# Question #1887e

Jan 29, 2015

An amphoteric compound is a compound that can act either as an acid, or as a base in a chemical reaction.

A first example of water's amphoteric character is the fact that it can self-ionize to a small extent. What that means is that a water molecule is capable of donating one of its protons (${\text{H}}^{+}$) to a neighbouring water molecule, forming the hydroxyde and hydronium ions.

${H}_{2} O + {H}_{2} O r i g h t \le f t h a r p \infty n s {H}_{3} {O}^{+} + O {H}^{-}$

One of the two water molecules acts as an Bronsted-Lowry acid and donates a proton to the other water molecule, which subsequently acts as a Bronsted-Lowry base.

Water can act as an acid or as a base in various chemical reactions, the most common two examples involving the ammonia and hydrochloric acid reactions.

$N {H}_{3 \left(a q\right)} + {H}_{2} {O}_{\left(l\right)} r i g h t \le f t h a r p \infty n s N {H}_{4 \left(a q\right)}^{+} + O {H}_{\left(a q\right)}^{-}$

Here water acts as a Bronsted-Lowry acid because it donates a proton to ammonia.

$H C {l}_{\left(g\right)} + {H}_{2} O \left(\left(l\right)\right) r i g h t \le f t h a r p \infty n s {H}_{3} {O}_{\left(a q\right)}^{+} + C {l}_{\left(a q\right)}^{-}$

This time water acts a Bronsted-Lowry base because it accepts a proton from hydrochloric acid.

To be exact, in reactions that involve the transfer of protons, the term is actually amphiprotic.

Water can act as a Lewis acid or as a Lewis base as well; in such reactions, water can either accept a pair of electrons, which makes it a Lewis acid, or donate a pair of electrons, which makes it a Lewis base.

Here's another example of water's amphiprotic (amphoteric) nature: