I have read the definition of vapour on Wikipedia, but I don't seem to have a clear idea about the difference between a vapour and a gas. I know what it means when it says, "gas at a temperature lower than its critical temperature".

enter image description here

In this graph, the blue curve, means the part where gaseous and liquid phase coexist, so did the scientists coined the term Vapour for only this reason so, that we can have orderly fashion of different states of matter ?

My question is Are we naming vapour and gas to the same thing just at different temperature or they are distinct in some other way too? Are there some other things which justify that vapour and gas are actually different things than just a gas below and above critical temperature ?

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    $\begingroup$ To OP, yes vapour refer to a gas in equilibrium with its liquid (Indeed we have vapour pressure...) or - as you said - can be brought in that condition by mean of compression only. That is. $\endgroup$ – Alchimista Apr 19 at 13:41
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    $\begingroup$ Beside the most upvoted answer (gas which is liquifiable by pressure), I heard another usage of vapor: that is colloid liquid in gaseous media. But I am not sure, if it is a correct terminology on English. $\endgroup$ – peterh Apr 19 at 21:18
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    $\begingroup$ @peterh, Colloid consisting of a liquid dispersed in a gas is called aerosol. Vapor is a single phase. $\endgroup$ – M. Farooq Apr 19 at 23:58
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    $\begingroup$ There is even the complication of steam. Sorry to bring it up :( $\endgroup$ – Alchimista Apr 20 at 14:21
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    $\begingroup$ Seems like that phase-diagram is mislabeled: the entire vapor/gas areas should be "gas", and then the gas-side of the phase-transition lines could also be called "vapor". I get the feeling that whoever labeled that graph was thinking roughly that in labeling the entire area beneath the phase-transition line "vapor". $\endgroup$ – Nat Apr 21 at 1:29

tl;dr "Gas" and "vapor" aren't mutually exclusive. Generally:

  • a gas is any material that'd fill a volume to its boundaries; and

  • a vapor is a gas-like material that's associated with a condensed-state transition.

It's a bit misleading for a state-diagram to label a region "vapor" in a manner that might imply that a vapor's not a gas.

State-diagrams should refer to "gas", not "vapor".


  • A gas is material that would fill any volume up to the boundaries, ignoring gravity and the like.

  • A vapor is a gas or gas-like material that's associated with a condensed-state transition.

These aren't alternatives; something can be a gas or/and a vapor. Examples of each combination:

Not vapor Vapor
Not gas: A rock. Suspended droplets in fog.
Gas: Helium. Steam.

State-diagrams are primarily about states-of-matter, i.e. the gas/liquid/solid thing, so usually a state-diagram should label things as "gas", not "vapor".

We can talk about "vapor" to stress a relationship with a condensed phase.

Often we talk about

  • vapor pressure,

  • vapor/liquid equilibrium,

  • vaporization,

etc., in terms of "vapor", rather than "gas", to stress the relationship with the condensed phase.

To be clear: once something vaporizes, it's a gas. That said, we might continue to refer to it as a vapor as well, stressing the physical closeness to the condensed state.

Discussion: Where would vapor be on a state-diagram?

Since "vapor" isn't a state-of-matter, it doesn't really have its own region in a state-diagram. Instead, it'd overlap with the gas-state near boundaries with condensed states (such as liquid and solid states).

A gas would be most vapor-like right at a boundary with a condensed state. The further a state is from a transition between a condensed-state and gas-state, the less vapor-like it'd be.

  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. Please note that further comment may be deleted without further warnings. $\endgroup$ – Martin - マーチン Apr 21 at 17:51

Vapor is a much older word alluding to dampness and it was not coined by scientists. It is in use since the 1300s. The actual meaning of meaning of vapor is "Matter in the form of a steamy or imperceptible exhalation; esp. the form into which liquids are naturally converted by the action of a sufficient degree of heat. This is the original 13th century meaning of vapor. The German equivalent of vapor is Dampf (which is relatively clearer in terms of feeling- imagine entering a basement which is damp from water vapor). Gas is relatively a new word as per the unabridged Oxford English Dictionary, and it entered English in the 1600s.

In fact, the OED mentions the distinction "Gas: A substance in a state in which it expands freely to fill the whole of a container, having no fixed shape (unlike a solid) and no fixed volume (unlike a liquid); spec. (distinguished from a vapour) such a substance above a critical temperature such that it cannot be liquefied by the application of pressure alone"

You can also think of the general meanings of vapor in scientific usage:

A substance which is a liquid at room temperature or under ambient conditions, and if it is heated enough so that the liquid evaporates (same root of word), you can call that as vapor of that substance.

Examples: Mercury is a liquid metal, when it is heated enough in a an electric bulb, it becomes a gas, hence the name mercury vapor lamp. It is not called a mercury gas lamp.

It is not necessary that heat be involved. You will find the usage "cold vapor method for Hg analysis" again for mercury which means that Hg was a liquid in a sample but by blowing a gas over it, it has been evaporated.

The recent meaning of vapor from OED, "Vapor: In modern scientific use, a fluid that fills a space like a gas but, being below its critical temperature, can be liquefied by pressure alone."

This is clear enough. Now apply this distinction to your phase diagram and notice the position of the critical point. The vapor "state" is below the critical point.

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    $\begingroup$ But nobody calls CO2 nor N2O vapour. $\endgroup$ – Poutnik Apr 19 at 16:59
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    $\begingroup$ Well, perhaps because nobody usually works with liquified carbon dioxide and liquid carbon dioxide does not exist under ambient conditions. But imagine if you had a transparent chamber full of liquid CO2, below its critical point, it would be fine to call it CO2 vapor in equilibrium with liquid CO2. Google Scholar shows the usage of carbon dioxide vapor in equilibrium with its liquid. $\endgroup$ – M. Farooq Apr 19 at 17:05
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    $\begingroup$ IMHO, usage of the term vapour is context dependent and is used for gas in context of liquid evaporation, condensation or equilibrium. So atmospheric CO2 is not considered vapour, even if it is below its critical point. OTOH, atmospheric gaseous water is considerer vapour, as out is related to water evaporation and condensation. $\endgroup$ – Poutnik Apr 19 at 17:17
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    $\begingroup$ You are right, again, I think the reason is that carbon dioxide does not exist in liquid phase under ambient conditions. The context is always important. $\endgroup$ – M. Farooq Apr 19 at 17:26
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    $\begingroup$ In general "gaseous" state of substances which are liquid/solid at room temperature is called a "vapour" in the Layman's term. Scientific definition might involve critical point but the normal usage doesn't consider it. For example, fluorine gas and chlorine gas vs Bromine Vapours and Iodine vapours. $\endgroup$ – Nisarg Bhavsar Apr 20 at 1:30

The normal use distinguishes "vapour" from permanent gas

At normal lab conditions there is a (fairly obvious) distinction between things that could exist as liquids and things where no liquid phase is possible. Oxygen, for example, is a permanent gas, but dichloromethane is not. But the vapour pressure of dichloromethane is pretty high and there will be a fair amount of it above the liquid in a flask of the substance.

In this sense it is useful to distinguish between a vapour and a gas.

But the distinction is sometimes a little blurry. Butane would normally be called a gas but will liquefy under moderate pressure; Nitrogen is a gas but is commonly used in laboratories as a very cold liquid.

But, under normal lab conditions both butane and nitrogen would be described as "gas" but the "gas" above boiling dichloromethane during a distillation would normally be described as a "vapour".

So the terms are a little useful, but not very precise in any technical sense.

Clarification note

The chart in the question uses a more stringent criterion for the regions called gas and vapour that only labels a region gas at a temperature above the critical point where there cannot be any distinction between the two at any pressure. I think conventional conversational use by many chemists is broader than that and considers normal atmospheric pressure as well as temperature which is more useful and pragmatic at lab conditions.

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    $\begingroup$ I would appreciate if you could be more clear as to what you are trying to convince because it did not strike me directly. $\endgroup$ – Goarkz Apr 19 at 15:29
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    $\begingroup$ @Goarkz I'm arguing that the vapour/gas distinction is not a precise technical term but a convenient way to differentiate between substances that depends on their normal state under lab conditions. Things that are normally gases are "gas'; things that are normally liquid have their gas phase described as 'vapour'. $\endgroup$ – matt_black Apr 19 at 15:49
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    $\begingroup$ Alright got it now, But then you see the graph that I have used from Wikipedia, they have a distinct place for vapour, then whats the explanation for it? $\endgroup$ – Goarkz Apr 19 at 15:58
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    $\begingroup$ @Goarkz As far as I can tell their distinction is less useful but more precise: they only label the region where there is no distinction between gas and liquid at any pressure. My definition is closer to normal usage which considers both pressure and temperature. $\endgroup$ – matt_black Apr 20 at 8:36
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    $\begingroup$ Yes. Actually after reading all the answers, now I see why you used the examples which you used. $\endgroup$ – Goarkz Apr 20 at 12:04

I'm surprised the OED has such a strict definition for gas. I could not find a strict definition in the IUPAC color books (certainly not in the gold book). Presumably these words are in such common use that their definition is assumed understood or easily found. The analytical compendium (orange book) and physical chemistry book (green book) mention vapour (or vapor) in entries for the meaning of the abbreviation g:

g gas or vapour

The green book (physical chemistry) also has an entry for vapour pressure and for the abbreviation vap:

vaporization, evaporation (liquid→gas) vap

and definitions for a variety of acronyms that include the word vapor.

There is no suggestion there that gas should be restricted to the description of substances in their supercritical phase.

Curiously the IUPAC books do not adopt a uniform spelling convention. Some choose the spelling vapour, others vapor.

In common usage, a vapor is a gaseous substance emanating from a condensed phase. The scientific meaning is roughly consistent with common use. Vapor implies the existence of a condensed phase that is the source or destination of the gas, or with which the gas may be in equilibrium; while gas does not make such an assumption.

The origin of vapor is presumably Latin with earlier roots in Ancient Greek.

The origin of the word gas appears to be the Greek word chaos by way of Dutch:

Coined by chemist Van Helmont. Perhaps inspired by geest (“breath, vapour, spirit”) or by chaos (“chaos”), from Ancient Greek χάος (kháos, “chasm, void”).

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    $\begingroup$ "spec. (distinguished from a vapour), ...", offset with a semicolon, is OED's way of saying that this distinction isn't necessarily made, but that if someone is treating "gas" and "vapor" as two different things, then this is the distinction that they probably mean. $\endgroup$ – hobbs Apr 19 at 21:26

In college, I had a thermodynamics teacher who was awesome. He had a way of explaining things that were accurate and easy to understand. He explained this difference to us this way:

A gas will not condense into a liquid with an isothermal compression. (i.e. an Ideal Gas)

A vapor will form liquid when isothermally compressed.

His example: When you boil water, you get vapor. Hold a spoon over the kettle and it will get water droplets on it.

Steam is heated to a much higher temperature. "If you hold a spoon in from of a steam jet, there will be no condensation. Although, I suggest you not do that because you will be burned." (He tried to have a sense of humor.)

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    $\begingroup$ That's consistent with the definition of permanent gas $\endgroup$ – Buck Thorn Apr 20 at 18:41
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    $\begingroup$ Back when I was taking thermo (and when I was a practicing Chemical Engineer), the distinction between gaseous water and water vapor was similar. Dry steam (thermal-engineering.org/what-is-dry-steam-definition) is a gas; it's superheated beyond a point of being in equilibrium with liquid water at that pressure. Wet steam is vapor. $\endgroup$ – Flydog57 Apr 20 at 21:57
  • $\begingroup$ Isothermal compression seems like a reasonable rule-of-thumb, though it'd seem a bit off to me. For example, we wouldn't tend to say that a nebula is composed of vapor, regardless of its temperature. Isothermal-compression points in the right direction, but taken literally it would over-extend to near-vacuum conditions. $\endgroup$ – Nat Apr 21 at 1:38
  • $\begingroup$ Except, we aren't talking astrophysics. We're talking about earth at 1atm. In a vacuum, things behave differently. Seems like, if that's what the OP was referring too,, that we be stated in the question. $\endgroup$ – Scottie H Apr 21 at 15:20

Vapor (or steam for H2O) can be used for the gaseous form of a particular substance which at normal T and P are solid or liquid. They are all gases although not always ideal (steam is certainly not ideal below its critical T).


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