Question

How can water be a lewis base?

Answer 1

Water can be a Lewis acid AND base.

Water (`H_2O`) is amphoteric, meaning that it can "self ionize".
`H_2O` + `H_2O` = `H_3O^+` + `OH^-`

One of the water molecules acts as a Lewis acid (`H_3O^+`) and one acts as a Lewis base (`OH^-`)

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 `OH^-` may donate electrons to another acid besides the `H_3O^+` ion, therefore reacting with it.