Oxygen is the most abundant element in Earth's crust and the central gas for all combustion and biological respiration. At room temperature it is a colorless, odorless gas with a density around 1.429 g/L, chemically reactive and able to form oxides with nearly every element. Its largest industrial consumption is top-blowing oxygen in steel converters: pure oxygen injected into the molten bath oxidizes away excess carbon, silicon, manganese and phosphorus, turning pig iron directly into steel—this is the fundamental reason that basic oxygen converters replaced open-hearth furnaces. The chemical industry uses pure or oxygen-enriched air to produce ammonia, ethylene oxide via ethylene oxidation, coal-derived syngas, and incineration of organic waste, all of which improve reaction efficiency and cut energy use.
Medical oxygen is used in emergency care, anesthesia, at high altitude and for patients with chronic respiratory disease; central hospital oxygen supply and home oxygen concentrators have become standard. Welding and cutting shops use oxy-acetylene and oxy-propane flames to melt or sever metals, which is the main method for on-site steel-structure erection and demolition. In water and wastewater treatment, pure-oxygen aeration is more efficient than air and is used for high-strength organic wastewater and river remediation. Aerospace uses liquid oxygen as the most common rocket oxidizer, paired with liquid hydrogen, kerosene or methane to deliver high thrust. Oxygen is also heavily consumed in glass melting, pulp bleaching, silica production and tire oxygen filling.
Silicon dioxide (SiO2) is the main component of quartz, sand and rock crystal, and is the foundational raw material for glass, cement, ceramics and silicon manufacturing, as well as the core material for optical lenses, optical fibers and quartz crucibles. Water (H2O) is the classic compound of hydrogen and oxygen, covering drinking, irrigation, cooling, power generation and chemical solvents across virtually all sectors. Iron(III) oxide (Fe2O3) is hematite: the principal ironmaking ore, and in the form of iron-oxide red pigment it is used in red paints, colored asphalt, floor tiles and ceramic glazes. Other common oxygen compounds include sulfur dioxide (SO2), mainly from coal combustion and volcanoes and also the feedstock for the sulfuric-acid industry, and carbon dioxide (CO2), widely used in carbonated beverages, welding shielding, greenhouse fertilization and dry-ice cold chains.
