Oxyhydrogen is a mixture of hydrogen and oxygen gases. This gaseous mixture is used for torches to process refractory materials and was the first 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 needed to avoid an oxidizing flame. This mixture may also be referred to as Knallgas, 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". "Brown's gas" and HHO are terms for oxyhydrogen mainly encountered in fringe science.
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. The minimum energy required to ignite such a mixture with a spark is about 20 microjoules. At standard temperature and pressure, oxyhydrogen can burn when it is between about 4% and 95% hydrogen by volume. When ignited, the gas mixture converts to water vapor and releases energy, which sustains the reaction: 241.8 kJ of energy for every mole of burned. The amount of heat energy released is independent of the mode of combustion, but the temperature of the flame varies. The maximum temperature of about is achieved with an exact stoichiometric mixture, about hotter than a hydrogen flame in air. 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.
Production
A pure stoichiometric mixture may be obtained by water electrolysis, which uses an electric current to dissociate the water molecules: William Nicholson was the first to decompose water in this manner in 1800. In theory, the input energy of a closed system will always equal the output energy, as the first law of thermodynamics states. However, in practice no systems are perfectly closed, and the energy required to generate the oxyhydrogen will always exceed the energy released by combusting it, even at maximum practical efficiency, as the second law of thermodynamics implies.
Applications
Lighting
Many forms of oxyhydrogen lamps have been described, such as the limelight, which used an oxyhydrogen flame to heat a piece of lime to white hotincandescence. Because of the explosiveness of the oxyhydrogen, limelights have been replaced by electric lighting.
An oxyhydrogen torch is an oxy-gas torch that burns hydrogen with oxygen. It is used for cutting and welding metals, glasses, and thermoplastics. Due to competition from arc welding and the acetylene-fueled cutting torch, the oxyhydrogen torch is seldom used today, but it remains the preferred cutting tool in some niche applications. Oxyhydrogen was once used in working platinum, because at the time, only it could burn hot enough to melt the metal. These techniques have been superseded by the electric arc furnace.
Brown's gas is associated with various exaggerated claims. It is often called "HHO gas", a term popularized by fringe physicist Ruggero Santilli, who claimed that his HHO gas, produced by a special apparatus, is "a new form of water", with new properties, based on his fringe theory of "magnecules". Many other pseudoscientific claims have been made about Brown's gas, like an ability to neutralize radioactive waste, help plants to germinate, and more. However, it is well known that hydrogen ions form the basis of pH balance in any solution, which can explain why this form of water may help seeds to obtain their germinated states in some cases. Oxyhydrogen is often mentioned in conjunction with vehicles that claim to use water as a fuel. The most common and decisive counter-argument against producing this gas on board to use as a fuel or fuel additive is that more energy is always needed to split water molecules than is recouped by burning the resulting gas. Additionally, the volume of gas that can be produced for on-demand consumption through electrolysis is very small in comparison to the volume consumed by an internal combustion engine. An article in Popular Mechanics reported that Brown's gas does not increase the fuel economy in automobiles. "Water-fueled" cars should not be confused with hydrogen-fueled cars, where the hydrogen is produced elsewhere and used as fuel or where it is used as fuel enhancement.
