Nitrogen is the most abundant element in Earth's atmosphere, making up roughly 78 percent of air by volume. At room temperature it is a colorless, odorless gas with a density around 1.251 g/L; because the N≡N triple bond is extremely strong, its chemistry is remarkably stable. Its largest industrial role is as feedstock for ammonia synthesis: pure nitrogen produced from air separation, together with hydrogen, enters the Haber-Bosch loop and is converted into ammonia, then into urea, ammonium nitrate and compound fertilizers that support global agriculture. Liquid nitrogen, at about -196°C, is the cheapest, most readily available cryogen and is widely used for quick-freezing food, the long-term storage of biological samples and of sperm and ova, cryotherapy in hospitals, and superconducting cables and cryogenic experiments.
In chemical, electronics and food industries, nitrogen is heavily used as an inert shielding gas because of its stability: to prevent oxidation of flammable or explosive materials, to protect polysilicon and optical fibers from oxygen contamination at high temperature, and to prevent rancidity and discoloration of oils and pigments in packaged snacks and milk powder. In steel and metallurgy, combined-blown converters use nitrogen to stir molten steel and improve composition uniformity. In petroleum refining and pipeline transport, nitrogen purges and displaces flammable gases to make inspection and maintenance safer. In tires and aircraft fuel systems, nitrogen charging reduces oxidative corrosion and deflagration risks caused by oxygen and moisture. Nitrogen is chosen precisely because it is nearly unlimited in supply, cheap, inert, and can be liquefied into an inexpensive cryogen.
Nitrous oxide (N2O), or laughing gas, was historically used as an inhaled anesthetic in dentistry and surgery; today it is more commonly used as a silicon-nitride source in semiconductor CVD processes, as a rocket oxidizer, in nitrous-injection systems for racing, and in small amounts as a whipped-cream propellant. Ammonia (NH3) goes beyond fertilizers as the feedstock for nitric acid, soda ash, urea, acrylonitrile and many amines; liquid ammonia itself is also a refrigerant in large cold-storage plants and thermal power stations, and a reducing agent in selective catalytic reduction of flue-gas NOx. Nitric acid (HNO3) is one of the most important industrial strong acids, used to manufacture nitrogen fertilizers, explosives, nitro coatings and adipic acid, and is a key raw material for petrochemicals, defense production and precious-metal refining; dilute nitric acid is also a common oxidizing pickling reagent in laboratories.
