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On Earth, oxygen is more common than any other element. It is an invisible, odourless gas that makes up 21 per cent of air. Oxygen is found in water, minerals, and almost all living things. It is essential to life. Ordinary oxygen molecules contain two oxygen atoms. Ozone, a three-atom form, is found high up in the atmosphere. Oxygen moves through the environment via the OXYGEN CYCLE.


Divers wear a SCUBA (self-contained underwater breathing apparatus), so they can breathe under water. The SCUBA contains a cylinder of compressed air, which divers carry on their backs. The air is compressed (or squeezed) into the cylinder to increase the amount of air the divers can carry. Divers breathe through a regulator, which decompresses the air as it comes out of the cylinder.


In 1774, this chemist announced his discovery of oxygen. He didn’t realize that Swedish chemist Carl Scheele (1742–1786) had found it first, a year or two previously. They both showed that air is not one element. Priestley also discovered how to combine carbon dioxide with water to make fizzy water.


Almost all living things, including humans, need oxygen to survive. Both plants and animals take in oxygen from their surroundings to release energy. Underwater plants and animals cannot use the oxygen in air – instead they use oxygen dissolved in water. The oxygen cycle continuously circulates oxygen through the environment, so it is always available to all living things.


Plants are able to use the energy of sunlight to convert carbon dioxide (CO2) and water (H2O) into carbohydrates and oxygen (O2) in a process called photosynthesis. This oxygen is taken in by plants and animals to provide energy, releasing carbon dioxide and water. This process is called respiration.

Copyright © 2007 Dorling Kindersley

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