Nitrogen

Category: non-metal Atomic number: 7 Colour: none Melting point: −210°C (−346°F) Boiling point: −196°C (−320°F) First identified: 1772

Nitrogen is a gas that makes up 78 per cent of the atmosphere of the Earth, and that we use in a huge variety of ways, from airbags to spray-on cream to the ‘laughing gas’ used in hospitals. However, we have only known of its independent existence for about 250 years.

Scientists in the eighteenth century became fascinated by the make-up of the air we breathe. The Scottish chemist Joseph Black had isolated carbon dioxide in the 1750s. He called it ‘fixed air’, as it could be released from minerals such as limestone when treated with acid. However, it was also known as ‘mephitic air’, meaning ‘poisonous’, because it seemed to suffocate animals that were immersed in it. If the oxygen in an enclosed space is all burned up, the remaining air has a similar effect, but in this case it is nitrogen (along with the other non-oxygen gases, including carbon dioxide, in the atmosphere) that is responsible.

Henry Cavendish was investigating mephitic air when he first discovered nitrogen, but the discovery is generally credited to Black’s student, Daniel Rutherford, who carried out similar experiments and published his findings in 1772. Cavendish, a methodical, slightly obsessive man, carried out numerous repetitions of an experiment in which he separated out the components of air. First, he passed air over heated charcoal, which converted the oxygen to carbon dioxide. Then he dissolved the carbon dioxide using an alkali solution. This left behind a separate gas, whose main component would later be named nitrogen (‘nitre-forming’) because it formed potassium nitrate (also known as saltpetre or nitre), a key component in early gunpowder.

Nitrogen played an important role in the development of explosives. Nitroglycerine, a liquid that will explode on impact, is formed when glycerine reacts with nitric acid. Alfred Nobel invented a much safer explosive, dynamite, when he found a way to absorb nitroglycerine into the soft rock ‘kieselguhr’ (also called diatomaceous earth).

The explosive nature of nitrogen is exploited when sodium azide is used in car airbags. A compound of sodium and nitrogen, it can be triggered by a spark to explode and decompose into nitrogen gas and sodium metal – the nitrogen rapidly inflates the bag.

Nitrogen can also come in handy if you want to freeze a piece of fruit. Liquid nitrogen is used in the flash-freezing process: you can watch a banana that has been frozen in nitrogen being smashed into tiny pieces with a hammer in various online videos. And nitrogen can be used to preserve fruit, too; if the fruit is stored in a non-refrigerated, sealed box of nitrogen, the types of decay that are caused by reactions with oxygen are prevented, and it will keep for up to two years.

Nitrogen is also used in the widgets that cause beer cans to froth – a ball containing nitrogen, with a small hole in it, is left in the can, then a dash of liquid nitrogen is added to the beer in the compression process. This expands when the can is sealed, so that the nitrogen is compressed into the ball – when the can is opened, the pressure is released, triggering a burst of gas through the beer. Spray-on cream also relies on compressed nitrogen: in this case, nitrous oxide (‘laughing gas’) is absorbed into the cream, which is compressed in the can. When the compression is temporarily released, the pressure forces the cream out of the can.

Green plants and algae absorb nitrates, which help to form DNA and the amino acids that help in the creation of protein; thus nitrogen is also a key element for our living organisms. Animals consume nitrogen through their diet, and the nitrogen is eventually released back into the atmosphere. Microbes and bacteria in the soil then convert nitrogen back into nitrates (and this process can be encouraged by the addition of chemical fertilizers made from ammonia, which is a compound of nitrogen and hydrogen).