what's the difference between gas and vapour? [duplicate]








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[What is the difference between "vapour" and "gas"?](/questions/150235/what-is-the-difference-between-vapour-and-gas)

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In the lecture, our teacher said that the significant difference between gas and vapour is we can't liquefy gas just by compressing it but it's not the same in the case of vapour."
So, I'm confused about the possible reason behind it, Is it because vapour contains both the liquid and gaseous phase?




[Wikipedia: Vapor](https://en.wikipedia.org/wiki/Vapor)



> 
> In physics, a vapor(US) or vapour(UK, CA) is a substance in the gas phase at a temperature lower than its critical temperature, which means that the vapor can be condensed to a liquid by increasing the pressure on it without reducing the temperature.
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> 


Note that for some reasons, I have not found vapor definition in [IUPAC Gold Book](https://goldbook.iupac.org), the authoritative source of definitions of terms used in chemistry.


Gases(1) = vapors(1) + (permanent) gases(2)


Every vapor(1) is a gas(1), but a gas(2) is not a vapor(1).


Gases(2) that are not vapors(1) are historically called permanent gases (or just gases). That does not mean they cannot be liquefied.




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In everyday life of chemists, a vapor(2) is often used in a narrower sense as a gaseous phase of substance with boiling point above room temperature. It is eventually mixed with other gases like air, if the total gas pressure is greater that saturated vapor pressure.


Water forms both vapor(1) and vapor(2), while gaseous chlorine is vapor(1), but not vapor(2). Chlorine is often consider as a gas, but it can be liquefied at room temperature by pressure about $\pu{8 atm}$. Similarly for propane, isobutane or butane, components of $\text{LPG}$.


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