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  • Oxygen is a colourless, odourless gas which, when combined with other elements, is the most plentiful element in the Earth’s crust.

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

  • In the lower atmosphere, oxygen exists almost entirely as diatomic molecules (O2). In the upper atmosphere, oxygen is present most predominantly as the triatomic ozone, O3 (which shields the Earth from the Sun’s ultraviolet radiation) and monatomic oxygen, O (which in the Northern hemisphere is responsible for the green display known as the Aurora Borealis or Northern Lights).

  • During respiration, animals (and lower plants, such as algae and fungi) convert oxygen from the air into carbon dioxide whereas green plants assimilate this carbon dioxide in the presence of sunlight to produce oxygen.

  • Oxygen accounts for 21 % by volume of dry air and results from the above process of photosynthesis.

  • Oxygen, combined as H2O, is the major component of water accounting for almost 90 % of its mass.

  • Oxygen dissolves in water to the extent of 3 parts of oxygen to 100 parts of water, by volume. This solubility is essential for respiration of marine life.

  • At - 183 ℃ (-297 ℉), oxygen condenses to a pale blue liquid which is strongly paramagnetic and can be held between the poles of a magnet.

  • Oxygen does not burn. It enables the combustion of other substances and, in its liquid state, oxygen is the oxidant for rocket fuel.

  • Joseph Priestley discovered oxygen on 1st August 1774 by heating red mercuric oxide which decomposes to metallic mercury releasing the pure gas.

  • Priestley noted that an adult mouse, when placed in a sealed vessel filled with oxygen, could live for a much longer time than when placed in the same volume of ordinary air. When removed from the depleted vessel, seemingly dead, the mouse was revived when warmed by the fire.

  • On breathing the newly discovered gas, Priestley wrote: -


“My breast felt particularly light and easy for some time afterwards. Who can tell, but that in time, this pure air may become a fashionable article of luxury. Hitherto, only two mice and myself have had the privilege of breathing it”.


  • Priestley’s discovery of oxygen was actually pre-empted by Carl Scheele whose surviving laboratory notes reveal that he had prepared the gas in 1772. This discovery was written in a book in 1773 but was not published until four years later.

  • The name ‘oxygen (oxygène), from the Greek for ‘acid former’, was coined by Antoine Lavoisier in 1779 who recognised the gas as an element.


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