Under ordinary conditions hydrogen is a colorless, odorless, tasteless gas that is only slightly soluble in water; it is the least dense gas known. It is the first element in Group 1 of the periodic table. Ordinary hydrogen gas is made up of diatomic molecules (H2) that react with oxygen to form water (H2O) and hydrogen peroxide (H2O2), usually as a result of combustion. A jet of hydrogen burns in air with a very hot blue flame. The flame produced by a mixture of oxygen and hydrogen gases (as in the oxyhydrogen blowpipe) is extremely hot and is used in welding and to melt quartz and certain glasses. Hydrogen gas must be used with caution because it is highly flammable; it forms easily ignited explosive mixtures with oxygen or with air (because of the oxygen in the air). At high temperatures hydrogen is a chemically active mixture of monohydrogen (atomic hydrogen) and the normal diatomic hydrogen (see allotropy).
Hydrogen has a great affinity for oxygen and is a powerful reducing agent (see oxidation and reduction). It reacts with nitrogen to form ammonia. With the halogens it forms compounds (hydrogen halides) that are strongly acidic in water solution. With sulfur it forms hydrogen sulfide (H2S), a colorless gas with an odor like rotten eggs; with sulfur and oxygen it forms sulfuric acid. It combines with several metals to form metal hydrides such as calcium hydride. Combined with carbon (and usually other elements) it is a constituent of a great many organic compounds, such as hydrocarbons, carbohydrates, fats, oils, proteins, and organic acids and bases.
It is theoretically possible for hydrogen to exhibit the properties of a metal, such as electrical conductivity. Although researchers have been able to squeeze hydrogen into liquid and crystalline solid states through applications of intense heat, cold, and pressure, the metallic form eluded them until 1996. By compressing liquid hydrogen to nearly 2 million atmospheres pressure and a temperature of 4,400°K, a team at the Lawrence Livermore National Laboratory created metallic hydrogen for a millionth of a second. While there is no practical application for the accomplishment, proof of the existence of a metallic form of hydrogen may have implications for theories of how Jupiter's magnetic field is produced.
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