* Definition of an acid: Acids are substances that donate hydrogen ions (H+) when dissolved in water.

* Dissociation of HNO3: When HNO3 dissolves in water, it dissociates into hydrogen ions (H+) and nitrate ions (NO3-):

* HNO3 (aq) → H+ (aq) + NO3- (aq)

The presence of hydrogen ions (H+) in the solution is what makes HNO3 acidic.