It is a very strong acid and decomposes on boiling or in presence of sunlight. It acts as a strong oxidizing agent. It oxidizes nonmetals and metalloids to their respective oxy-acids, i.e., C to H₂ CO₃,S to H₂SO₄,P to H₃PO₄,I₃ to HIO₃,As to H₃AsO₄ (arsenic acid) and Sb to H₃SbO₄ (antimonic acid), while nitric acid itself is reduced to NO₂ .

I₂ + 10 HNO₃ ——→ 2HIO₃ + 10NO₂ + 4H₂O

Nitric acid reacts with metals to form nitrates and is itself reduced to NO, N₂O, NO₂ or NH₃ (which further reacts with HNO₃ to give NH₄NO₃ ) depending upon the concentration of the acid, activity of the metal and the temperature of the reaction.

(i) Very active metals such as Mn, Mg, Ca , etc. give H₂ on treatment with very dilute HNO₃ (2%).

(ii) Less active metals like Cu, Hg, Ag, Pb etc. give NO with dil. HNO₃ . Zinc, however, gives N₂O with dil HNO₃ and NH₄NO₃ with very dilute HNO₃ .

Zn + 10HNO₃ ( dilute ) ——→ 4Zn ( NO₃ )₂ + N₂O + 5H₂O

Zn + 10HNO₃ ( very dilute ) ——→ 4Zn ( NO₃ )₂ + NH₄NO₃ + 3H₂O

Similarly, Fe and Sn react with dilute nitric acid to give NH₄NO₃ .

(iii) Conc. HNO₃ gives NO₂ both with active metals ( Zn,Pb etc.) and less active metals ( Cu, Hg, Ag etc.)

Cu + 4HNO₃ (Conc. ) ——→ Cu ( NO₃ )₂ + 2NO₂ + 2H₂O

Tin is, however, oxidized by conc. HNO₃ to metastannic acid ( H₂SnO₃ ) .

Sn + 4HNO₃ ——→ H₂SnO₃ + 4NO₂ + H₂O