Nitrogen

Lore & Background

Nitrogen compounds have a very long history, with ammonium chloride known to Herodotus and well known by the Middle Ages. Alchemists knew nitric acid as aqua fortis and the mixture of nitric and hydrochloric acids as aqua regia, celebrated for dissolving gold. Though he did not recognise it as an entirely different chemical substance, he clearly distinguished it from Joseph Black's 'fixed air' (carbon dioxide). Nitrogen was also studied at about the same time by Carl Wilhelm Scheele, Henry Cavendish, and Joseph Priestley, who referred to it as burnt air or phlogisticated air. French chemist Antoine Lavoisier referred to nitrogen gas as 'mephitic air' or azote, from the Greek word άζωτικός (azotikos), 'no life', because it is asphyxiant. Elemental nitrogen is usually produced from air by pressure swing adsorption technology. About 2/3 of commercially produced elemental nitrogen is used as an inert gas for commercial uses such as food packaging, and much of the rest is used as liquid nitrogen in cryogenic applications. Many industrially important compounds, such as ammonia, nitric acid, organic nitrates, and cyanides, contain nitrogen. The extremely strong triple bond in elemental nitrogen (N≡N), the second strongest bond in any diatomic molecule after carbon monoxide, dominates nitrogen chemistry. This causes difficulty for both organisms and industry in converting N2 into useful compounds, but it also means that burning, exploding, or decomposing nitrogen compounds to form nitrogen gas releases large amounts of often useful energy. Synthetically produced ammonia and nitrates are key industrial fertilisers, and fertiliser nitrates are key pollutants in the eutrophication of water systems.

Reader's Guide

Nitrogen's significance spans multiple domains. As the most abundant gas in Earth's atmosphere, it is fundamental to life, being a constituent of amino acids, proteins, nucleic acids (DNA and RNA), and adenosine triphosphate. The human body contains about 3% nitrogen by mass, the fourth most abundant element after oxygen, carbon, and hydrogen. The nitrogen cycle describes the movement of the element from the air into the biosphere and back. These same nitrates became feedstock for explosives in the World Wars of the 20th century. Nitrogen compounds are also vital in pharmacology: many drugs are mimics or prodrugs of natural nitrogen-containing signal molecules, such as organic nitrates nitroglycerin and nitroprusside that control blood pressure by metabolising into nitric oxide. Notable nitrogen-containing drugs include caffeine, morphine, and synthetic amphetamines. The element's strong triple bond makes it both difficult to convert into useful compounds and valuable as an energy store, as decomposing nitrogen compounds releases large amounts of energy. Its inertness makes it useful for food packaging and cryogenic applications as liquid nitrogen.

Did You Know?

• The name 'azote' was suggested by Antoine Lavoisier from the Greek word for 'no life', as nitrogen is an asphyxiant gas.
• The triple bond in elemental nitrogen (N≡N) is the second strongest bond in any diatomic molecule, after carbon monoxide.
• About 2/3 of commercially produced elemental nitrogen is used as an inert gas for food packaging.

The Physics Behind a Frigid Liquid

Liquid nitrogen exists as a colorless, highly mobile fluid whose viscosity is roughly half that of acetone and about one-sixth that of room-temperature water. What makes its behavior so distinctive is the molecular architecture of nitrogen itself. Even after liquefaction, each molecule remains a diatomic N₂ unit, and the only forces holding neighboring molecules together are weak van der Waals interactions. Because these intermolecular attractions are so feeble, the substance requires temperatures near absolute zero to condense, giving it an unusually low boiling point of approximately −196 °C (77 K). Pushing it further into a vacuum chamber can bring it down to its freezing point of −210 °C (63 K). One practical consequence of these properties is the Leidenfrost effect: when liquid nitrogen touches a surface significantly warmer than its boiling point, it flashes into a blanket of insulating gas bubbles that actually shields the object from further rapid cooling. Engineers working around this limitation often use a slush of liquid and solid nitrogen together, which transfers heat more aggressively than the liquid alone.

From Air to Insulated Vessels: Production and Storage

Industrially, liquid nitrogen is obtained through fractional distillation of liquid air, a process that separates the component gases based on their differing boiling points. Once produced, the fluid must be handled with considerable care because, as a cryogenic substance, it rapidly freezes any living tissue it contacts. Standard storage relies on vacuum flasks in which the liquid is held at a steady 77 K by its own slow, continuous boiling; depending on the vessel's size and design, this arrangement can maintain the liquid for anywhere from a few hours to several weeks. More advanced pressurized super-insulated vacuum vessels have pushed that envelope dramatically, cutting losses to as little as two percent per day and enabling transport over much longer durations. Beyond its role as a coolant, liquid nitrogen also serves as a compact, easily transported reservoir of dry nitrogen gas. Because the gas is released simply by allowing the liquid to boil, no separate pressurization equipment is needed, making it a convenient backup source in systems such as hypoxic air fire-prevention setups.

A Chilling Versatility: Applications Across Science, Medicine, and Industry

The specific heat of liquid nitrogen (1040 J·kg⁻¹·K⁻¹) and its heat of vaporization (200 kJ·kg⁻¹) make it an extraordinarily versatile open-cycle refrigerant. In medicine, it is used in cryotherapy to destroy warts and actinic keratoses, and in cryopreservation to store blood, sperm, eggs, and surgically excised tissue for future study. In the laboratory and in industry, it cools charge-coupled cameras for astronomical imaging, keeps high-temperature superconductors below their critical threshold, and maintains the low-temperature environment surrounding the liquid helium in NMR spectrometers and MRI machines. Engineers use it to shrink-fit mechanical components, freeze water and oil pipes for cryogenic isolation when no valve is available, and stabilize loose tunnel soils by freezing the water within them. In the kitchen, chefs like Heston Blumenthal at The Fat Duck flash-freeze desserts at the table, producing smaller ice crystals and a silkier texture than conventional chilling. Bartenders exploit the same rapid cooling to chill glasses or create a dramatic smoky vapor in cocktails.

The Hidden Danger: Expansion, Accidents, and a 2006 Catastrophe

The most sobering aspect of liquid nitrogen is its expansion ratio. At room temperature, one volume of the liquid can generate roughly 694 volumes of gas, meaning that vaporization inside a sealed or poorly vented space can produce an almost unimaginable pressure. On January 12, 2006, at Texas A&M University, the pressure-relief devices on a liquid nitrogen tank were already malfunctioning and were subsequently sealed shut. With no safe path for the expanding gas, pressure built until the tank ruptured catastrophically. The blast was powerful enough to drive the tank straight through the ceiling above it and to shatter a reinforced concrete beam directly below, illustrating just how violent an uncontrolled release can be. The same expansion physics underlies the danger of accidental ingestion: if liquid nitrogen is swallowed, the rapid phase change inside the body can cause severe and potentially fatal internal damage. Even in culinary settings, where the dramatic smoky effect is a selling point, operators must treat every pour with the seriousness of a high-pressure hazard.