Answer. In 1923, the Danish chemist Bronsted and the English chemist Lowry independently presented their theories of acids and bases on the basis of proton-transfer. According to this concept:
An acid is a substance, molecule or ion that can donate a proton (H+) to another substance.
A base is a substance that can accept a proton (H+) from another substance.
For example, HC1 acts as an acid while NH3 acts as a base:
HCl(aq) + NH3(aq) ————— > NH4+(aq) + Cl-(aq)
acid base
Similarly, when HC1 dissolves in water; HCl acts as an acid and H2O as a base.
HCl(aq) + H2O(aq) H3O(aq) + Cl-(aq)
Acid Base Conjugate acid Conjugate base
The substance that can behave as an acid, as well as, a base is called amphoteric compound.
According to Bronsted- Lowry concept, acid and base always work together to transfer a proton. That means a substance can act as an acid (proton donor) only when another substance simultaneously behaves as a base (proton acceptor). Hence, a substance can act as an acid as well as a base, depending upon the nature of the other substance. For example, H2O acts as a base when it reacts with CH3COOH
CH3COOH(aq)+ H2O(aq) ————> CH3COO-(aq) + H3O+(aq)
Acid Base Conjugate base Conjugate acid
While water acts as an acid when it reacts with ammonia
H2O(1) + NH3(aq) ———> NH4+(aq)+ OH-(aq)
Acid Base Conjugate acid Conjugate base
According to above two examples, water is an amphoteric compound because it acts as an acid as well as a base.