All air breathing animals and plants use oxygen present in air by the process of respiration. This is known as aerobic respiration. During respiration, food is burnt in living cells by using oxygen to release energy for all metabolic activities. Without breathing in of air (and the oxygen it contains), life would cease to exist.When the quantity of oxygen is insufficient, oxygen packed in cylinders is used as an aid to breathing. For e.g., by patients in hospitals, at high latitudes by mountaineers, under water by divers, by astronauts walking in space etc.
We all know that oxygen is needed to burn fuels. Atmospheric oxygen is used for the combustion of fuel to generate energy in automobiles, aeroplanes, ships, generators, industrial process etc. In space there is no air and hence no oxygen. Yet liquid oxygen-known as LOX, is stored within the spacecraft and used to burn the fuel for producing the required thrust in space.
For welding and cutting of metals
In oxy-acetylene and oxy-hydrogen blow torches, oxygen is used for producing very high temperatures required to melt metals (Fig.13.17). The temperature of oxy-acetylene blow pipe is around 3000oC, and that of the oxy-hydrogen is around 2800oC.
In the process of welding, the two metal pieces are brought together, and the flame is applied at the junction. The high temperature of the flame melts the metals at the junction. When the flame is removed the metal pieces fuse and solidify to get joined together.When the flame is used for cutting a metal into two, it is applied to one part till it becomes red hot. Then the quantity of oxygen is increased. The oxygen oxidises the red hot part of the metal, and the oxide being very brittle, can be broken off by hitting it with a hammer.
For production of iron and steel
Usually, in blast furnaces the oxygen in hot air is used. When hot oxygen is directly used, the extraction of iron and steel is much faster.
For explosives in mines
In mines, oxygen is used as a part of explosives for blasting rocks.
In chemical industry
In the chemical industry, oxygen is used in the manufacture of nitric acid, sulphuric acid etc.