I know that the heating of water drives the auto ionization of water. It also drives the evaporation of water. My question is where do the ions go as the water evaporates as it is heated up. Ions can't be isolated in the solid phase. Do the ions enter the gas phase?
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asked Aug 17, 2014 at 20:13
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2$\begingroup$ I suspect that the hydronium and hydroxide react to regenerate water before it enters the gas phase. I don't have any way to prove that. It just seems to me that if you have pure water, and some of it ionizes, the ions will be too attracted to each other to want to vaporize. I assume if you heated the water up so fast that it instantly vaporized you could get ions into the gas phase, but I don't have any proof for that either. That's why this is a comment. $\endgroup$– user137Aug 17, 2014 at 20:26
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$\begingroup$ That's what I suspect too; we'll just have to wait for a better answer! $\endgroup$– DissenterAug 17, 2014 at 20:26
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$\begingroup$ If an ion has sufficient kinetic energy to reach the gas phase what is stopping it from evaporating? It should be far enough away from other ions in solution to make the force holding it in solution comparable to hydrogen bonding holding water in the liquid phase. Cars near the ocean rust faster because of the salt in the air. Have you ever accidentally inhaled the vapor coming off boiling aqueous NaOH? It burns. I'd say the ions enter the gas pahse the same as water molecules. $\endgroup$– Brinn BelyeaAug 17, 2014 at 21:10
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1$\begingroup$ There is nothing against ions vaporising. It is far more likely for them do so at least as neutral pairs, though, because of the strong attraction between opposing charges. In the case of autoionised water, this would necessarily mean recombination into molecular $\ce{H2O}$. Uncompensated ions in the gas phase are also possible in certain circumstances, but not common. $\endgroup$– Nicolau Saker NetoAug 17, 2014 at 22:06
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$\begingroup$ @NicolauSakerNeto I would suspect, that via the Grotthuss mechanism it is almost impossible to vaporise hydronium and/or hydroxyl ions. $\endgroup$– Martin - マーチン ♦Aug 18, 2014 at 3:03
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1 Answer
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The auto-ionization is an equilibrium process, means that it works in both direction. As water evaporates, the auto-ionization converts back the ions into water molecules. You can take it as an example to le chatelier's principle. You gradually remove water from the system, therefore the concentration of hydroxide and oxonioum would go up if they do not turn to water.
Ions in gas phase are rather high energy things, because the dielectric properties of air are much less favorable in stabilizing charges than water can. It is much more easy to evaporate water molecules.
