According to the Arrhenius theory Acids produce hydrogen ions (H+) in solution. e.g. HNO3(aq) → H+(aq) + NO3–(aq), H+ was produced thus it’s an Arrhenius acid.Bases produce hydroxide ions (OH–) in solution.e.g. KOH(aq) → K+(aq) + OH–(aq), OH– was produced, thus it’s an Arrhenius base.
According to the Lowry-Brønsted theory, an acid is a proton (H+ ion) donor, and a base is a proton (H+ ion) acceptor.
HNO3 donated a proton to H2O, it is a Lowry-Brønsted acid, and H2O accepted the proton, so it is a Lowry-Brønsted base.