Oxyhydrogen is a mixture of hydrogen gas (H₂) and oxygen gas (O₂). This gas mixture is used in torches to work with very hard materials and was the first gas mixture used for welding. In theory, a ratio of 2 parts hydrogen to 1 part oxygen is enough for best results. However, in practice, a ratio of 4 parts hydrogen to 1 part oxygen or 5 parts hydrogen to 1 part oxygen is needed to prevent a flame that can damage materials.

This mixture is also called Knallgas in some regions, such as Scandinavia and Germany, which means "bang-gas" in those languages. Some sources use the term "Knallgas" to describe any fuel mixed with the exact amount of oxygen needed for complete burning, making the 2:1 oxyhydrogen mixture called "hydrogen-knallgas."

Terms like "Brown's gas" and HHO are names used for oxyhydrogen in unscientific contexts. However, the formula x H₂ + y O₂ is preferred because HHO can be confused with water (H₂O).

Properties

Oxyhydrogen burns when it reaches its autoignition temperature. For a balanced mixture in air at normal atmospheric pressure, autoignition happens at about 570 °C (1,058 °F). At standard temperature and pressure, a spark needs about 0.007 mJ of energy to ignite the mixture. This mixture can burn when hydrogen makes up between 4% and 95% of the gas by volume.

When ignited, the gas mixture changes into water vapor and releases energy. This energy helps keep the reaction going, producing about 241.8 kJ of energy (LHV) for every mole of hydrogen burned. The amount of heat released does not depend on how the gas burns, but the flame temperature changes. The highest flame temperature, about 2,800 °C (5,100 °F), occurs with a balanced mixture. This is about 700 °C (1,300 °F) hotter than a hydrogen flame in air. If the mixture has too much of one gas or is mixed with an inert gas like nitrogen, the heat spreads through more material, lowering the flame temperature.

Oxyhydrogen is explosive and can explode when ignited, releasing a large amount of energy. This is sometimes shown in classrooms, where teachers fill a balloon with the gas because hydrogen and oxygen are easily available.

Production by electrolysis

A mixture with the exact amounts of hydrogen and oxygen can be made using water electrolysis. This process uses electricity to break apart water molecules into hydrogen and oxygen gas. William Nicholson was the first person to do this in 1800. According to the first law of thermodynamics, in a closed system, the energy put in should equal the energy produced. However, in real situations, no systems are completely closed. The energy needed to create hydrogen and oxygen gas is more than the energy released when they burn, even when working as efficiently as possible. This is explained by the second law of thermodynamics (see Electrolysis of water#Efficiency).

Applications

Many types of oxyhydrogen lamps have been created, including the limelight. This lamp used an oxyhydrogen flame to heat quicklime until it glowed very brightly. However, oxyhydrogen is very explosive, so limelights are no longer used. Electric lighting has replaced them.

The oxy-hydrogen blowpipe was first studied by Carl Wilhelm Scheele and Joseph Priestley in the late 1700s. Later, in the late 1700s and early 1800s, scientists such as Bochard-de-Saron, Edward Daniel Clarke, and Robert Hare developed the blowpipe. It produced a flame hot enough to melt very hard materials like platinum, porcelain, fire brick, and corundum. This tool was important for many scientific fields. It is used in the Verneuil process to make synthetic corundum.

An oxyhydrogen torch, also called a hydrogen torch, burns hydrogen (the fuel) with oxygen (the oxidizer). It is used to cut and weld metals, glass, and thermoplastics.

Because of competition from arc welding and other oxy-fuel torches, like acetylene-fueled cutting torches, oxyhydrogen torches are rarely used today. However, they are still preferred in some special applications.

Oxyhydrogen was once used to work with platinum because it was the only method that could melt the metal at 1,768.3 °C (3,214.9 °F). These methods have been replaced by the electric arc furnace.

Pseudoscientific claims

Oxyhydrogen is linked to many exaggerated claims. It is sometimes called "Brown's gas" or "HHO gas," a name promoted by scientist Ruggero Santilli, who claimed his HHO gas, made using a special device, is "a new form of water" with different properties based on his unproven theory of "magnecules."

Other unscientific claims about oxyhydrogen include the idea that it can neutralize radioactive waste or help plants grow.

Oxyhydrogen is often discussed in connection with vehicles that claim to use water as fuel. A major reason why producing this gas on a vehicle for fuel or as a fuel additive is not practical is that more energy is required to split water molecules than is gained by burning the gas. Also, the amount of gas created through electrolysis for immediate use is much smaller than what an internal combustion engine would need.

An article in Popular Mechanics in 2008 stated that oxyhydrogen does not improve fuel efficiency in cars.

"Water-fueled" cars should not be confused with "hydrogen-fueled" cars, where hydrogen is made in another place and used as an energy source, or where hydrogen is used to improve fuel performance.