Liquid oxygen (O) is pale blue and extremely cold. Although nonflammable by itself, oxygen is a strong oxidizer. Oxygen is the second largest component of the atmosphere, comprising 20.8% by volume. Oxygen is necessary to support life. Oxygen will react with nearly all organic materials and metals usually forming an oxide. Liquid oxygen is a cryogenic liquid. Liquid oxygen has a boiling point of -297.3°F (-183.0°C or 90.21 K). Oxygen is often stored as a liquid, although it is used primarily as a gas. Liquid storage is less bulky and less costly than the equivalent capacity of high-pressure gaseous storage. A typical storage system consists of a cryogenic storage tank, one or more vaporizers, a pressure control system, and all piping necessary for the fill, vaporization, and supply functions.
Several chemists had produced oxygen prior to its discovery in 1774, but they failed to recognize it as a distinct element. Joseph Priestley and Carl Wilhelm Scheele both independently discovered oxygen, but Priestly is usually given credit for the discovery. They were both able to produce oxygen by heating mercuric oxide (HgO). Priestley called the gas produced in his experiments “dephlogisticated air”‘ and Scheele called his “fire air.” Antoine Lavoisier who incorrectly believed that oxygen was necessary to form all acids created the name oxygen.
Oxygen accounts for nearly half of the mass of the earth’s crust, two-thirds of the mass of the human body, and nine tenths of the mass of water. Large amounts of oxygen can be extracted from liquefied air through a process known as fractional distillation. Oxygen can also be produced through the electrolysis of water or by heating potassium chlorate (KClO3).
Oxygen is a highly reactive element and is capable of combining with most other elements. It is required by most living organisms and for most forms of combustion. Impurities in molten pig iron are burned away with streams of high-pressure oxygen to produce steel. Oxygen can also be combined with acetylene (C2H2) to produce an extremely hot flame used for welding. Liquid oxygen, when combined with liquid hydrogen, makes an excellent rocket fuel. Oxygen is also a component of hundreds of thousands of organic compounds.