1. According to Lowry-Bronsted theory, an acid is defined as a substance that has a tendency to donate a proton to another substance and base is a substance that has a tendency to accept a proton from other substance.
2. An acid is a proton donor and a base is a proton acceptor.
3. When HCI is dissolved in H2O, HCI donates a proton to H2O. Thus HCI behaves as an acid and H2O is a base.
HCI + H2O ⇌ H3O+ + Cl-