this is because nitric acid has two properties: acidic prop. and oxidizing prop.
for acidic properties:
the hydrogen ions in HNO3 is hving reaction with the other substancesthe hydrogen ions are ionized
for oxidizing property:
this involve the idea of giving out and receiving electrons
HNO3 [NO3-] take/receive electrons from the reacted substance like metals
this is so-called the oxidizing power