sodium hydrogen carbonate: see sodium bicarbonate.

potassium hydrogen tartrate: see cream of tartar.

hydrogen-ion concentration: see pH.

hydrogen sulfide, chemical compound, H₂S, a colorless, extremely poisonous gas that has a very disagreeable odor, much like that of rotten eggs. It is slightly soluble in water and is soluble in carbon disulfide. Dissolved in water, it forms a very weak dibasic acid that is sometimes called hydrosulfuric acid. Hydrogen sulfide is flammable; in an excess of air it burns to form sulfur dioxide and water, but if not enough oxygen is present, it forms elemental sulfur and water. Hydrogen sulfide is found naturally in volcanic gases and in some mineral waters. It is often formed during decay of animal matter. It is a part of many unrefined carbonaceous fuels, e.g., natural gas, crude oil, and coal; it is obtained as a byproduct of refining such fuels. It may be made by reacting hydrogen gas with molten sulfur or with sulfur vapors, or by treating a metal sulfide (e.g., ferrous sulfide, FeS) with an acid. Hydrogen sulfide reacts with most metal ions to form sulfides; the sulfides of some metals are insoluble in water and have characteristic colors that help to identify the metal during chemical analysis. Hydrogen sulfide also reacts directly with silver metal, forming a dull, gray-black tarnish of silver sulfide (Ag₂S).

hydrogen peroxide, chemical compound, H₂O₂, a colorless, syrupy liquid that is a strong oxidizing agent and, in water solution, a weak acid. It is miscible with cold water and is soluble in alcohol and ether. Although pure hydrogen peroxide is fairly stable, it decomposes into water and oxygen when heated above about 80°C;; it also decomposes in the presence of numerous catalysts, e.g., most metals, acids, or oxidizable organic materials. A small amount of stabilizer, usually acetanilide, is often added to it. Hydrogen peroxide has many uses. It is available for household use as a 3% (by weight) water solution; it is used as a mild bleaching agent and medicinally as an antiseptic. The 3% solution is sometimes called ten volume strength, since one volume of it releases ten volumes of oxygen when it decomposes. Hydrogen peroxide is available for commercial use in several concentrations. Highly concentrated solutions were first used in World War II by the military, e.g., in fuels for rockets and torpedoes. It is used as a bleaching agent for textiles, e.g., wool and silk, and in paper manufacture. It is also used in chemical manufacture. Hydrogen peroxide is prepared commercially by oxidation of alkylhydroanthraquinones and by electrolysis of ammonium bisulfate. It can also be prepared by reaction of barium peroxide with sulfuric acid and is prepared (with acetone) by oxidation of isopropanol. Hydrogen peroxide was discovered (1818) by L. J. Thenard.

hydrogen fluoride, chemical compound, HF, a colorless, fuming liquid or colorless gas that boils at 19.54°C;. It is miscible with water and is soluble in benzene, toluene, and concentrated sulfuric acid. Hydrofluoric acid is a water solution of hydrogen fluoride; hydrofluoric acid containing 35.35% hydrogen fluoride by weight is an azeotrope with a constant boiling point of 120°C;. Whether gaseous, liquid, or in solution, hydrogen fluoride is a dangerous chemical and must be handled with caution, since it attacks the skin and other tissue. Hydrogen fluoride has a number of properties that distinguish it from the other hydrogen halides. It polymerizes, forming molecules such as H₂F₂ and H₆F₆; this explains in part its relatively high boiling point. It is a relatively weak acid. It attacks glass, reacting with the silica, SiO₂, to form the gas silicon tetrafluoride, SiF₄, and water; this leaves the surface of the glass etched. Major industrial uses of hydrogen fluoride include the synthesis of fluorocarbons (e.g., Freon and teflon) and the production of aluminum fluoride and synthetic cryolite for use in aluminum refining. It is also employed in refining uranium for use as a nuclear fuel, in manufacturing various organic chemicals, in producing stainless steel, and for various other applications. Hydrogen fluoride is produced commercially by heating purified fluorspar (calcium fluoride) with concentrated sulfuric acid to produce the gas, which may be condensed by cooling or dissolved in water. Hydrogen fluoride is available commercially either in an anhydrous (water-free) state or in water solutions of various concentrations. Because it attacks glass, it is usually stored in steel tanks, cylinders, or drums, or, in small amounts, in plastic bottles.

hydrogen cyanide, HCN, colorless, volatile, and extremely poisonous chemical compound whose vapors have a bitter almond odor. It melts at -14°C; and boils at 26°C;. It is miscible in all proportions with water or ethanol and is soluble in ether. Its water solution is a weak acid (see acids and bases) commonly known as hydrocyanic acid or prussic acid. Its salts are called cyanides. Hydrogen cyanide may be synthesized directly from ammonia and carbon monoxide or from ammonia, oxygen (or air), and natural gas. It is a byproduct of the production of coke from coal and is recovered (along with hydrogen sulfide) from coke-oven exhaust gases. It may also be prepared by reacting a cyanide salt, e.g., calcium cyanide, with a strong acid, e.g., sulfuric acid, or by thermal decomposition of formamide. Because impure hydrogen cyanide can undergo spontaneous explosive polymerization and decomposition, a small amount of stabilizer (usually phosphoric acid) is added to it. The principal use of hydrogen cyanide is in the manufacture of organic chemicals, e.g., acrylonitrile, methyl methacrylate, and adiponitrile, that are used in producing synthetic fibers and plastics. It is also used in the chemical laboratory, and is sometimes used in agriculture as a fumigant. Hydrogen cyanide is found in nature in some vegetable substances, e.g., bitter almond, peach stones, cherry and cherry laurel leaves, and sorghum; it is usually combined in glycoside molecules (see sugar) and is released when they are broken down by enzymes during metabolism.

hydrogen chloride, chemical compound, HCl, a colorless, poisonous gas with an unpleasant, acrid odor. It is very soluble in water and readily soluble in alcohol and ether. It fumes in moist air. It is not flammable, and the liquid is a poor conductor of electricity. Hydrogen chloride is prepared commercially by the reaction of sulfuric acid with sodium chloride (common salt); niter cake, a mixture of sodium bisulfite and sulfuric acid that is a byproduct of nitric acid manufacture, is sometimes used in place of sulfuric acid. Hydrogen chloride is also produced as a byproduct of the manufacture of chlorinated organic chemicals. It can be prepared directly by reaction of hydrogen and chlorine gases; the reaction is very exothermic and takes place readily in sunlight or at elevated temperatures. Although anhydrous (water-free) hydrogen chloride is commercially available as a high-pressure compressed gas in steel cylinders, most of the gas produced is dissolved in water to form hydrochloric acid (see acids and bases), a commercially important chemical. Pure grades of hydrochloric acid are colorless, but technical grades, commonly called muriatic acid, are often yellow-colored because of impurities such as dissolved metals. Most hydrochloric acid produced has a concentration of 30% to 35% hydrogen chloride by weight. The major use of hydrochloric acid is in the manufacture of other chemicals. It is also used in large amounts in pickling (cleaning) metal surfaces, e.g., iron before galvanizing. It reacts with most common metals, releasing hydrogen and forming the metal chloride; with most metal oxides and hydroxides it reacts to form water and the metal chloride. Hydrochloric acid is also used in small amounts in processing glucose and other foods and for various other uses. Concentrated solutions are strong acids and highly corrosive. Hydrochloric acid is not an oxidizing agent but can be oxidized by very strong oxidizing agents, liberating chlorine gas. In dilute solutions of the acid the hydrogen chloride is almost completely dissociated into hydrogen and chloride ions. A solution containing 20.24% hydrogen chloride by weight is azeotropic, boiling at a constant temperature of 110°C; at atmospheric pressure. Hydrogen chloride also forms monohydrates, dihydrates, and trihydrates that are liquids at room temperature.

hydrogen bomb or H-bomb, weapon deriving a large portion of its energy from the nuclear fusion of hydrogen isotopes. In an atomic bomb, uranium or plutonium is split into lighter elements that together weigh less than the original atoms, the remainder of the mass appearing as energy. Unlike this fission bomb, the hydrogen bomb functions by the fusion, or joining together, of lighter elements into heavier elements. The end product again weighs less than its components, the difference once more appearing as energy. Because extremely high temperatures are required in order to initiate fusion reactions, the hydrogen bomb is also known as a thermonuclear bomb.

The first thermonuclear bomb was exploded in 1952 at Enewetak by the United States, the second in 1953 by Russia (then the USSR). Great Britain, France, and China have also exploded thermonuclear bombs, and these five nations comprise the so-called nuclear club—nations that have the capability to produce nuclear weapons and admit to maintaining an inventory of them. The three smaller Soviet successor states that inherited nuclear arsenals (Ukraine, Kazakhstan, and Belarus) relinquished all nuclear warheads, which have been removed to Russia. Several other nations either have tested thermonuclear devices or claim to have the capability to produce them, but officially state that they do not maintain a stockpile of such weapons; among these are India, Israel, and Pakistan. South Africa's apartheid regime built six nuclear bombs but dismantled them later.

The presumable structure of a thermonuclear bomb is as follows: at its center is an atomic bomb; surrounding it is a layer of lithium deuteride (a compound of lithium and deuterium, the isotope of hydrogen with mass number 2); around it is a tamper, a thick outer layer, frequently of fissionable material, that holds the contents together in order to obtain a larger explosion. Neutrons from the atomic explosion cause the lithium to fission into helium, tritium (the isotope of hydrogen with mass number 3), and energy. The atomic explosion also supplies the temperatures needed for the subsequent fusion of deuterium with tritium, and of tritium with tritium (50,000,000°C; and 400,000,000°C;, respectively). Enough neutrons are produced in the fusion reactions to produce further fission in the core and to initiate fission in the tamper.

Since the fusion reaction produces mostly neutrons and very little that is radioactive, the concept of a "clean" bomb has resulted: one having a small atomic trigger, a less fissionable tamper, and therefore less radioactive fallout. Carrying this progression further would result in the suggested neutron bomb, which would have a minimum trigger and a nonfissionable tamper; there would be blast effects and a hail of lethal neutrons but almost no radioactive fallout; this theoretically would cause minimal physical damage to buildings and equipment but kill most living things. The theorized cobalt bomb is, on the contrary, a radioactively "dirty" bomb having a cobalt tamper. Instead of generating additional explosive force from fission of the uranium, the cobalt is transmuted into cobalt-60, which has a half-life of 5.26 years and produces energetic (and thus penetrating) gamma rays. The half-life of Co-60 is just long enough so that airborne particles will settle and coat the earth's surface before significant decay has occurred, thus making it impractical to hide in shelters. This prompted physicist Leo Szilard to call it a "doomsday device" since it was capable of wiping out life on earth.

Like other types of nuclear explosion, the explosion of a hydrogen bomb creates an extremely hot zone near its center. In this zone, because of the high temperature, nearly all of the matter present is vaporized to form a gas at extremely high pressure. A sudden overpressure, i.e., a pressure far in excess of atmospheric pressure, propagates away from the center of the explosion as a shock wave, decreasing in strength as it travels. It is this wave, containing most of the energy released, that is responsible for the major part of the destructive mechanical effects of a nuclear explosion. The details of shock wave propagation and its effects vary depending on whether the burst is in the air, underwater, or underground.

See disarmament, nuclear and nuclear weapons; see also nuclear energy.

hydrogen [Gr.,=water forming], gaseous chemical element; symbol H; at. no. 1; at. wt. 1.00794; m.p. -259.14°C;; b.p. -252.87°C;; density 0.08988 grams per liter at STP; valence usually +1.

The Isotopes and Forms

Atmospheric hydrogen is a mixture of three isotopes. The most common is called protium (mass no. 1, atomic mass 1.007822); the protium nucleus (protium ion) is a proton. A second isotope of hydrogen is deuterium (mass no. 2, atomic mass 2.0140), the so-called heavy hydrogen, often represented in chemical formulas by the symbol D. The deuterium nucleus, or ion, is called the deuteron; it consists of a proton plus a neutron. The two isotopes are found in atmospheric hydrogen in the proportion of about 1 atom of deuterium to every 6,700 atoms of protium. Protium and deuterium differ slightly in their chemical and physical properties; for example, the boiling point of deuterium is about 3°C; lower than protium. The properties of compounds they form differ depending on the ratio of the two isotopes present.

Deuterium oxide (D₂O), the so-called heavy water, is present in ordinary water; the concentration of deuterium oxide is increased by electrolysis of the water. The melting point (3.79°C;), boiling point (101.4°C;), and specific gravity (1.107 at 25°C;) of deuterium oxide are higher than those of ordinary water. Deuterium oxide is used as a moderator in nuclear reactors. Deuterium is also of importance because of the wide use it has found in scientific research; for example, chemical reaction mechanisms have been studied by the use of deuterium atoms as tracers (i.e., deuterium is substituted for atoms of ordinary hydrogen in compounds), making it possible to follow the course of individual molecules in a reaction.

Tritium (mass no. 3, atomic mass 3.016), a third hydrogen isotope, is a radioactive gas with a half-life of about 121/4 years; it is often represented in chemical formulas by the symbol T. It is produced in nuclear reactors and occurs to a very limited extent in atmospheric hydrogen. It is used in the hydrogen bomb, in luminous paints, and as a tracer. The tritium nucleus, or ion, is called the triton; it consists of a proton plus two neutrons. Tritium oxide (T₂O) has a melting point (4.49°C;) higher than that of deuterium oxide.

Besides being a mixture of three isotopes, hydrogen is a mixture of two forms, an ortho form and a para form, which differ in their electronic and nuclear spins. At room temperature atmospheric hydrogen is about 3/4 ortho-hydrogen and 1/4 para-hydrogen. The two forms differ slightly in their physical properties.

Properties

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.

Sources and Commercial Preparation

While hydrogen is only about one part per million in the atmosphere, it is the most abundant element in the universe. It is believed that hydrogen makes up about three quarters of the mass of the universe, or over 90% of the molecules. It is found in the sun and in other stars, where it is the major fuel in the fusion reactions (see nucleosynthesis) from which stars derive their energy.

Hydrogen is prepared commercially by catalytic reaction of steam with hydrocarbons, by the reaction of steam with hot coke (carbon), by the electrolysis of water, and by the reaction of mineral acids on metals. Millions of cubic feet of hydrogen gas are produced daily in the United States alone.

Uses

Hydrogen was formerly used for filling balloons, airships, and other lighter-than-air craft, a dangerous practice because of hydrogen's explosive flammability; there were disastrous fires, e.g., the immolation of the German airship Hindenburg at its mooring at Lakehurst, N.J., in 1937. Helium is preferable for use in lighter-than-air craft since it is not flammable. Hydrogen is used in the Haber process for the fixation of atmospheric nitrogen, in the production of methanol, and in hydrogenation of fats and oils. It is also important in low-temperature research. It can be liquefied under pressure and cooled; when the pressure is released, rapid evaporation takes place and some of the hydrogen solidifies.

Discovery of Hydrogen and Its Isotopes

Although hydrogen was prepared many years earlier, it was first recognized as a substance distinct from other flammable gases in 1766 by Henry Cavendish, who is credited with its discovery; it was named by A. L. Lavoisier in 1783. Deuterium was discovered by H. C. Urey, F. G. Brickwedde, and G. M. Murphy in 1932, although its existence had been suspected for some years. Deuterium oxide was also discovered by Urey and was first obtained in nearly pure form by G. N. Lewis. Tritium was synthesized by Ernest Rutherford, L. E. Oliphant, and Paul Harteck in 1935.

Atomic Number:	Atomic Number: 1
Atomic Symbol:	Atomic Symbol: H
	Name of Element: Hydrogen
Atomic Weight:	Atomic Weight: 1.00794
Electron Configuration:	Electron Configuration: 1

Isotope of hydrogen, chemical symbol written as ³H or T, with atomic number 1 but atomic weight approximately 3. Its nucleus contains one proton and two neutrons. Tritium is radioactive (see radioactivity), with a half-life of 12.32 years. Its occurrence in natural water in an amount 10⁻¹⁸ that of ordinary hydrogen is believed to be due to the action of cosmic rays. Some tritium is used in self-luminous materials (e.g., for watch dials) and as a radioactive tracer in chemical and biochemical studies. Nuclear fusion of deuterium and tritium at high temperatures releases enormous amounts of energy. Such reactions have been used in nuclear weapons and experimental power reactors. Seealso heavy water.

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or oil of vitriol

Dense, colourless, oily, corrosive liquid inorganic compound (H₂SO₄). A very strong acid, it forms ions of hydrogen or hydronium (H⁺ or H₃O⁺), hydrogen sulfate (HSO₄⁻), and sulfate (SO₄²⁻). It is also an oxidizing (see oxidation-reduction) and dehydrating agent and chars many organic materials. It is one of the most important industrial chemicals, used in various concentrations in manufacturing fertilizers, pigments, dyes, drugs, explosives, detergents, and inorganic salts and acids, in petroleum refining and metallurgical processes, and as the acid in lead-acid storage batteries. It is made industrially by dissolving sulfur trioxide (SO₃) in water, sometimes beyond the saturation point to make oleum (fuming sulfuric acid), used to make certain organic chemicals.

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or H-bomb or thermonuclear bomb

Weapon whose enormous explosive power is generated by the nuclear fusion of hydrogen isotopes. The high temperatures required for the fusion reaction are produced by detonating an atomic bomb (which draws its energy from nuclear fission). The bomb's explosion produces a blast that can destroy structures within a radius of several miles, an intense white light that can cause blindness, and heat fierce enough to set off firestorms. It also creates radioactive fallout that can poison living creatures and contaminate air, water, and soil. Hydrogen bombs, which may be thousands of times more powerful than atomic bombs, can be made small enough to fit in the warhead of a ballistic missile (see ICBM) or even in an artillery shell (see neutron bomb). Edward Teller and other U.S. scientists developed the first H-bomb and tested it at Enewetak atoll (Nov. 1, 1952). The Soviet Union first tested an H-bomb in 1953, followed by Britain (1957), China (1967), and France (1968). Most modern nuclear weapons employ both fusion and fission.

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Lightest chemical element, chemical symbol H, atomic number 1. A colourless, odourless, tasteless, flammable gas, it occurs as the diatomic molecule H₂. Its atom consists of one proton (the nucleus) and one electron; the isotopes deuterium and tritium have an additional one and two nuclear neutrons, respectively. Though only the ninth most abundant element on Earth, it represents about 75percnt of all matter in the universe. Hydrogen was formerly used to fill airships; nonflammable helium has replaced it. It is used to synthesize ammonia, ethanol, aniline, and methanol; to treat petroleum fuels; as a reducing agent (see reduction) and to supply a reducing atmosphere; to make hydrogen chloride (see hydrochloric acid) and hydrogen bromide; and in hydrogenation (e.g., of fats). Liquid hydrogen (boiling point −423 °F [−252.8 °C]) is used in scientific and commercial applications to produce extremely low temperatures and as a rocket propellant and a fuel for fuel cells. Combustion of hydrogen with oxygen gives water as the sole product. The properties of most acids, especially in water solutions, arise from the hydrogen ion (H⁺, also referred to as the hydronium ion, H₃O⁺, the form in which H⁺ is found in a water environment). Seealso hydride; hydrocarbon.

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