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Thinking of this question, I thought that the phenomenon of water evaporation into the air is totally analogous to the solid (say, salt) that is dissolved into a liquid (water).

Indeed, in this analogy, the original doubt would translate thus: We know that salt melts at 801 °C. Now, when we dissolve it in water (at room temperature)... what becomes of its state: it's still solid salt or liquid?

I thought that the traditional picture of states of matter (solid, liquid, gas) applies to pure or homogeneous substances. When some substance is dissolved into some solvent, it looses its individuality, and it makes little sense to speak of the state of the solute: the solute is neither solid or liquid salt, but part of a liquid solution. From Wikipedia:

The solution more or less takes on the characteristics of the solvent including its phase

This makes sense to me, and I tended to believe that the same applies to water vapour in the air: it was a liquid that has been dissolved into a gas. (Steam, on the other hand, is gaseous water). And the formation of tiny droplets when the air cools (condensation) would be analogous to the precipitation of solids in liquid solutions. Afterwards, I found that some people agree with this ("the air is the solvent and water is the solute").

However, the same Wikipedia does not seem to agree:

If the solvent is a gas, only gases are dissolved...

So I'm confused.

  • When a solid dissolves into a liquid, it's right say that the solute is in the liquid state? Or solid? Or neither?

  • The case of water vapour in the air (at room temperature) is, analogously, a liquid (solute) disolved in a gas (air)?

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  • $\begingroup$ Water vapor is indeed a gas. Liquid water droplets that happen to be suspended for some time in air are not dissolved in the gas - they are not thermodynamically stable in that configuration. $\endgroup$
    – Jon Custer
    Commented Jul 21, 2015 at 19:27

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The melting point of nitrogen is greater than that of oxygen. Is gaseous oxygen really liquid oxygen dissolved in gaseous nitrogen? No. It is gaseous oxygen.

"Water vapour in the air" (your phrase) is just that - vapour. Not liquid droplets "dissolved" in air.

You may be confused by the fact that wet steam does contain water droplets, and the steam from a kettle is wet. However, it is perfectly possible to produce dry steam, and this stuff is enormously dangerous when let loose - it's invisible but will cook flesh almost instantly.

Your biggest mistake is to assume that evaporation is equivalent to a solid being dissolved, specifically salt into water. The difference can be illustrated by the fact that water will evaporate into any gas (with the obvious caveat that it must not react with the gas), and the rate is controlled by temperature and the partial pressure of water vapor in the surrounding atmosphere. Salt, on the other hand, simply will not dissolve in gasoline, and in fact will not dissolve in any non-polar liquid.

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  • $\begingroup$ Suppose we boil a vessel with water. This turns water into vapor which gets into the atmosphere (air above the vessel). Isn't now the water vapor diffused? Why we can't consider it as a solute in a gaseous solution? $\endgroup$
    – Anton
    Commented Feb 5, 2021 at 18:39
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A solution means it is a 'homogenous' mixture. That is its composition and properties are uniform throughout the mixture. In case of water vapour in air(and also dust in air) it is not a homogenous mixture therefore cannot be considered as a solution. Also refer answer of whatRoughBeast for a brief answer.

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