We know water is floating around in the air. Why is it called a vapor and not gas? I’ve also heard CO[sub]2[/sub] is a vapor, and that O[sub]2[/sub] is a gas. 
Someone clear this up for me.
I believe that a vapour consists of droplets, whereas a gas consists of (atomic or molecular)particles.
From http://dictionary.reference.com/
So generally, if it’s a solid or liquid at normal conditions (presumably STP) and you come accross it in gaseous form, then it’s a vapor (i.e.water vapor). If it’s a vapor at normal conditions then it’s a gas. Right?
Vapor n.
The gaseous state of a substance that is liquid or solid under ordinary conditions.
The dictionary is your friend. 
Oh yeah, as Mangetout says, apparently particulate matter is also called vapor.
As far as I know the two are synonyms in general life, but in chemistry terminology, a vapour is something that, under normal conditions is not a gas, but which conditions have made into a gas, ie it has been vapourised.
There are three states of matter; solid, liquid and gas and under STP (standard temperature and pressure) every chemical compound and element has a natural state.
In general, you increase temperature or reduce pressure to turn change from a solid to a liquid and then to a gas and cool things down, or increase pressure to turn them from a gas to a liquid to a solid.
There are exceptions; iodine I[sub]2[/sub] does not have a liquiid state and when you heat solid iodine you get iodine gas.
So to give my opinion on your OP, water in the air is vapour because at STP, it is a liquid, but it has been turned into a gas, or vapourised. Whereas, O[sub]2[/sub] is a gas at STP and it has not been vapourised.
Didn’t we do this just a few days ago, crafter_man? In a thread in which you participated?
http://boards.straightdope.com/sdmb/showthread.php?s=&threadid=175040
Well I didn’t really participate it in very much, other than defining what an ideal gas was. I should have read the whole thing; I didn’t even know the gas vs. vapor question was being discussed.
But isn’t water ordinarily not a liquid under “normal” conditions? It’s been my experience that water usually “wants” to be a gas at ambient conditions. If I place a small cup of water on my desk, the water will eventually disappear…
Crafter_man, the gradual dribbling away of molecules is very different from the boiling away of a gas. Water definitely “wants” to be a liquid, but thermodynamics dictates that there will always be a few molecules with enough energy to break away.
I think the “vapor” question is historical. The word is actually Latin, and means “steam.” While today’s scientists use the word to describe a suspension of droplets or particles in air or gas, and distinguish it from an actual gas, that distinction was not well understood when the word was coined. Hence, “water vapor” is historically sound, but it’s inaccurate by today’s definitions.
G’day
A vapour can be condensed (to a liquid or solid) by compressing it without reducing the temperature.
A gas cannot be condensed by compression alone: you have to cool it as well.
So whether a given substance is a gas or a vapour depends on the temperature: below its critical temperature it is a vapour (and capable of existing in equilibrium with a solid or liquid of the same chemical composition); above its critical temperature it is a gas (and any solid or liquid will already have sublimed, melted, evaporated and/or boiled away).
Do not confuse either vapour or gas with a dispersion or suspension of droplets or powder.
Regards,
Agback
I’m still unclear about this.
Right now in my office there are millions of water molecules floating around in the air. Do these water molecules “want” to be a liquid or gas? It would seem to me that, if they wanted to be a liquid, they would simply condense into the liquid phase.
I guess what I’m saying is that I like this definition better:
I understand that water vapor can be liquefied simply by compressing it until it’s partial pressure reaches the saturation pressure. I believe this is also true for CO[sub]2[/sub], but not O[sub]2[/sub]. Using this definition, is CO[sub]2[/sub] considered a vapor, and is O[sub]2[/sub] considered a gas?
I think Agback explained it quite clearly. Thanks.
Under ‘normal’ conditions (with temperature above the freezing point and below the critical point of water) water ‘wants’ to be a liquid in equilibrium with a vapour of exactly a certain partial pressure (the saturated vapour pressure of water at ambient temperature). If there isn’t enough to fill the room with vapour at saturated vapour pressure, it prefers to be a vapour with a partial pressure below saturated vapour pressure, without the liquid.
At room temperature and pressure, water exists as a vapour in equilibrium with a liquid. If the partial pressure of the vapour is below the saturated vapour pressure of water at that temperature, more water will evaporate. If the partial pressure of the vapour exceeds the saturated vapour pressure of water at that temperature, more vapour will condense (typically forming ‘dew’ on cool surfaces).
The saturated vapour pressure of a substance depends on temperature: raise the temperature and the saturated vapour pressure will rise. If you raise the temperature high enough that saturated vapour pressure exceeds teh prevailing air pressure, the water vapour will exert enough pressure to blow bubbles int eh liquid: the water will boil.
Regards,
Agback
When a lady gets the vapors, she rests on a convenient fainting couch.
When she gets gas, she leaves the room.
Simple: a “vapor” is any gas whose temperature is below boiling.
If the weather outside was 70 kelvin, then we’d have to call the air by the name “vapor” rather than “gas.” (N2 boils at 77K, O2 boils at 90K)
For example, if you have an unsealed tank full of air, and the air contains some “water vapor”, then if you heat the tank and its contents above 100C, the “water vapor” within the tank doesn’t change a bit. However, the terminology changes. We’re now supposed to stop using the term “vapor” and must call it by the name “gas.”
Where all this stuff DOES make a difference is in situations where some of the liquid substance is present. If our tank has a pool of water in it as well as the air and the “water vapor,” then when we try to heat it above 100C, the liquid water boils away and drives the air out of the tank, leaving a gas cloud made of H20 molecules.
Or the reverse: if we have a very hot tank which is entirely full of H2O gas, and if we cool it below 100C, then much of that gas will condense on the walls of the tank, and only a little gas will remain in the space. And now we’re supposed to call that gas by the name “vapor.”
PS, the popular definition of the word vapor is “any vaporous matter.” In other words, if you spray water into the air with a high pressure pinhole sprayer, then in the popular sense of the word, those tiny droplets of water are a “vapor.” They’re a “vapor” even though they are actually liquid droplets. Throw a handful of photocopier toner across the room, and the room fills with black “vapor.” Great confusion arises when people believe that the term “water vapor” refers to very small droplets. Nope. It refers to a dry gas, and has nothing to do with suspended liquid droplets. (Ah, but sometimes when you cool a gas in the presence of nucleation centers, some suspended liquid droplets form. Arrg! Those droplets aren’t vapor, since they’re a liquid! The popular definition collides and mixes with the strict scientific definition, and nobody can pry them apart again.)
Thanks for all the responses. Looks like I’ve learned a few things here about the mysterious behavior of water. So can someone confirm the following?
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Let’s say I have an oven set to 200 °C. I pump water into it, all the while maintaining the oven (and the inside walls) at 200 °C. Are you saying that, no matter how much water I pump into the oven, the water will be in the gas phase?
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I have a piston and cylinder. I fill the chamber with air, then slowly compress it. Assuming the temperature remains at ambient, will water condense out of the air after I reach a certain pressure? Is this why water accumulates in the bottom of air compressors (and rusts them out)?
That depends on whether your oven is airtight. If it is not, then the gases and vapours inside will leak out, and the pressure will never rise above 1 atmosphere. But the saturated vapour pressure of water at 200 C is above one atmosphere, so you will never achieve a saturated vapour. Whatever water you introduce will boil away.
But if the oven is airtight (and therefore more of a pressure cooker) you will be able to achieve a situation with liquid water at 200 C in equilibrium with a high-pressure vapour (“live steam”).
Condensation depends on the partial pressure of the vapour, not the total pressure, but yes.
Here this evening the relative humity is 62% (the weather report just told me so). That means that the partial pressure of the water vapour in the air here now is 62% of the saturated vapour pressure of water at the prevailing temperature (which is 19.7 C). If I were to take a sample of the air in this room in an airtight cylinder, and use a piston to compress it to 62% of its original volume, but maintain its temperature of 19.7 C, I would raise the pressure in the cylinder to 1.613 atmospheres. I would also, incidentally, raise the pressure of the water vapour to satuartion point. Any further compression wouls induce condensation on nucleation sites and cool surfaces.
Regards,
Agback