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In my textbook, humid air is given as an example to liquid in gas solution. But isn't humidity the amount of water vapour (gas) and therefore should be gas in gas solution?

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    $\begingroup$ A solution is a single phase by definition. So as soon as there is liquid water (droplets) in air, it is not a solution. $\endgroup$ May 7 at 9:04
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    $\begingroup$ A asking B why C has written something wrong will not provide the answer unless B knows the history or context of such writing. $\endgroup$
    – Poutnik
    May 7 at 11:15
  • $\begingroup$ Would be nice if you post the identity of the source textbook. $\endgroup$
    – Buck Thorn
    May 9 at 11:05

2 Answers 2

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Humid air is just a mixture of gases. It is not a solution (neither of a gas nor of a liquid) in the sense that a solvent (air) is needed to dissolve a solute (water). In this sense, water vapour is not dissolved in air; it is just another gas that is mixed with the other gases in the air.

The idea of air "holding" water or being "saturated" with water is misleading. 100 % relative humidity does not correspond to a solubility limit of water in air. It just corresponds to the equilibrium vapour pressure of water at the given temperature.

The equilibrium concentration of water in the atmosphere depends on temperature. It is almost independent of the amount of air (or other gases) that is present and therefore it is almost independent of the total ambient pressure. A vacuum has approximately the same equilibrium concentration of water vapour as the same volume filled with air. At $10\ \mathrm{^\circ C}$, it is about $9.4\ \mathrm{g/m^3}$, at $20\ \mathrm{^\circ C}$, it is about $17.3\ \mathrm{g/m^3}$, and at $30\ \mathrm{^\circ C}$, it is about $30.4\ \mathrm{g/m^3}$.

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Note that not always the water will be in gaseous state (in form of water vapor) in humid air. The moisture we speak of in humid air may be in gaseous state (as water vapor) or liquid state (miniscule water droplets). The former will be considered a homogenous mixture and latter a heterogenous mixture. In case of extremely humid air, the moisture will mostly be in liquid form (as water droplets) and that's when you will consider humid air as liquid-in-gas colloidal system. In less humid air, the moisture will most likely be water vapor rather than water droplets.

You can see my previous answer or this physics.SE question for more information.

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  • $\begingroup$ "In case of extremely humid air, the moisture will mostly be in liquid form (as water droplets) and that's when you will consider humid air as liquid-in-gas solution" As you say it yourself, this is a heterogenous mixture and therefore not a solution. $\endgroup$ May 7 at 9:56
  • $\begingroup$ @Snijderfrey it is a colloidal solution and not true solution. I have edited my answer just to be clear. $\endgroup$ May 8 at 4:16

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