Nitric acid is normally considered to be a strong acid at ambient temperatures. There is some disagreement over the value of the acid dissociation constant, though the pKa value is usually reported as less than –1. This means that the nitric acid in solution is fully dissociated except in extremely acidic solutions. The pKa value rises to 1 at a temperature of 250 °C.
Nitric acid can act as a base with respect to an acid such as sulfuric acid.
HNO3 + 2H2SO4 NO2+ + H3O+ + 2HSO4–; K ~ 22 The nitronium ion, NO2+, is the active reagent in aromatic nitration reactions. Since nitric acid has both acidic and basic properties it can undergo an autoprotolysis reaction, similar to the self-ionization of water
2HNO3 NO2+ + NO3– + H2O  Oxidizing properties
Nitric acid is a strong oxidizing agent as shown by its large positive reduction potential (E0r).
NO3- + 2 H+ + e- → NO2 + H2O, E0r = 0.79 VNO3- + 4 H+ + 3 e- → NO + 2 H2O, E0r = 0.96 V Being a powerful oxidizing agent, nitric acid reacts violently with many non-metallic compounds and the reactions may be explosive. Depending on the acid concentration, temperature and the reducing agent involved, the end products can be variable. Reaction takes place with all metals except the noble metals series and certain alloys. As a general rule, oxidizing reactions occur primarily with the concentrated acid, favoring the formation of nitrogen dioxide (NO2).
 Reactions with metals
Nitric acid reacts with most metals. This characteristic has made it a common agent to be used in acid tests. Some precious metals, such as pure gold do not react with nitric acid, though pure gold does react with aqua regia, a mixture of concentrated nitric acid and hydrochloric acid. However, some less noble metals (Ag, Cu, ...) present in some gold alloys relatively poor in gold such as colored gold can be easily oxidized and dissolved by nitric acid, leading to colour changes of the gold-alloy surface. Nitric acid is used as a cheap means in jewelry shops to quickly spot low-gold alloys (< 14 carats) and to rapidly assess the gold purity.
Strongly electropositive metals, such as magnesium react with nitric acid as with other acids, reducing the hydrogen ion.
Mg + 2 H+ → Mg2+ + H2 With less electropositve metals the products depend on temperature and the acid concentration. For example, copper reacts with dilute nitric acid at ambient temperatures with a 3:8 stoichiometry.
3 Cu + 8 HNO3 → 3 Cu2+ + 2 NO + 4 H2O + 6 NO3- The nitric oxide produced may react with atmospheric oxygen to give nitrogen dioxide. With more concentrated nitric acid, nitrogen dioxide is produced directly in a reaction with 1:4 stoichiometry.
Cu + 4 H+ + 2 NO3− → Cu2+ + 2 NO2 + 2 H2O  Passivation
Although chromium (Cr), iron (Fe) and aluminium (Al) readily dissolve in dilute nitric acid, the concentrated acid forms a metal oxide layer that protects the metal from further oxidation, which is called passivation. Typical passivation concentrations range from 18% to 22% by weight.
 Reactions with non-metals
Being a powerful oxidizing acid, nitric acid reacts violently with many organic materials and the reactions may be explosive.
Reaction with non-metallic elements, with the exceptions of nitrogen, oxygen, noble gases, silicon and halogens, usually oxidizes them to their highest oxidation states as acids with the formation of nitrogen dioxide for concentrated acid and nitric oxide for dilute acid.
C + 4 HNO3 → CO2 + 4 NO2 + 2 H2O or
3 C + 4 HNO3 → 3 CO2 + 4 NO + 2 H2O