Oxygen is a gas that is important for living things and supports combustion. It makes up 21% of the earth's atmosphere.
It can be formed when green plants carry out photosynthesis, a process which carbon dioxide is converted into oxygen by the plants.
It is also formed when saltpeter is heated and decomposed into oxygen gas.
Liquid oxygen and liquid hydrogen makes a very energetic fuel for rockets.
Melting point: −219 °C; −362.2°F; 54.2 K
Boiling point: −183 °C; −297.4 °F; 90.2 K
Oxygen gas was discovered in 1771 by Carl Wilhelm Scheele. He called the gas "fire air" because it was the only substance that supports combustion. In 1777 Antoine Lavoisier named it Oxygen.