Image by Greg Robson

There is more oxygen in the earth’s crust than any other element and it makes up 65% of the human body – mostly in the form of water (H2O). In order to survive, animals need oxygen in the form of water, in its pure gaseous form (O2) and as part of hundreds of other compounds that make up our bodies.

Pure oxygen, which is just two atoms joined together to make O2, doesn’t look like much. In fact it’s totally transparent and we can’t smell or taste it either. It only makes up 21% of the air but it is one of the most reactive elements and the cause of explosive reactions in which things burn. Any fire will go out and a mortar shell filled with TNT can’t explode if there isn’t enough oxygen in the air.

Image by Waugsberg

O2 is also what causes metal things to go rusty if you leave them out. When they get wet, iron objects react with the oxygen in the air to form an iron-oxygen compound known as rust! If you took an old car to the moon, it wouldn’t go rusty no matter how much water you sprayed on it as there is no oxygen there.

There is another form of pure oxygen on earth – ozone – which is made of three oxygen atoms joined together. Unlike O2, ozone (O3)is very reactive and damaging to your lungs if you breathe it in. You can find ozone in car exhaust fumes and it is considered a pollutant down here near the earth’s surface. Up in the atmosphere however, a layer of ozone around the earth is actually extremely important for shielding us from the sun’s ultra-violet radiation.

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Earth has far more oxygen in the atmosphere than other planets because life here evolved a reaction called photosynthesis. The bacteria and plants that evolved photosynthesis were able to grow using only carbon dioxide, water and sunlight. As a side-effect they produced oxygen and soon the Earth’s atmosphere was filled with it. Towards the end of the Carboniferous era (about 300 million years ago) there was so much oxygen around that insects and amphibians were able to grow to terrifyingly enormous sizes. That’s how you ended up with two and a half metre long millipedes and giant dragonflies with 75cm wingspans!

A man called Joseph Priestly discovered oxygen and he noticed that a mouse sealed in a high-oxygen environment seemed more active and lived longer. A flame would also burn brighter in this strange new substance, and when he tried breathing in the gas himself (a dangerous past-time for a chemist!) he claimed that “I fancied that my breast felt peculiarly light and easy for some time afterwards.”

In the modern day you might be given extra oxygen if you are in hospital and having trouble breathing. Airplanes often have an emergency supply of oxygen masks which activate in case of a sudden drop in cabin pressure. Some professional athletes even breathe in oxygen in order to boost their performance in sports like American football. Does this really work? Scientists think that breathing in oxygen before a game probably doesn’t make any difference once you get on the pitch, but if the athletes think it helps then they might play better anyway! They have to be careful though, because breathing in too much pure oxygen at high pressures can actually be poisonous.

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