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?

  • 2
    $\begingroup$ Generally, if you are confused by stuff, taught in school, the first thing you should do is reviewing available offline and online sources. Asking here should be rather the last than the first step how to get the answer. $\endgroup$
    – Poutnik
    Nov 28, 2022 at 7:14

1 Answer 1


Wikipedia: 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.

Note that for some reasons, I have not found vapor definition in IUPAC Gold Book, 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.

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 $\text{LPG}$.

  • $\begingroup$ This is an interpretation on general use of the terms. A vapor is a gas in possible equilibrium with its liquid [or solid] at a temperature below its critical T and either in contact with the liquid or at the equilibrium vapor pressure. If not at a sufficient pressure to condense and not in contact with the liquid the vapor behaves as a gas [nonideal] and the term becomes colloquial. For example, water in the atmosphere is "water vapor", CO2 in the atmosphere is a gas or simply "carbon". $\endgroup$
    – jimchmst
    Nov 29, 2022 at 22:00
  • $\begingroup$ Out of space. [water gas! has an entirely different, archaic, meaning] This is in American English; I have no idea what is going on across the pond [we buy "gas" they buy "petrol", hopefully after fossil fuels are gone that problem will vanish] or in other languages. Language can cause minor problems that can be alleviated by concentrating on the actual chemical and physical properties of gases and the equilibria among solids, liquids and "vapors" [or "gases"]. $\endgroup$
    – jimchmst
    Nov 29, 2022 at 22:11

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