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A number of books mention "solid-in-gas" and "liquid-in-gas" types of gaseous solutions.

However, for something to dissolve in a gas, it must be in its vapour phase. So, for instance, why wouldn't iodine vapours in air be a gas-in-gas solution?

Or, for that matter, since water vapour is gaseous water, why wouldn't water vapour in air be a gas-in-gas solution?

One reason I can think of is that we probably mention the main state these substances are in at room temperature. But then, water vapour also exists at room temperature (though water is obviously liquid).

How do I understand it?

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    $\begingroup$ Can you give the titles and authors of such books? $\endgroup$
    – Karl
    Dec 12, 2021 at 18:04
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    $\begingroup$ I actually found a paper using the term "gaseous solution", from 1928. pubs.acs.org/doi/pdf/10.1021/ja01391a006 $\endgroup$
    – Karl
    Dec 12, 2021 at 22:23
  • $\begingroup$ @Karl a gas mixture is a solution. It is just trivial so the explicit term is unusual, and became desuete if ever was common. IUPAC reserves the term to solutions in a liquid or a solid,for the above reason. A mixture of gas is always one phase. $\endgroup$
    – Alchimista
    Dec 13, 2021 at 8:41
  • $\begingroup$ @Alchimista IUPAC reserves the term for liquid or solid cases, ergo a gas mixture is NOT a solution. qed ;) $\endgroup$
    – Karl
    Dec 13, 2021 at 21:38

2 Answers 2

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Solid/liquid-in-gas solution is factical nonsense, being communication shortcut for a vapour of solid/liquid in a gas.

Solid sublimates/liquid evaporates into space not occupied by a condensed phase. Gas has nothing to do with that.

Mixture of gases is and is not solution, depending on the point of view.

It is a solution because it is a homogenous fluid mixture.

It is not a solution, because air/major gas does not participate in bringing the solid/liquid to gaseous phase at all. There is no action of dissolving, tearing a molecule from the condensed phase by molecules of gas. The vapour would be there even if there was no other gas at all. They just share common space, the gas being a bystander.

We do not say air is solution of oxygen and minor gases in nitrogen. There is no difference between "permanent gases" and vapours here.

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You must look at the phases in your system. Whenever there is only one phase, but there are two or more chemical species, then you have a solution.

Beside this, all the rest is semantic. Some can be regulated by IUPAC, some not. Refer to air as a solution would sound unusual. A gas mixture - unless reacting to give a liquid or solid product - is always one phase, so the term gaseous solution isn't even considered by the IUPAC's definition of solution: https://goldbook.iupac.org/terms/view/S05746.

About "solid in gas" you are correct, once in the gas phases the iodine or naphthalene vapours aren't solid anymore. However, note that the same applies to sugar or salt in water. You can certainly speak of a solid (dissolved) in water although the actual solution obviously has no actual solid anymore, and even no salt but ions. In this case, the use is common because it is what operationally we do, we take a spoon of salt and dissolve it, even in the kitchen, and so we speak.

"Solid in gas (or liquid)" as it is and without context would mean smoke (or suspension), so we are not dealing with solutions.

I hope this is clear enough for you to understand that you were somehow over thinking the subject.

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    $\begingroup$ I like you answer, perhaps you could add that the term "gaseous solution" is pretty outdated. And I would like to point out all gases are 100% miscible (unless they react strongly with each other). $\endgroup$
    – Karl
    Dec 13, 2021 at 21:44
  • $\begingroup$ @Karl I will, as OP pose a side question about gas in gas. $\endgroup$
    – Alchimista
    Dec 14, 2021 at 7:50

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