Many polar protic solvents exist and ionic compounds are able to dissolve inside these substances for the same reason as water. So, theoretically, if a substance were to steal a $H^+$ ion from a polar protic solvent other than water, the end result would be an acid. So, my question is: Is it possible that such an acid can exist? If yes, then why is it rarely mentioned? If no, then what makes water so special in creating acids?


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    $\begingroup$ There are plenty of reactions run in acetic acid $\endgroup$ – Waylander Nov 16 '18 at 9:12
  • $\begingroup$ I don't understand this question, and the grammar needs improvement, as the first sentence ends in a hanging "as...". $\endgroup$ – Buck Thorn Nov 16 '18 at 11:22
  • $\begingroup$ @Try Hard What about this: Is there a polar protic solvent of the form $H_2X$ where $X$ denotes anything other than one oxygen atom, that can react with a molecule $Y$ to form $HY^++HX^-$? $\endgroup$ – Kyky Nov 17 '18 at 3:37
  • $\begingroup$ There are amphoteric molecules, such as aminoacids, which do something similar. en.wikipedia.org/wiki/Amphoterism $\endgroup$ – Buck Thorn Nov 17 '18 at 19:08

Yes, and Brønsted, Lowry and others generalized the concept of acid to be a proton donor and base a proton acceptor, no matter what solvent system. (There is also the Lewis definition of an acid, which has an empty orbital capable of accepting an electron pair from a Lewis base. These two definitions may overlap for certain chemicals.)

As to the reason that these are not "common" definitions of acid and base, consider that the human body is largely aqueous, that water is plentiful on Earth, and that for thousands of years chemistry relied on our senses - acids taste sour, for example (though I don't recommend the taste test in the lab!), and alkalis saponify fats in skin, so feel slippery.

Organic chemists often use solvents other than water, and rather reactive (and noxious!) chemicals such as the Lewis acid $\ce{BF3}$, but the average grammar-school student is content with making vinegar and baking soda "geysers" (and has a longer life expectancy if she or he avoids using $\ce{BF3}$ in a science project).

  • $\begingroup$ Follow-up question: Is water the only solvent to have a $pK_a$ the same as $pK_b$? If not, what other examples are there? $\endgroup$ – Kyky Nov 16 '18 at 10:09
  • $\begingroup$ By definition, pKa is the inverse (log) of pKb, and they are the same only in a neutral system. Pure water, or water with a neutral salt dissolved, has pKa and pKb the same, as do other neutral systems. See thoughtco.com/ph-pka-ka-pkb-and-kb-explained-4027791 $\endgroup$ – DrMoishe Pippik Nov 16 '18 at 19:32

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