Nitric acid (HNO3), historically referred to as Aqua Fortis (strong water), is a highly corrosive, toxic, and fumes-emitting mineral acid. It is a monoprotic acid, meaning it releases a single hydrogen ion (H^+) per molecule when dissolved in water. Beyond its strong acidity, nitric acid is distinguished by its powerful oxidizing capabilities, which dictate its unique chemical interactions with metals and organic matter.
Physical and Chemical Properties
Physical Characteristics
- Appearance and Color: Pure nitric acid is a colorless liquid. However, upon standing, it frequently develops a yellowish-brown tint. This coloration occurs due to the photochemical decomposition of the acid into nitrogen dioxide gas (NO2), which remains dissolved in the liquid.4HNO3(aq) Light / Heat→ 4NO2(g) + O2(g) + 2H2O(l)
- Boiling and Melting Points: Pure anhydrous nitric acid melts at -42°C and boils at 83°C. Commercial concentrated nitric acid forms a constant boiling azeotrope with water at 68% concentration by mass, boiling stably at 120.5°C.
Chemical Properties and Dissociation
In an aqueous solution, nitric acid ionizes completely to yield hydronium (H3O^+) and nitrate (NO3^-) ions, making it a strong Arrhenius acid.
Exceptional Oxidizing Reactions
Unlike hydrochloric or sulfuric acids, nitric acid rarely liberates hydrogen gas when reacting with metals. Because the nitrate ion (NO3^-) is a stronger oxidizing agent than the hydrogen ion (H^+), it oxidizes the evolved hydrogen gas into water (H2O) and is itself reduced to various oxides of nitrogen (NO2, NO, N2O).
- Reaction with Copper (Concentrated Acid): Yields dense brown fumes of nitrogen dioxide.Cu(s) + 4HNO3(conc.) → CuSO4(aq) + 2NO2(g) ↑ + 2H2O(l)
- Reaction with Copper (Dilute Acid): Yields nitric oxide gas.3Cu(s) + 8HNO3(dil.) → 3Cu(NO3)2(aq) + 2NO(g) ↑ + 4H2O(l)
- The Hydrogen Exception: Only two metals—Manganese (Mn) and Magnesium (Mg)—react with extremely dilute (around 1%) cold nitric acid to liberate hydrogen gas, because the weak concentration minimizes the oxidizing power of the nitrate ion.Mg(s) + 2HNO3(v. dil.) → Mg(NO3)2(aq) + H2(g) ↑
Industrial Production: The Ostwald Process
On a commercial scale, nitric acid is manufactured using the Ostwald Process, which was developed by the German chemist Wilhelm Ostwald. This process utilizes the catalytic oxidation of ammonia derived from the Haber process.
Stage 1: Catalytic Oxidation of Ammonia
Ammonia gas (NH3) is mixed with atmospheric oxygen and passed over a Platinum-Rhodium gauge catalyst at a high temperature (800°C) to form nitric oxide (NO).
Stage 2: Oxidation of Nitric Oxide
The nitric oxide gas is cooled and reacted with additional oxygen to produce nitrogen dioxide gas (NO2), which appears as dense brown fumes.
Stage 3: Absorption in Water
The nitrogen dioxide gas is passed into an absorption tower where it reacts with water in the presence of excess air to form concentrated nitric acid, recycling a portion of nitric oxide back into the loop.
Passivation of Metals (The Passive State)
A highly vital chemical phenomenon for competitive examinations is the passivation of certain metals by concentrated nitric acid. When metals like Iron (Fe), Aluminum (Al), Chromium (Cr), and Nickel (Ni) are dipped into concentrated nitric acid, they initially react for a fraction of a second, but the reaction stops completely. This occurs because the intensely powerful oxidizing nature of concentrated HNO3 instantly forms a microscopic, highly dense, and non-porous layer of metal oxide on the surface of the metal. This oxide skin acts as a protective shield, preventing any further contact between the acid and the underlying metal, rendering the metal chemically passive.
Core Applications and Industrial Uses
Fertilizer Production
The predominant consumer market for nitric acid is the global agricultural sector. It is used to synthesize Ammonium Nitrate (NH4NO3), a highly soluble chemical fertilizer with a dual-nitrogen delivery profile (supplying both ammonium and nitrate ions directly to crops).
Explosives and Munitions
Due to the high energy stored within nitrogen-oxygen chemical bonds, nitric acid is the primary nitrating agent used to manufacture military and commercial explosives:
- Trinitrotoluene (TNT) & Nitroglycerin: Produced by nitrating toluene and glycerol respectively using a mixture of concentrated nitric and sulfuric acids.
- Ammonium Nitrate Fuel Oil (ANFO): Widely used as a bulk industrial blasting explosive in mining and civil engineering projects.
Component of Aqua Regia
Nitric acid is a critical component of Aqua Regia (Royal Water), a mixture consisting of concentrated HCl and concentrated HNO3 in a precise 3:1 volume ratio. Within this mixture, nitric acid serves as the primary oxidizer that allows noble metals like gold (Au) and platinum (Pt) to dissolve into soluble chloride complexes.
Rocket Propellants
In space exploration and defense engineering, fuming nitric acid (such as Inhibited Red Fuming Nitric Acid, or IRFNA) is used as a liquid storable oxidizer in rocket engines, specifically when paired with hypergolic rocket fuels like hydrazine.
Last Modified: May 26, 2026