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Considering the Arrhenius theory of acids and bases:

  1. Is every existing compound either an acid or a base?
  2. Why is a compound such as $ \ce {NH_3}$ considered a base, if it doesn't even have an oxygen atom and a base is supposed to release $ \ce {OH-}$ in water?
  3. If $X$ is a base according to Arrhenius theory, will $X$ still be a base according to the other two theories (and the same if $X$ were an acid)?

According to the Brønsted-Lowry theory:

  1. There are no absolute acid and bases, but only acid-base reaction in which one compound acts as a base and the other as an acid. So, is every reaction of two compounds an acid-base one?
  2. Except for water, are there any other substances which in some reactions act as bases while in others as acids?
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    $\begingroup$ Ammonia is the very reason chemists had to rethink the Arrhenius theory. $\endgroup$ – M.A.R. Feb 11 '15 at 21:01
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A couple of thoughts:

There are mainly 3 different definitions of acids and bases:

  1. The Arrhenius formulation: Acid = proton donor in water, Based = hydroxide ion donor in water
  2. The Brønsted-Lowry formulation: Acid = potential proton donor, Base = potential proton acceptor
  3. The Lewis formulation: Acid = Electron pair acceptor, Base = Electron pair donor

The Lewis definition is the most general, followed by the BL and finally by the Arrhenius definition.

You are right, for example HCl is considered a strong acid in water, but it is very weakly acidic in glacial acetic acid. If you follow the Lewis definition a large number of reactions will be considered acid base reactions. An exception are free-radical based reactions

There are many compounds that are amphoteric, but it really also depends on the definition you use.

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