An acid-base pair is a set of two species that transform into each other by the gain or loss of a proton.HNO3(aq) + H2O(l) ⇌ H3O+(aq) + NO3-(aq)HNO3 is an acid as it can donate a proton and H2O is the base as it accepts a proton. But the reaction is reversible.H3O+(aq) + NO3-(aq) ⇌ HNO3(aq) + H2O(l)In the reverse reaction H3O+ is the acid and NO3- is the base. HNO3 is an acid and NO3- is its conjugate base H2O acts as a base and H3O+ is its conjugate acid.