hydraulic press Machine consisting of a cylinder fitted with a piston (see piston and cylinder) that uses liquid under pressure to exert a com¬ pressive force upon a stationary anvil or baseplate. The liquid is forced into the cylinder by a pump. The hydraulic press is widely used in indus¬ try for forming metals and for other tasks where a large force is required. It is manufactured in a wide variety of styles and sizes and in capacities ranging from 1 ton (0.9 metric ton) or less to 10,000 tons (9,000 metric tons) or more. See also punch press.
hydraulics Branch of science concerned with the practical applications of fluids, primarily liquids, in motion. It is related to fluid mechanics, which in large part provides its theoretical foundation. Hydraulics deals with such matters as the flow of liquids in pipes, rivers, and channels and their confinement by dams and tanks. Some of its principles apply also to gases, usually when variations in density are relatively small. The scope of hydraulics extends to such mechanical devices as actuators and control systems. See also Bernoulli's principle, Pascal's law, pump.
hydride Vhl-.dridV Inorganic compound of hydrogen with another ele¬ ment. Three common types are differentiated by their bonding. In saline (ionic) hydrides (see ionic bond), the hydrogen is an anion, H - , and behaves like a halogen. Saline hydrides such as sodium hydride (NaH) and calcium hydride (CaH 2 ) react vigorously with water, giving off hydro¬ gen gas (H 2 ), and are used as portable sources of it. Metallic hydrides, such as titanium hydride (TiH 2 ), are alloylike materials (see alloy) with some properties of metals, such as luster and electrical conductivity. Cova¬ lent hydrides (see covalent bond) are mostly compounds of hydrogen and nonmetallic elements; they include water, ammonia, hydrogen sulfide (H 2 S), and methane. In polymeric hydrides, the hydrogen forms bridges
between other atoms (e.g., hydrides of boron and aluminum). Those hydrides give off large amounts of energy when burned and may be use¬ ful as rocket fuels.
hydrocarbon Any of a class of organic compounds composed only of carbon and hydrogen. The carbon atoms form the framework, and the hydrogen atoms attach to them. Hydrocarbons, the principal constituents of petroleum and natural gas, serve as fuels, lubricants, and raw materials for production of plastics, fibres, rubbers, solvents, explosives, and indus¬ trial chemicals. All burn to carbon dioxide and water with enough oxygen or to carbon monoxide without it. The two major categories are aliphatic, with the carbon atoms in straight or branched chains or in nonaromatic rings, and aromatic (see aromatic compound). Aliphatic compounds may be saturated (paraffins) or, if any carbon atoms are joined by double or triple bonds, unsaturated (e.g., olefins, alkenes, alkynes). All but the sim¬ plest hydrocarbons have isomers (see isomerism). Ethylene, methane, acety¬ lene, benzene, toluene, and naphthalene are hydrocarbons.
hydrocephalus N.hl-dro-'se-fo-losV Accumulation of cerebrospinal fluid (CSF) in the ventricles (cavities) of the brain. Hydrocephalus is caused by overproduction of CSF, congenital blockage that prevents drainage (see neural tube defect), or complications of head injuries or infections. Nor¬ mally, CSF circulates through the brain and spinal cord and drains into the circulation. In infants and young children, hydrocephalus causes the brain and skull to enlarge because the fontanels have not yet closed. Without surgery to divert the excess fluid into the blood or abdomen, accumulating fluid eventually compresses the brain, causing convulsions, mental retardation, and death.
hydrochemistry See chemical hydrology
hydrochloric acid or muriatic acid Solution in water of hydrogen chloride (HC1), a gaseous inorganic compound. It is a strong acid, virtu¬ ally completely dissociated (see dissociation) into hydronium cations (H 3 0 + ) and chloride anions (Cl - ), and is corrosive and irritating. The acid reacts with most metals, to produce hydrogen and the metal’s chloride, and with oxides, hydroxides, and many salts. It is used extensively in indus¬ trial processing of metals and concentrating of some ores; in boiler scale removal, food processing, metal cleaning, and pickling; and as a chemi¬ cal intermediate, laboratory reagent, and alcohol denaturant (see ethanol). Hydrochloric acid is present in the stomach’s gastric juice and is involved in the devlopment of peptic ulcers.
hydroelectric power Electricity produced from generators driven by water turbines that convert the energy in falling or fast-flowing water to mechanical energy. Water at a higher elevation flows downward through large pipes or tunnels (penstocks). The falling water rotates turbines, which drive the generators, which convert the turbines’ mechanical energy into electricity. The advantages of hydroelectric power over such other sources as fossil fuels and nuclear fission are that it is continually renew¬ able and produces no pollution. Norway, Sweden, Canada, and Switzer¬ land rely heavily on hydroelectricity because they have industrialized areas close to mountainous regions with heavy rainfall. The U.S., Russia, China, India, and Brazil get a much smaller proportion of their electric power from hydroelectric generation. See also tidal power. See illustration opposite on following page.
hydrogen Lightest chemical element, chemical symbol H, atomic num¬ ber 1. A colourless, odourless, tasteless, flammable gas, it occurs as the diatomic molecule H 2 . 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 75% 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 metha¬ nol; to treat petroleum fuels; as a reducing agent (see reduction) and to supply a reducing atmosphere; to make hydrogen chloride (see hydro¬ chloric 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, H 3 0 + , the form in which H + is found in a water environment). See also hydride; hydrocarbon.
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hydrogen bomb ► hydrothermal ore deposit I 917
hydrogen bomb 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 cre¬ ates 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.
hydrogen bonding Interaction involving a hydrogen atom located between a pair of other atoms having a high affinity for electrons; such a bond is weaker than an ionic bond or covalent bond but stronger than van der Waals forces. Hydrogen bonds can exist between atoms in different molecules or in parts of the same molecule. One atom of the pair (the donor), generally a fluorine, nitrogen, or oxygen atom, is covalently bonded to a hydrogen atom (—FH, —NH, or —OH), whose electrons it shares unequally; its high electron affinity causes the hydrogen to take on a slight positive charge. The other atom of the pair, also typically F, N, or O, has an unshared electron pair, which gives it a slight negative charge. Mainly through electrostatic attraction, the donor atom effectively shares its hydrogen with the acceptor atom, forming a bond. Because of its extensive hydrogen bonding, water (H 2 0) is liquid over a far greater range of temperatures that would be expected for a molecule of its size. Water is also a good solvent for ionic compounds and many others because it readily forms hydrogen bonds with the solute. Hydrogen bonding between amino acids in a linear protein molecule determines the way it folds up into its functional configuration. Hydrogen bonds between nitrogenous bases in nucleotides on the two strands of DNA (guanine pairs with cytosine, adenine with thymine) give rise to the double-helix structure that is crucial to the transmission of genetic information.