It is well known that H2O spontaneously autoionizes to H+ and OH- when it is in liquid phase. I was wondering whether it does the same in gas phase, either to the same extent as in liquid phase or at all.
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Yes, water is ionized to some extent in all phases. One study even extends to 1,000 K!
A concise discussion of the change in ionization with temperature at Chemguide also explains the variation of pH of neutral water with temperature. [The concept of pH 7 being neutral is based on Kw (in mol2 dm-6) of ~10-14 at ~300 K.]
In general, the higher the temperature, the more a compound is ionized.
answered Jan 12, 2016 at 21:59
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$\begingroup$ Thing is, in actual vapour and low temp. auto-ionisation is as good as non-existent. $\endgroup$– MithoronCommented Dec 23, 2019 at 14:03
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$\begingroup$ @Mithoron, read the answer: "water is ionized to some extent". "as good as non-existent," is not none. $\endgroup$ Commented Jul 3, 2022 at 3:09
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$\begingroup$ OP asked to what extent. Later you talk about liquid water. While it's true that raising temp. raised deg. of autoionization, vaporisation lowers it extremely. $\endgroup$– MithoronCommented Jul 3, 2022 at 13:35