Ammonia

Ammonia, with the chemical formula NH₃, is an inorganic compound made of nitrogen and hydrogen. It is a stable binary hydride and the simplest pnictogen hydride, existing as a colourless gas that has a sharp, pungent odour. This substance sees broad use in fertilizers, refrigerants, explosives, cleaning agents, and as a starting material for many other chemicals. In future energy systems, renewable ammonia is viewed as a key energy carrier.

Biologically, ammonia is a common nitrogenous waste and plays a major role in meeting the nutritional needs of land-based organisms because it serves as a precursor to fertilizers. Roughly 70% of industrially produced ammonia goes into making fertilizers in various forms—such as urea and diammonium phosphate—and pure ammonia is also applied directly to soil. Ammonia, either directly or indirectly, acts as a building block for synthesizing many chemicals. In numerous countries, it is classified as an extremely hazardous substance due to its toxicity, which damages cells and tissues; for this reason, most animals excrete it in urine as dissolved urea.

Ammonia is produced biologically through nitrogen fixation, but far more is generated industrially via the Haber process. This process revolutionized agriculture by providing cheap fertilizers. Conventional ammonia production emits about 500 million tonnes of CO₂ per year, and global industrial output in 2021 reached 235 million tonnes. Industrial ammonia is transported by road in tankers, by rail in tank wagons, by sea in gas carriers, or in cylinders. Ammonia occurs naturally and has been detected in the interstellar medium. It boils at −33.34 °C (−28.012 °F) at one atmosphere of pressure, though the liquid can often be handled in the lab without external cooling. Household ammonia, or ammonium hydroxide, is a solution of ammonia in water.

The name "ammonia" comes from the Egyptian deity Amun (called Ammon in Greek). Priests and travelers at those temples burned soils rich in ammonium chloride—derived from animal dung and urine. In Book XXXI of his *Natural History*, Pliny mentions a salt called *hammoniacum*, named for its source near the Temple of Jupiter Amun in the Roman province of Cyrenaica. However, Pliny’s description of the salt does not match ammonium chloride; according to Herbert Hoover’s commentary on Georgius Agricola’s *De re metallica*, it was likely common sea salt. Regardless, that salt gave ammonia and ammonium compounds their name. Substances containing ammonia or similar to it are called ammoniacal.

Ammonia is found throughout the Solar System—on Earth, Venus, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto, among other places. On smaller, icy bodies like Pluto, ammonia can act as a geologically important antifreeze: a mixture of water and ammonia can have a melting point as low as −100 °C (−148 °F; 173 K) if the ammonia concentration is high enough, allowing such bodies to retain internal oceans and active geology at far lower temperatures than water alone would permit.

In the atmosphere, ammonia comes from both human and natural sources. Agriculture is the dominant source, accounting for about 80%, including fertilizers, urea from livestock, and manure (in order of contribution). Biomass burning—from forest fires and agricultural waste—releases roughly equal amounts of ammonia into the air. Ammonia can also be emitted naturally from soils and plants when its concentration in the soil or plant exceeds that in the air; this is part of the bi-directional exchange model. The main sink for atmospheric ammonia is rain. In soil, ammonia-oxidizing archaea (AOA) and bacteria (AOB) convert ammonia into nitrite. Through these processes, ammonia’s atmospheric lifetime ranges from 12 to 48 hours.

Atmospheric ammonia reacts with sulfuric acid and nitric acid to form sulfate-nitrate aerosols, with ammonium sulfate being prominent due to its low vapour pressure. These aerosols affect climate and can harm human health. Ammonia can also be oxidized by the hydroxyl radical: NH₃ + ·OH → ·NH₂ + H₂O. The lifetime of ammonia through oxidation is much longer than through deposition—as low as 30 days. The resulting amino radical can then be oxidized by multiple compounds to form greenhouse gases like nitrous oxide. Oxidation of ammonia may also produce nitric oxide, significantly increasing NOx levels in rural areas, where relative levels are typically lower than in urban areas.

Ammonia is a colourless gas with a characteristically pungent smell. It is lighter than air, with a density 0.589 times that of air. Strong hydrogen bonding between molecules makes it easy to liquefy. Gaseous ammonia turns into a colourless liquid that boils at −33.1 °C (−27.58 °F) and freezes into colourless crystals at −77.7 °C (−107.86 °F). Little data exists for very high temperatures and pressures, but the liquid-vapor critical point occurs at 405 K and 11.35 MPa. In solid form, ammonia has a cubic crystal symmetry (Pearson symbol cP16, space group P2₁3 No. 198, lattice constant 0.5125 nm). Liquid ammonia has strong ionizing powers due to its high dielectric constant (ε of 22 at −35 °C). It also has a very high standard enthalpy change of vaporization (23.5 kJ/mol; for comparison, water’s is 40.65 kJ/mol, methane’s 8.19 kJ/mol, and phosphine’s 14.6 kJ/mol), and it can be transported in pressurized or refrigerated vessels.