# 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 $\ce{OH-}$.

We define Arrhenius acidity with respect to water

That definition is new to me, whatever it is. The definition I know is:

An Arrhenius acid is a substance that dissociates in water to form $\ce{H+}$ ions.

An Arrhenius base is a substance that dissociates in water to form $\ce{OH-}$ ions.

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.

• Aren't there other molecules which can donate both $OH^-$ and $H^+$? (I can imagine some organic ones, though I have no examples at hand.) Those would also be Arrhenius amphoteric, right? – Paŭlo Ebermann Oct 3 '16 at 21:16
• @PaŭloEbermann I’m pretty sure there aren’t. Frankly, I don’t know a single organic molecule that dissociates to give $\ce{OH-}$. (Side note: Use mhchem for chemical expressions; it prevents italics and inserts better spacing.) – Jan Oct 3 '16 at 21:31