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This welding torch burns acetylene gas in oxygen.This welding torch burns acetylene gas in oxygen. Oxygen is one of the commonest chemical elements on Earth. It forms about half the weight of the planet’s hard outer rock layer or crust, as it is found in most minerals, including quartz and feldspar. A colourless and odourless gas, it makes up one fifth of the air. Joined with hydrogen it forms the water which covers more than two thirds of Earth’s surface. Oxygen joins up easily with many other elements in chemical reactions to form compounds called oxides. This kind of reaction is called oxidation. Sometimes this is a slow process, as seen when iron turns to rust (iron oxide) in damp air. But if oxygen reacts very quickly, a process called combustion, or burning, takes place—and flames, light and heat are the result.

A scuba diver breathes oxygen from a tank on her back.A scuba diver breathes oxygen from a tank on her back.


We cannot see, smell or taste oxygen, yet it is essential for life. Oxygen is a vital part of chemical changes inside each microscopic living cell, which break apart food substances to obtain the energy for life. This is why oxygen is essential for all living things (except for a few specialized types of microbes): they must breathe oxygen to stay alive. Respiration, the action of breathing, is a chemical reaction: the nutrients from the food we eat reacts with the oxygen we breathe in to produce energy.

Electrolysis of water using a voltameterElectrolysis of water using a voltameter


Oxygen is the third most abundant element in the Universe, after hydrogen and helium.

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