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I found the following definition of Henry's Law in my textbook:

"The partial pressure of the gas in vapour phase is proportional to the mole fraction of the gas in the solution."

My questions:
(a) Why should the undissolved gas be in vapour phase?

(b) As i understand it, a gas is called a vapour if it is below its critical temperature.

(c) But from other sources on the Internet (such as Wikipedia )there is no such restriction on the temperature for Henry's law to hold[I mean the temperature is not supposed to be below the critical temperature of gas to use Henry's law].

(d) Can a gas above it's critical temperature be dissolved in a liquid?

P.S:
(a) I am having trouble understanding why the word vapour phase is used in the definition. Although I do understand what Henry's Law is trying to say.

(b) I tried to search all over the internet but could not find any satisfactory answer.

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  • $\begingroup$ Rather vapour/gas in gaseous phase.//1 Why should not ? //2 + usually if it is possibnle to have it as liqiud at give circumstances as well // 3 no such restiction // Even helium with crit. T few K dissolves in liquids $\endgroup$
    – Poutnik
    Apr 22, 2021 at 10:26
  • $\begingroup$ @Poutnik [1]I have no problem with the gas being a vapour but stating explicitly in the definition means that if it is not a vapour then we can't use the Henry's law and that is what bugs me. [2] I can't find a satisfactory example of helium being dissolved in liquid at a temperature above its critical temperature on the net ; please clarify. $\endgroup$
    – JustCurious
    Apr 22, 2021 at 13:42
  • $\begingroup$ Henry's law is explictly about dissolving of gas in supposedly nonvolatile solvent. This law is closely related to the Raoult law, considering volatile liquid components and their vapours. // What example do you need ? All gases dissolve in liquids, regardless of their critical temperature, just their Henry constants differ. $\endgroup$
    – Poutnik
    Apr 22, 2021 at 14:09
  • $\begingroup$ @Poutnik [1]So the definition in the book (or rather the phrase "gas in vapour phase") is unnecessary? [2] I am confused as to why the word "vapour phase" has been used...(pls see ques part a,b and c)...[3]Any well established example of a gas dissolving in a liquid when the temperature is more than the gas's critical temperature. $\endgroup$
    – JustCurious
    Apr 22, 2021 at 14:17
  • $\begingroup$ 1/ a gas is a component of gaseous phase, or part of mixed gas without mentioning the phase. 2/ There are sometimes disagreements when to call a gas vapour, but expression "vapour phase" is not used. 3/ The fact that you live is such an example. You do breathe oxygen, don't you ? What is its critical temperature ? $\endgroup$
    – Poutnik
    Apr 22, 2021 at 15:07

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