reservoir
A hydro station generates power by the controlled release of water from the reser¬ voir of a dammed river. The flowing water turns a turbine, which powers a genera¬ tor, creating electricity. The voltage is stepped up by a transformer to allow long¬ distance power transmission.
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hydrogenation Mil-.dra-jo-'na-shonV Chemical reaction between molecular hydrogen (H 2 ) and another element or a compound, usually in the presence of a catalyst. It may involve adding hydrogen at the sites of double or triple bonds (see bonding) to make them single bonds (i.e., to saturate an unsaturated compound; see saturation), or to aromatic com¬ pounds to make them cyclic hydrocarbons. Edible oils with unsaturated fatty acids are liquid at room temperature; food manufacturers use hydro¬ genation to convert a fraction to saturated fatty acids to make the product more solid. A second type of hydrogenation involves breaking up a com¬ pound (hydrogenolysis, or destructive hydrogenation) and is of great importance in the petroleum industry. Numerous processes in gasoline and petrochemical manufacturing are based on it.
hydrologic \,hl-dr3-'la-jik\ cycle Cycle that involves the continuous circulation of water in the Earth-atmosphere system. Water is transferred from the oceans through the atmosphere to the continents and back to the oceans by means of evaporation, transpiration, precipitation, interception, infiltration, subterranean percolation, overland flow, runoff, and other complex processes. Although the total amount of water within the cycle remains essentially constant, its distribution among the various processes is continually changing.
hydrology Scientific discipline concerned with the waters of the Earth, including their occurrence, distribution, circulation via the hydrologic cycle, and interactions with living things. It also deals with the chemical and physical properties of water in all its phases.
hydrolysis \hl-'dra-ta-s9s\ Chemical reaction in which water (H 2 0 or HOH) and another reactant exchange functional groups to form two prod¬ ucts, one containing the H and the other the OH. In most hydrolyses involving organic compounds, the other reactants and products are neu¬ tral; for example, an ester can be hydrolyzed to form a carboxylic acid and an alcohol. Such reactions are often accelerated by enzymes (as in much of digestion and metabolism in general) or other catalysts. In hydrolyses of compounds with ionic bonds, the nonwater reactants are salts, acids, or bases, participating in dissociation reactions.
hydrometallurgy Extraction of metal from ore by dissolving the metal (as one of its salts) and then recovering it from the solution. The opera¬ tions usually involved are leaching (dissolving in water), commonly with additional agents; separating the waste and purifying the leach solution; and precipitating the metal or one of its pure compounds from the leach solution by chemical or electrolytic means. Though hydro metallurgy originated in the 16th century, its principal development took place in the 20th century. The development of ion exchange, solvent extraction, and other processes now permits more than 70 metallic elements to be pro¬ duced by hydrometallurgy, including most gold, much silver, and large tonnages of copper and zinc.
hydroponics Cultivation of plants in nutrient-enriched water, with or without the mechanical support of an inert medium such as sand or gravel. Fertilizer solution is pumped through the system periodically. As the plants grow, concentration of the solution and frequency of pumping are increased. A wide variety of vegetables and florist crops can be grown satisfactorily in gravel. Automatic watering and fertilizing saves on labor, but installation costs are high and fertilizer solution must be tested fre¬ quently. Yields are about the same as for soil-grown crops.
hydrosphere Discontinuous layer of water at or near the Earth’s sur¬ face. It includes all liquid and frozen surface waters, groundwater held in soil and rock, and atmospheric water vapour. Virtually all of these waters are in constant circulation through the hydrologic cycle. Although the com¬ ponents of the hydrosphere are undergoing continuous change of state and location, the total water budget remains in balance. The components of the hydrosphere have been seriously affected by the water-polluting activi¬ ties of modern society.
hydrostatics Branch of physics that deals with the characteristics of flu¬ ids at rest, particularly with the pressure in a fluid or exerted by a fluid (gas or liquid) on an immersed body. In applications, the principles of hydrostatics are used for problems relating to pressure in deep water (pressure increases with depth) and high in the atmosphere (pressure less¬ ens with altitude).
hydrotherapy External use of water for medical treatment. Wet heat helps relieve pain, improves circulation, and promotes relaxation. Wet cold causes blood vessels to close, reducing swelling and pain after injury. Underwater exercise helps strengthen weak muscles, restore joint motion after injury, clean and heal burned flesh, aid muscle function after stroke, and treat arthritic deformity and pain. Whirlpool baths and showers are also used. Hydrotherapy is usually employed by specialists in physical MEDICINE AND REHABILITATION.
hydrothermal ore deposit Any concentration of metallic minerals formed by the release of solids from hot mineral-laden water (hydrother¬ mal solution). The solutions are thought to arise in most cases from the action of deeply circulating water heated by magma. Another source of heating that may be involved includes energy released by radioactive decay.
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918 I hydroxide ► hypertext
hydroxide Any compound with one or more functional groups made up of one atom each of hydrogen and oxygen, bonded together and acting as the hydroxide anion (OH - ). Hydroxides include the familiar alkalies of laboratory and industrial processes. Those of the alkali metals (lithium, sodium, potassium, rubidium, and cesium), the strongest bases, are the most stable and soluble; those of the alkaline earth metals (calcium, barium, and strontium), also soluble strong bases, are less stable. The hydroxides of most other metals are only slightly soluble but neutralize acids; some are “amphoteric,” reacting with both acids and bases. In compounds in which OH is un-ionized and covalently bonded (e.g., in methanol, CH 3 OH), it is known as a hydroxyl group.
hyena Any of three species of coarse-furred, doglike carnivores (fam¬ ily Hyaenidae) found in Asia and Africa. Actually more closely related to cats than to dogs, they have four toes on each foot, long forelegs, non- retractile claws, and enormously strong jaws and teeth. They live alone or in packs and may be active by night or day. Hyenas are noted for scav¬ enging but will also attack live prey. The spotted, or laughing, hyena, whose calls alternately resemble wailing and maniacal laughter, ranges through much of sub-Saharan Africa. Yellowish or grayish with dark spots, it is about 6.5 ft (1.8 m) long, including the 12-in. (30-cm) tail, and weighs up to 175 lbs (80 kg). It has been known to attack people and even carry off young children.