So far we have seen water acting as a (very weak) base to form H3O+. If we added a strong base, such as sodium hydride, to water, the base would deprotonate the water to give hydroxide ion, HO⁻, and here the water would be acting as an acid. It’s amusing to notice that hydrogen gas is the conjugate acid of hydride ion, but more important to note that hydroxide ion is the conjugate base of water.

Water is a weak acid and a weak base so we need a strong acid like HCl to give much H₃O⁺, and a strong base, like hydride ion, to give much hydroxide ion.