For example, when liquid water evaporates, my instinct is to say that of course it's become gas, but I'm a bit unsure because, if I understand correctly, evaporation occurs because air dissolves the water, and it's not clear to me whether that counts as a phase change. Intuitively it seems like it should should water dissolves in the air or salt dissolved in water, etc. have different properties before and after being dissolved. And yet, the way I've always heard it explained, phase changes are specifically due to changes in temperature and/or pressure, not due to chemical interactions with another another substance.

On the other hand, from what I remember from chem 101 and 102, when we considered chemical reactions occuring between solids dissolved in a liquid (usually acids and bases dissolved in water), we usually just labeled them as aqueous, meaning "in solution", whereas for non-aqueous substances, we'd label them with the relevant state of matter, solid, liquid, or gas. Does that mean dissolved substances constitute their own state of matter? Or that it's simply not meaningful to talk about the state of the solute independent of the solvent?

I also saw this thread, Is it appropriate to say "solid-in-gas solution" and "liquid-in-gas solution"?, where someone says, "Whenever there is only one phase, but there are two or more chemical species, then you have a solution", which heavily implies there's a fundamental relationship between phase changes and solutions, but it also doesn't seem quite right

  • $\begingroup$ It is usually helpful to look for a proper agreed upon definition such as in the IUPAC gold book goldbook.iupac.org/terms/view/… which states 'A liquid or solid phase containing more than one substance, when for convenience one (or more) substance, which is called the solvent, is treated differently from the other substances, which are called solutes......' $\endgroup$
    – porphyrin
    Commented Apr 10, 2023 at 13:09

1 Answer 1


Water vapour in air is a solution, but not in the same sense as solutions in water.

Water vapour in air is solution in sense of homogenous mixture, where molecules move freely and independently.

Salts in water dissociate (are dissociated by water) to ions. Having a net charge, they form a nonhomogeneous electrostatic gradient.Ions are hydrated by water molecules that have an electric dipole and therefore attracted to the center of such a gradient. So here we see strong interaction and dependent motion.

Capacity for water vapour at given temperature is not a shared property of water and air, like solubility of table salt in water. It is a property of water alone. Water need not air at all to evaporate and to stay as vapour.

Presence of air has very tiny effect on the maximal absolute humidity in $\pu{g / m3}$, due secondary phenomena, that are significant only at conditions near critical conditions.

Like nitrous oxide versus it's 1:1 mixture with oxygen. The former is at room temperature and pressure $\pu{100 atm}$ liquid, while it is gaseous at the same partial pressure $\pu{100 atm}$ in the latter.


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