Ammonia (NH3)

Ammonia (NH3) is a colourless gas with a sharp, choking smell that anyone who has cleaned an oven or scooped a litter tray will recognise. Each molecule is made of one nitrogen atom joined to three hydrogen atoms. Despite its sharp smell, ammonia is one of the most important chemicals in the modern world. It is the starting point for almost all artificial fertilisers, which grow nearly half of the worlds food. It is also used in cleaning, refrigeration, plastics and explosives. Without ammonia, we could not feed 8 billion people.

  • FormulaNH31 nitrogen + 3 hydrogen
  • StateGas at room temperatureBoils at -33 C
  • SmellSharp, chokingEasy to detect at low levels
  • Produced worldwideAround 180 million tonnes/yearSecond-highest of any chemical
  • Main useFertilisers80 per cent of production
  • DiscoveredJoseph Priestley, 1774British chemist

The ammonia molecule

The NH3 molecule is shaped like a tiny pyramid (tetrahedron with one corner missing). The nitrogen sits at the top, the three hydrogens form a triangle at the bottom, and the nitrogen has a "lone pair" of electrons sticking out the open corner.

That lone pair makes ammonia a base: it can grab an H+ ion from any acid, forming NH4+ (ammonium ion). Dissolved in water, ammonia produces a basic solution (pH around 11) that fizzes when added to acids.

How ammonia is made

Almost all of the worlds ammonia is made by the Haber-Bosch process, invented in Germany in 1909. The recipe combines:

  • Nitrogen gas (N2) from the air (78 per cent of the atmosphere)
  • Hydrogen gas (H2), usually made from methane and steam
  • High temperatures (around 450 degrees Celsius)
  • High pressure (around 200 atmospheres)
  • An iron catalyst

N2 + 3 H2 -> 2 NH3

The reaction is slow and only goes part-way to completion, but at industrial scale it still produces about 180 million tonnes of ammonia per year, more than any other chemical except sulfuric acid.

Fact The Haber-Bosch process uses about 2 per cent of the worlds total energy supply. About half of all the nitrogen atoms in your body have, at some point in their history, passed through a Haber-Bosch reactor. Without it, the world simply could not grow enough food to feed everyone alive today.

Ammonia and fertilisers

Plants need nitrogen to build proteins and DNA, but they cannot use the nitrogen gas in the air directly. They need it in the form of nitrates or ammonium ions in the soil. Most modern fertilisers are made by turning ammonia into one or another nitrogen compound that plants can absorb:

  • Ammonium nitrate: made by reacting ammonia with nitric acid. A common solid fertiliser.
  • Urea: made by reacting ammonia with CO2. The most-produced fertiliser worldwide.
  • Ammonium phosphate: provides both nitrogen and phosphorus.

Without these synthetic fertilisers, traditional farming could probably support about 4 billion people. We have 8 billion. The difference is fed by Haber-Bosch ammonia.

Other uses of ammonia

  • Cleaners: ammonia solution cuts through grease, dirt and grime. You will find it in many glass cleaners and floor polishes. The strong smell is a warning of its strength.
  • Refrigeration: ammonia is an efficient refrigerant gas used in large industrial freezers, ice rinks and food cold stores. It is being adopted again as a "green" alternative to some other refrigerants that damage the ozone layer.
  • Plastics: ammonia is a starting ingredient for many plastics including nylon and melamine.
  • Explosives: ammonium nitrate is also used in mining explosives. Carelessly stored, it has caused some terrible accidental disasters (Beirut in 2020, Texas City in 1947).
  • Pharmaceuticals: many medicines start from ammonia.
  • Pulp and paper: ammonia helps process wood pulp into paper.
  • Wastewater treatment: ammonia chemistry is involved in removing pollutants from water.

Ammonia in nature

Ammonia is found in living systems too:

  • Urea in urine: as your body breaks down protein, the nitrogen-containing waste is converted to urea (a relative of ammonia) and excreted in urine. Stale urine smells of ammonia because the urea slowly breaks down back into NH3.
  • Decaying matter: dead plants and animals release ammonia as bacteria break them down.
  • Bird and bat guano: piles of bird droppings give off ammonia, which is why ancient guano deposits were mined for fertiliser in the 1800s.
  • Plant roots: some plants (especially legumes like beans and peas) host special bacteria in their roots that can convert N2 from the air directly into ammonia, naturally fertilising the soil.
Did you know? Ammonia is one of the most-detected molecules in space. Astronomers have found ammonia clouds around stars, comets, the atmospheres of Jupiter and Saturn, and even in the dust clouds where new stars and planets are forming. It is one of the basic ingredients of the universe.

Why ammonia is dangerous

Concentrated ammonia is poisonous. Even small amounts in the air can sting your eyes and lungs. Strong solutions used in industry can cause chemical burns and serious injury.

Two main hazards to remember:

  • Never mix ammonia-based cleaners with bleach (sodium hypochlorite). The two react to produce chloramine gas, which can damage lungs and be fatal in a small enclosed room.
  • Use ammonia cleaners with plenty of ventilation, ideally with windows open.
Try this Smell-test (carefully, with adult permission) the difference between cleaning ammonia and other cleaners. Open a jar of ammonia-based glass cleaner just a tiny crack. The sharp, eye-watering smell is ammonia. Then compare it to vinegar (acetic acid) and washing-up liquid (a near-neutral surfactant). You can detect ammonia at concentrations of just a few parts per million, which is why even tiny leaks at industrial sites are easy to spot.
Deeper dive: from war research to feeding the world

The Haber-Bosch process began as a competition for German national survival. Before 1914, Germany imported most of its nitrate fertilisers from natural deposits in Chile. When the First World War started, the British navy blockaded German ports and cut off the supply. Without nitrates, Germany could not grow enough food, and (just as worrying for its generals) could not make enough explosives.

Fritz Haber, a brilliant German chemist, had already shown that nitrogen could be combined with hydrogen to make ammonia in lab experiments. Industrialist Carl Bosch and engineer Alwin Mittasch then spent years scaling up the process to industrial size. By 1913, the first ammonia plant at Oppau in Germany was producing tonnes a day. By 1915, the process was supplying enough nitrates to keep the German war effort going.

After the war, the same process became the foundation of modern agriculture. The huge ammonia plants built for war kept running, but now their output went to fertilisers. Farm yields shot up. Famines that had been a normal part of European history became rare. Population grew from 1.6 billion in 1900 to 8 billion today, fed largely by Haber-Bosch ammonia.

Haber himself had a complicated legacy. He won the Nobel Prize in 1918 for his ammonia work, then was driven into exile in 1933 when the Nazi government persecuted Jewish scientists. He died in 1934. His wife Clara, also a chemist, had committed suicide in 1915 in protest at his earlier work developing poison gas weapons. The Haber-Bosch process feeds the world today, but its inventor lived to see what happens when chemistry serves both peace and war.

For more, see what is a base and catalysts.