Answer. A substance that can behave as an acid as well as a base is called amphoteric specie. For example, H2O behaves as amphoteric specie. According to Bronsted Lowry concept H2O behaves as a base because it accepts a proton (H+) when it reacts with HC1.
HCl(aq) + H2O (l) H3O+(aq) + CI-(aq)
On the other hand, H2O behaves as an acid because it donates a proton (H+) when it reacts with NH3
H2O(1) + NH3(aq) NH4+(aq) + OH-(aq)