"Knallgas" redirects here. For bacteria which oxidize hydrogen, see Knallgas-bacteria.

Nineteenth century electrolytic cellfor producing oxyhydrogen.

Oxyhydrogen is a mixture of hydrogen (H₂) and oxygen (O₂) gases. This gaseous mixture is used for torches for the processing of refractorymaterials and was the first^[1] gaseous mixture used for welding. Theoretically, a ratio of 2:1 hydrogen:oxygen is enough to achieve maximum efficiency; in practice a ratio 4:1 or 5:1 is required to avoid an oxidizing flame.^[2]

This mixture may also be referred to as knallgas (German; "bang-gas"), although some authors define knallgas to be a generic term for the mixture of fuel with the precise amount of oxygen required for complete combustion, thus 2:1 oxyhydrogen would be called "hydrogen-knallgas".^[3]

Brown's gas^[4] and HHO are fringe science terms for a 2:1 mixture of oxyhydrogen; its proponents claim that it's endowed with special properties

Properties

Oxyhydrogen will combust when brought to its autoignition temperature. For the stoichiometric mixture, 2:1 hydrogen:oxygen, at normal atmospheric pressure, autoignition occurs at about 570 °C (1065 °F).^[5] The minimum energy required to ignite such a mixture with a spark is about 20 microjoules.^[5] At standard temperature and pressure, oxyhydrogen can burn when it is between about 4% and 95% hydrogen by volume.^[5]

When ignited, the gas mixture converts to water vapor and releases energy, which sustains the reaction: 241.8 kJ of energy (LHV) for every mole of H₂ burned. The amount of heat energy released is independent of the mode of combustion, but the temperature of the flame varies.^[6] The maximum temperature of about 2800 °C is achieved with an exact stoichiometric mixture, about 700 degrees hotter than a hydrogen flame in air.^[7][8][9] When either of the gases are mixed in excess of this ratio, or when mixed with an inert gas like nitrogen, the heat must spread throughout a greater quantity of matter and the temperature will be lower.^[6]