Why is water an Arrhenius acid?

Is water an Arrhenius acid? We define Arrhenius acidity with respect to water, but what will water itself be? It can donate $\ce{H+}$ as well as $\ce{OH-}$.





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> We define Arrhenius acidity with respect to water
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That definition is new to me, whatever it is. The definition I know is:



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> An Arrhenius acid is a substance that dissociates in water to form $\ce{H+}$ ions.
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> An Arrhenius base is a substance that dissociates in water to form $\ce{OH-}$ ions.
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The key concept to answer the question is the self-ionisation of water given in equation $(1)$.


$$\ce{H2O <<=> H+ + OH-}\tag{1}$$


This ionisation happens in water, and the ion product as written in equation $(2)$ is $10^{-14}$.


$$K\_\mathrm{w} = [\ce{H+}] [\ce{OH-}] = 10^{-14}\tag{2}$$


Thus, water qualifies as a substance that dissociates in water to form $\ce{H+}$ ions. It also qualifies as a substance that dissociates in water to form $\ce{OH-}$ ions. It is both an Arrhenius acid and an Arrhenius base and thus the only Arrhenius amphoteric compound.


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