Flexi Says: Water can act as a base in a Brønsted-Lowry acid-base reaction, as long as it reacts with a substance that is a better proton donor. Shown below is the reaction of water with the hydrogen sulfite ion.
HSO3-(aq)+H2O(l)⇄H3O+(aq)+SO42-(aq)
When a substance that is acting as a Brønsted-Lowry acid donates its proton, it becomes a base in the reverse reaction. In the reaction above, the hydrogen sulfite ion donates a proton to water and becomes a sulfite ion.
