Oxygen

Category: non-metal Atomic number: 8 Colour: none Melting point: −219°C (−362°F) Boiling point: −183°C (−297°F) First identified: 1770s

Together with carbon, oxygen is one of the most crucial components of life on Earth. We breathe in, absorb and exhale carbon dioxide. Our brain, DNA and cells, and pretty much every molecule in our body, rely on oxygen, and it makes up about 60 per cent of our body mass, mostly in the form of water, or H₂O.

However, in spite of oxygen being the third most abundant element in the universe, it was something of an accident that our planet ended up with such a profuse supply. Before any larger animals existed, plants and cyanobacteria took their energy from the sun, absorbing carbon dioxide and exhaling oxygen. As oxygen is a highly reactive element, a large part of that oxygen reacted with other elements to form compounds. For example, did you know that 46 per cent of the mass of the rocks on the planet is oxygen – humble sand is actually silicon dioxide, many metals that we extract are taken from oxides (for instance, iron often comes from hematite, aluminium from bauxite), and carbonates such as limestone also contain oxygen.

In addition, the excreted oxygen entered the atmosphere, of which it gradually reached a level of about 21 per cent, effectively terraforming planet Earth. It was dissolved oxygen in the water that allowed species to develop there, and over time life also evolved on land.

In the fifteenth century, Leonardo da Vinci had noticed that a candle wouldn’t burn without air and speculated that it contained something life-giving. The discovery of oxygen was made independently by three chemists in the 1770s. In 1774, Joseph Priestley collected oxygen by focusing sunlight on mercuric oxide, and observed that the resulting gas made a candle burn more brightly (like his colleague Henry Cavendish, he wrongly identified this as ‘dephlogisticated air’). In 1777, the Swedish scientist Carl Wilhelm Scheele published an account of discovering oxygen, which he had actually experimented with in 1771. And Antoine Lavoisier also identified oxygen – in fact, he recognized that this was indeed a new element, rather than air without phlogiston in it, although his name for it, ‘oxy-gène’, meaning ‘acid-forming’, was derived from the incorrect assumption that this gas would be present in all acids. In spite of that, the name stuck.

One of oxygen’s less appealing qualities is that, because it is so reactive and helps to support many microorganisms, it causes many forms of decay, including rotting food. For many years, scientists have battled to come up with ever more inventive ways of keeping it away from food – fruit can be stored in nitrogen, buried, kept in tins or vacuum packs; it can be frozen, dried, cured or bottled to prevent oxygen doing its dirty work.

Relight My Fire

One of the classic chemistry experiments is a test for pure oxygen. First, you place some pure oxygen (or air with a high density of oxygen) in a flask. Then you light a wooden splint before shaking it to extinguish the flame – at this point it will be glowing slightly and will glow a bit more orange if you blow on it, but it will not relight. However, if you put it momentarily into the flask of oxygen, it will immediately relight, showing how reactive oxygen is and how easily it feeds flames.

At higher altitudes, the concentration of oxygen in the atmosphere is lower, which is why we find it harder to breathe there. And remember that oxygen doesn’t just come in the dioxygen molecules we breathe (two oxygen atoms bonded together). It can also come as trioxygen, better known as ozone or O₃, which has three atoms. In the stratosphere, oxygen particles are constantly bombarded by UV radiation and split into individual atoms – these recombine with dioxygen molecules to form ozone molecules, which in their turn are hit by UV and split apart again. This is a continuous churning process and one that is crucial in protecting us from the sun’s dangerous UV rays. When we are lucky enough to be able to watch the aurora or the Northern or Southern Lights, the beautiful swirling patterns are the result of the solar wind colliding with oxygen molecules way up in the atmosphere.