oxygen

The most important use of oxygen is for breathing. Animals require
oxygen in their air (or water) to live.

Another use for oxygen is for oxidation reactions -- the most obvious
example is burning. Fires require oxygen to burn. Some fire extinguishers
work by preventing oxygen from reaching the burning materials. By adding
oxygen (more than is in the air) to a fire it will burn hotter. This is
how an oxy-acetylene torch works. The additional oxygen makes the flame
much hotter and it can effectively cut metal (by melting it).

Oxygen is of great importance in the chemical and the iron and steel industries. Its other major use is in steel production, for example in the Bessemer process.

Oxygen is utilized in medicine in the treatment of respiratory diseases and is mixed with other gases for respiration in submarines, high-flying aircraft, and spacecraft.

Liquid oxygen is used as an oxidizer in the fuel systems of large rockets

Hospitals need oxygen for patients who have difficulty in breathing. Tiny premature babies often need oxygen, and people with lung diseases are kept alive by daily sessions with their breathing machines.

Industry has many uses for oxygen. The “oxy-acetylene” torch burns the gas ethyne in oxygen to produce a temperature of about 4000° C. This torch is used for cutting and welding metal plates.

The steel industry uses oxygen to burn off impurities, such as carbon, sulphur and phosphorous. The removal of these impurities turns brittle cast iron into strong steel.

Airplanes which fly at high altitude must carry oxygen. At a height of 5 km, the air pressure is only half that at sea level. Climbers who are tackling high mountains take oxygen with them.

Divers carry oxygen cylinders when they are working under water. Astronauts must carry oxygen, and even unmanned space flights need oxygen. To lift spacecraft into orbit, rockets burn kerosene in oxygen.

Air or oxygen is used in the treatment of sewage. If it were not for this treatment, many rivers and lakes would be spoiled by sewage.