hydrogenoxygen

Hydrogen–oxygen is not a single chemical compound. The term can refer to the two elements hydrogen and oxygen, to their diatomic molecules H2 and O2 that exist in the gas phase, or to compounds formed from them, most notably water (H2O). In many scientific contexts the two elements are studied together because of their central roles in combustion, energy production, and atmospheric chemistry.

Hydrogen and oxygen have distinct properties but share some common features as diatomic gases. Hydrogen is the lightest element, a colorless, mostly nonpolar diatomic molecule that is highly flammable and forms compounds with many elements. Oxygen is more chemically reactive, is a colorless diatomic gas that supports combustion, and accounts for about 21% of Earth's atmosphere. When hydrogen and oxygen react, they form water and release energy, typically represented by the equation 2 H2 + O2 → 2 H2O. Under standard conditions water is a stable liquid, with a high heat capacity and exceptional solvent abilities.

Industrial production and applications differ for the two elements. Hydrogen is produced by methods such as steam methane reforming and water electrolysis; it is used as a fuel, a reducing agent in chemical synthesis, and a feedstock for ammonia production and petroleum processing. Oxygen is produced mostly by air separation or electrolysis and is widely used in steelmaking, medical applications, combustion support, and chemical manufacturing. Oxyhydrogen, the stoichiometric mixture of hydrogen and oxygen, is an explosive gas blend historically employed in welding and cutting, though it poses significant safety hazards and is tightly regulated.

Safety considerations are central to handling hydrogen and oxygen. Both gases are non-toxic, but hydrogen is highly flammable and can form explosive mixtures with air; oxygen supports and accelerates combustion, increasing fire risk in enclosed spaces.