NGC 604, a giant region of ionized hydrogen in the Triangulum Galaxy

Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of 1.00794 u (1.007825 u for hydrogen-1), hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly composed of hydrogen in its plasma state. Naturally occurring elemental hydrogen is relatively rare on Earth.

Hydrogen gas (now known to be H2) was first artificially produced in the early 16th century, via the mixing of metals with strong acids. In 1766–81, Henry Cavendish was the first to recognize that hydrogen gas was a discrete substance, and that it produces water when burned, a property which later gave it its name, which in Greek means "water-former." At standard temperature and pressure, hydrogen is a colorless, odorless, nonmetallic, tasteless, non-toxic, highly combustible diatomic gas with the molecular formula H2.

Industrial production is mainly from the steam reforming of natural gas, and less often from more energy-intensive hydrogen production methods like the electrolysis of water. Most hydrogen is employed near its production site, with the two largest uses being fossil fuel processing (e.g., hydrocracking) and ammonia production, mostly for the fertilizer market.

The Space Shuttle Main Engine burns hydrogen with oxygen, producing a nearly invisible flame at full thrust.

Hydrogen gas (dihydrogen or molecular hydrogen) is highly flammable and will burn in air at a very wide range of concentrations between 4% and 75% by volume. The enthalpy of combustion for hydrogen is −286 kJ/mol:

2 H2(g) + O2(g) → 2 H2O(l) + 572 kJ (286 kJ/mol)

Hydrogen gas forms explosive mixtures with air if it is 4–74% concentrated and with chlorine if it is 5–95% concentrated. The mixtures spontaneously explode by spark, heat or sunlight. The hydrogen autoignition temperature, the temperature of spontaneous ignition in air, is 500 °C (932 °F). Pure hydrogen-oxygen flames emit ultraviolet light and are nearly invisible to the naked eye, as illustrated by the faint plume of the Space Shuttle Main Engine compared to the highly visible plume of a Space Shuttle Solid Rocket Booster. The detection of a burning hydrogen leak may require a flame detector; such leaks can be very dangerous. The destruction of the Hindenburg airship was an infamous example of hydrogen combustion; the cause is debated, but the visible flames were the result of combustible materials in the ship's skin. Because hydrogen is buoyant in air, hydrogen flames tend to ascend rapidly and cause less damage than hydrocarbon fires. Two-thirds of the Hindenburg passengers survived the fire, and many deaths were instead the result of falls or burning diesel fuel.

H2 reacts with every oxidizing element. Hydrogen can react spontaneously and violently at room temperature with chlorine and fluorine to form the corresponding hydrogen halides, hydrogen chloride and hydrogen fluoride, which are also potentially dangerous acids.

The Industrial Gas Business is the supply of technical gases by one of three supply modes;

• On-site and pipeline,
• Bulk - (liquids and tube-trailer compressed gases)
• Cylinder or packaged gases.

It excludes gases used primarily for heating, such as natural gas and LPG, except where these are used for cutting and welding or supplied at very high purity for chemical or other use.

The defining characteristic of the Industrial Gas business is that whilst the cost of such gases in the final product is often small, the absence of the gas generally inhibits production.

The Industrial Gas Business is thus a service industry focused on the reliability and availability of the supply to end-users.

Cost Management

However, Industrial Gas Companies realise that costs can really only be managed by production and distribution expertise for any supply mode. Therefore, a successful Industrial Gas Company manages its costs by supply mode or product line but manages its customers and applications technologies by market sector.

In the case of larger volume users there is also an option to own an industrial gas plant and "make" their own gases, rather than the "buy" option of onsite or pipeline supply.

Most of the major Industrial Gas Companies will sell such plant to an end-user as will a large number of independent equipment suppliers.

The "make" versus "buy" decision is often at the heart of most new large volume industrial gas requirements. Plants owned by end-users are generally referred to as "captive".

Industrial Gas Companies view the conversion of "captive" capacity to "onsite" as a low risk opportunity to increase market penetration or market share.

There are six broad categories of gas

Industrial

The basic gases used in "industrial" applications, often in large quantities:

• Oxygen, nitrogen and argon - the "air gases"
• Acetylene and acetylene substitutes
• Carbon dioxide
• Hydrogen
• Carbon monoxide
• Air and "artificial" air

Medical

Gases intended for use in the medical field:

• Special grades of oxygen or breathing air
• Nitrous oxide
• Special grade carbon dioxide
• Breathing mixtures
• Blood analysis mixtures

Special

Rare, uncommon or speciality gases used in small quantities and delivered with strict quality assurance:

• Helium
• Xenon, Krypton, Neon - the noble gases
• Lighting mixtures
• Ultra-pure electronics gases
• Electronics dopants such as silane and arsine
• Calibration gas mixtures
• Research grade gas mixtures
• Sterilisation and fumigation gases

Fuel Gases

Gases such as LPG or its components from some companies but generally not considered when used for heating

• Butane
• Propane
• Cutting mixtures

Refrigerants

Gases used in the refrigeration cycle in cooling technologies are supplied by some companies.

Fire fighting gases are supplied by some companies

• Carbon dioxide
• Halon substitutes