Flexi Says: Water is capable of being either an acid or a base, a characteristic called amphoterism. An amphoteric substance is one that is capable of acting as either an acid or a base by donating or accepting hydrogen ions. In an aqueous solution, the water molecules readily dissociate in presence of acid or base as solute and split into H+ and OH- ions. When ammonia is dissolved in water, the water molecule donates a proton to the ammonia molecule. The resulting products are the ammonium ion and the hydroxide ion. The water is acting as a Brønsted-Lowry acid, while the ammonia is acting as a Brønsted-Lowry base. Water can also act as a base i.e. proton acceptor in a Brønsted-Lowry acid-base reaction, as long as it reacts with a substance that is a better proton donor such as sulfuric acid.