Fe, Cr, Ni and Al become passive in conc. HNO₃ (i.e., lose their normal reactivity) due to the formation of a thin protective layer of the oxide on the surface of the metal which prevents further action. Nitric acid has no action on noble metals ( Au, Pt ) but these metals dissolve in aqua regia (3 vol. HCI + 1 vol. HNO₃ ) forming their respective chlorides.

HNO₃ + 3HCI ——→ 2H₂O + NOCI + 2[CI]

Au + 3[CI] ——→ AuCI₃; Pt + 4 [CI] ——→ PtCI₄

These chlorides subsequently dissolve in excess of HCI forming their corresponding soluble complexes. Thus,

Sugar on oxidation with nitric acid gives oxalic acid. Nitric acid reacts with glycerin to give glycerol trinitrate or nitro glycerine, with toluene it gives 2, 4, 6-trinitrotoluene (T.N.T.) and with cellulose (cotton) it gives cellulose trinitrate (gun cotton). All these are used as explosives.