# How can water be a lewis base?

Water (${H}_{2} O$) is amphoteric, meaning that it can "self ionize".
${H}_{2} O$ + ${H}_{2} O$ = ${H}_{3} {O}^{+}$ + $O {H}^{-}$
One of the water molecules acts as a Lewis acid (${H}_{3} {O}^{+}$) and one acts as a Lewis base ($O {H}^{-}$)
The Lewis definition of a base (lewis base) is that it donates an electron pair, while a Lewis acid accepts it. When water is ionized, the $O {H}^{-}$ may donate electrons to another acid besides the ${H}_{3} {O}^{+}$ ion, therefore reacting with it.