the saturation limit!!! The saturation vapor pressure is a special case where the vapor pressure reaches its maximum value. Therefore saturation vapor pressure (aka saturated vapor pressure) symbol p0 really needs its own section.Softwarestorage (talk) 01:46, 8 August 2008 (UTC)

Proposal to delete section on "Examples of vapor Pressure"[edit]

This article is meant to explain what vapor pressure is, its relation to temperature and to boiling point, its units and its usages. There is really no reason to have picked 14 substances at random and provide their vapor pressures at some single specific temperature.

This is an encyclopedia article ... not a data handbook. Hundreds of substances and their vapor pressures across a range of temperatures can be found in data handbooks such as the Handbook of Chemistry and Physics, Perry's Chemical Engineers' Handbook and Lange's Handbook of Chemistry. Similar data is also readily available online at the NIST Chemistry WebBook website. So why does this article need a section on "Examples of vapor pressures"?? I propose that section should be deleted. mbeychok (talk) 07:57, 16 November 2008 (UTC)

Since no one has responded or raised any objections, I am proceeding to remove the section on "Examples of vapor pressures". mbeychok (talk) 03:57, 18 November 2008 (UTC)
The unit chapter is also rather useless, in my opinion. It is simply a duplicate of the table in in the Pressure article (where it belongs). What do you think? --WilfriedC (talk) 18:25, 18 November 2008 (UTC)
Hi, WilfriedC: Yes, the table is the same as the one in the Pressure article. That table is really the template {{Pressure units}} and that template was developed so that it could be used in multiple articles where it would be useful. It wasn't meant to be used in only one article. In my opinion, it serves a useful function in this article for those who won't take the time (or not know how) to go to the Pressure article. mbeychok (talk) 02:23, 19 November 2008 (UTC)
I had a look at the article before you deleted the "Examples of vapor pressure" table, and I am of the opinion that those examples contribute a lot to the quality of the article, because they help the reader to gain an idea of the order of magnitude the vapor pressure can be, and why some well-known substances behave in certain ways, in vacuum (for instance). Hence I raise my objection to that deletion. Blind cyclist (talk) 19:38, 29 September 2009 (UTC)
Since no one has responded or raised any objections, I am proceeding to restore the section on "Examples of vapor pressures". Note that I have waited much longer, after proposing this change, than the editor who so quickly deleted it. Blind cyclist (talk) 10:24, 17 October 2009 (UTC)
Blind cyclist, that dig about "editor who so quickly deleted it" was unwarranted. That editor (me) explained that vapor pressure data (or examples, if you wish) are readily and abundantly available in handbooks and online in the NIST Chemistry Webbook. Your only reason seems to be that you thought some randomly chosen examples would be helpful. That is what eventually "dumbs down" most technical articles in WP. mbeychok (talk) 16:16, 17 October 2009 (UTC)
The article already states that all liquids boil when their vp reaches the pressure of the environment. I don't see any need to keep the helium entry. The only useful information in there is the boiling point of He, clearly out of context. I am going to remove the entry. Kbrose (talk) 18:12, 8 February 2010 (UTC)
First of all, it wasn't a dig, and anyone can see it: you proposed a change and then moved to implement it after only 2 days. I would be surprised if you would argue with this. What I wouldn't be surprised and in fact expect you to argue about, is the usefulness of those examples. Throughout my studies of material science, I found such examples (of any physical characteristic) most enlighting, as they allowed me to realize the inner workings on atomic and molecular level, by comparing other properties of given example substances. I don't know whether the vapor pressure data are readily available or not, but it is conceivable that they will not be online at a certain point (just as many references already aren't). So that's another argument to keep the table where it is, instead of relying on the availability of outside sources. Furthermore, it's much nicer to have a number of substances with differing vapor pressures listed in the same place where the subject is discussed, rather than having to search for them somewhere else. Finally, I completely disagree that "this is what eventually dumbs down most technical articles". Besides, this isn't just a "technical article", it is supposed to have a degree of educative methodology, and I am of the opinion that examples are essential in that context. Blind cyclist (talk) 00:21, 18 October 2009 (UTC)
Opposed. The table is very useful and educational and give the interested reader a sense of variability of pressure and a sense for the property as it relates to some common substances. I ordered the table according to increasing pressure to enhance the illustrative power. Kbrose (talk) 01:11, 18 October 2009 (UTC)

I am in favor of the Examples section, but the Helium entry doesn't belong there. The vapor pressure is 1 atm, because the chosen temperature is the boiling point of helium. That's not a property of helium; every liquid has a vapor pressure of 1 atm at its boiling point. Spiel496 (talk) 18:58, 5 December 2009 (UTC)

First of all, the statement that "every liquid has a vapor pressure of 1 atm at its boiling point" is not quite correct. A liquid at boiling point has a vapor pressure equaling that of the environment. Second, and more importantly, the example for Helium should stay to illustrate just this very point.Blind cyclist (talk) 10:29, 9 December 2009 (UTC)
I have now added an explanation for the vapor pressure of Helium boiling point. But I am quite surprised that people feel happy to just come and delete things at will, without any consultation. Deleting should be hard and debated, not as easy as you do it. Don't wonder when outsiders are outraged at deletionism.Blind cyclist (talk) 10:57, 9 December 2009 (UTC)

Vapour pressure data examples of electrolyte solutions missing[edit]

Some data on the most typical electrolyte solution (NaCl) could be illustrative to the specific of electrolytes vs nonelectrolytes.-- (talk) 12:40, 9 October 2013 (UTC)

This is supposed to be an encyclopedia article, not some incomprehensible, esoteric technical essay by an anonymous visitor.[edit]

I am deleting the section added by an anonymous visitor entitled "Technical Explanation fo Vapor Pressure" (the excessive capitalization was by the anonymous visitor ... not by me).

It is completely unnecessary and esoteric essay. Here are but a few quotes from that section:

"sigmoidal behavior", "mathematical inevitability" and this prize bit of gobbledy-gook "Eliminating the unstable region of the equation of state curve using the equilibrium criteria that the chemical potentials of all phases present in a system must be equal, we see that there is one pressure a on a given isotherm at which the specific volume of the substance takes on two values."

99% of the people reading this article won't understand what in the heck that section was talking about. mbeychok (talk) 21:50, 14 October 2009 (UTC)

What is the normal boiling point of propane??[edit]

There is a discrepency between this page and the one for volatility regarding the normal boiling point of propane. This page says its -42.1 deg C and the volatility page has it listed as -43.7 deg C. Which is correct? (talk) 17:32, 29 October 2009 (UTC)

The volatility page misquoted its number as deg C, when in fact it used the F value. I changed it to the correct value in C. Kbrose (talk) 19:41, 29 October 2009 (UTC)

Saturated Air !!!!! ?????[edit]

'air at sea level, saturated with water vapor at 20 °C'. The author commits a common solecism by imagining that 'saturation' refers to a state of the air, whereas in fact it refers to a state of the water vapour in the air. 'Saturated air', to be consistent with the definition given earlier in the section, should be understood to be gaseous air in equilibrium with liquid air. '...air... saturated with water vapour' should be replaced by '..air containing saturated water vapour..'. This kind of muddled thinking undermines the authority of the article. —Preceding unsigned comment added by (talk) 06:48, 11 November 2010 (UTC)


This (really nice!) article has three different categories within chemistry! That seems excessive. Anyway, two of those, namely Category:Chemical properties and Category:Physical chemistry have a common subcategory, which seems even more to the point: Category:Thermodynamic properties. I'm going to move it to that subcategory. (talk) 00:27, 6 December 2010 (UTC)


In-line reference citations should be provided for each section, and preferably for each paragraph, to improve WikiProject Engineering quality classification.Thewellman (talk) 22:24, 1 July 2011 (UTC)

Mistake in introduction[edit]

In the introduction there is written: "In plain terms, a liquid evaporates at all pressures below its vapor pressure, while remaining stable at pressure above the vapor pressure."

That's not true! If liquids were stable at pressures above its vapor pressure, then your wet cloths would not dry under normal conditions. There would also be no clouds and some other minor problems.

Right: A liquid is evaporating at pressures above its vapor pressure, while at pressures below its vapor pressure it is boiling.

As my English is not that good, I don't want to change it directly in the article.

--LordOider (talk) 18:21, 1 February 2012 (UTC)

Four articles for one phenomenon[edit]

… namely, Saturated fluid, Vapor pressure, Saturation vapor density and Vapor–liquid equilibrium, IMHO is too many articles. I propose to reduce the number of articles to no more than two (but not necessarily to merge all this into one article). Incnis Mrsi (talk) 06:43, 6 August 2012 (UTC)
For two resulting articles, I can propose the following configuration:

  • "Vapor pressure" absorbs "Saturation vapor density" and is moved to a new title Saturated vapor (currently a redirect to "Saturated fluid"). Rationale: currently there is a large article about a property (pressure) of a fluid (the [saturated] vapor) which is not defined explicitly.
  • "Saturated fluid" is merged into "Vapor–liquid equilibrium" (because the latter is necessary to understand the former), except aforementioned redirect (and its spelling variants).

There will be two article: one about the equilibrium and its implementations. Another will be focused on the gas phase, with its properties. Incnis Mrsi (talk) 09:56, 6 August 2012 (UTC)

I agree with the idea of ending up with two article rather than four. However, I strongly disagree that the title of the Vapor pressure article should be changed. mbeychok (talk) 15:29, 6 August 2012 (UTC)
The Title of Vapor Pressure should be kept, because As a Perfumer, and not a Chemist, "Vapor Pressure" is something that needs to be learned, and that is the subject searched for by my profession seeking to learn how it relates to Perfumery. - Thanks — Preceding unsigned comment added by Pkiler (talkcontribs) 15:39, 20 August 2012 (UTC)
Even if the article title is changed, a redirect will be maintained to the page, so Perfumers and Chemists and Anyone Else For That Matter can easily find it. The more important concern is not what term is used by a specific group (e.g. perfumers, ant farm owners, sanitation workers, etc.) but rather what the most common term is overall. However, I think "vapor pressure" is the more common term in general, so I would support keeping that title. (talk) 16:25, 10 September 2012 (UTC)
With mergers "Saturated fluid" → "Vapor–liquid equilibrium" proceeded and "Vapor pressure" ↔ "Vapor–liquid equilibrium" rejected, the only remaining question is the fate of "Saturation vapor density". Merge it to "Vapor pressure", remain intact, or what? Incnis Mrsi (talk) 16:32, 10 September 2012 (UTC)
This user (Incnis Mrsi) has gone off the radar, and so an anonymous removal of their suggested merging shall occur. — Preceding unsigned comment added by (talk) 06:34, 1 November 2012 (UTC)
There seemed to be developing consensus. Why is there a need to rush the discussion?--Yannick (talk) 16:39, 4 November 2012 (UTC)

Since I referred to this discussion when redirected the Saturated fluid (edit | talk | history | links | watch | logs), I reiterate here: “Vapor–liquid equilibrium” is the only valid target for redirect, because it is this equilibrium what is necessary to understand before understanding what is a “saturated fluid”. Such edits as are purely degrading: a saturated fluid is not necessarily a vapour. Incnis Mrsi (talk) 20:52, 13 February 2013 (UTC)