Chelyabinsk \chel-'ya-b9nsk\ City (pop., 2002: 1,078,300), west- central Russia. Located 125 mi (200 km) southeast of Yekaterinburg on the Trans-Siberian Railroad, it is the capital of Chelyabinsk oblast. Founded as a frontier outpost on the site of a Bashkir village in 1736, its growth was greatly stimulated by the eastward evacuation of Russian industry in World War II.
chemical dependency See drug addiction chemical element See element, chemical
chemical energy Energy stored in the bonds of chemical compounds. Chemical energy may be released during a chemical reaction, often in the form of heat; such reactions are called exothermic. Reactions that require an input of heat to proceed may store some of that energy as chemical energy in newly formed bonds. The chemical energy in food is converted by the body into mechanical energy and heat. The chemical energy in coal is converted into electrical energy at a power plant. The chemical energy in a battery can also supply electrical power by means of electrolysis.
chemical engineering Academic discipline and industrial activity concerned with developing processes and designing and operating plants to change materials’ physical or chemical states. With roots in the inorganic and coal-based chemical industries of western Europe and the oil-refining industry in North America, it was spurred by the need to supply chemicals and products during the two World Wars. The field includes research, design, construction, operation, sales, and management activities. Chemi¬ cal engineers must master chemistry (including the nature of chemical reac¬ tions, the effects of temperature and pressure on equilibrium, and the effects of catalysts on reaction rates), physics, and mathematics. The engineering aspect, involving fluid flow (see deformation and flow) and heat and mass transfer, is broken down into “unit operations,” including vaporization, dis¬ tillation, absorption, filtration, extraction, crystallization, agitation and mix¬ ing, drying, and size reduction; each is described mathematically, and its principles apply to any material. Chemical engineers work not only in the chemical and oil industries but also in such processing industries as foods, paper, textiles, plastics, nuclear, and biotechnology.
chemical equation Method of writing the essential features of a chemi¬ cal reaction using chemical symbols (or other agreed-upon abbreviations). By convention, reactants (present at the start) are on the left, products (present at the end) on the right. A single arrow between them denotes an irreversible reaction, a double arrow a reversible reaction. The law of conservation of matter (see conservation law) requires that every atom on the left appear on the right (the equation must balance); only their arrange¬ ments and combinations change. For example, one oxygen molecule com¬ bining with two hydrogen molecules to form two water molecules is written 2H 2 + 0 2 —» 2H 2 0. The dissociation of salt into sodium and chlo¬ ride ions is written NaCl —> Na + + Cl - . See also stoichiometry.
chemical formula Expression of the composition or structure of a chemical compound. Formulas for MOLECULES USe CHEMICAL SYMBOLS with subscript numbers to show the num¬ ber of atoms of each element: 0 2 for molecular oxygen, 0 3 for ozone, CH 4 for methane, C 6 H 6 for benzene. Parentheses may enclose atoms that act as a group. General formulas show the proportions of atoms in members of a class (e.g., C„H 2 „ + 2 for alkanes). If the substance does not exist as molecules (see ionic bond), empirical formulas show the relative proportions of the constitu¬ ents (e.g., NaCl for sodium chlo¬ ride). Structural formulas show bonds (see bonding) between atoms in a molecule as short lines between symbols; they are particularly useful for showing how isomers differ. A projection formula also indicates the three-dimensional arrangement of the atoms (see Fischer projection; ste¬ reochemistry).
chemical hydrology or hydrochemistry Subdivision of hydrology that deals with the chemi¬ cal characteristics of the water on and beneath the surface of the Earth. Water in all forms is affected chemi¬ cally by the materials with which it
Cathedral of Saint Mary, Chelmsford, England.
THE J. ALLAN CASH PHOTOUBRARY
Chelsea soft-paste porcelain vase in the French Rococo style of Sevres ware with "mazarin blue" ground and a "reserve" panel painting by John Donaldson (after Francois Boucher), gold anchor mark, c. 1763; in the Vic¬ toria and Albert Museum, London.
COURTESY OF THE VICTORIA AND ALBERT MUSEUM, LONDON
© 2006 Encyclopaedia Britannica, Inc.
chemical reaction ► Chen Shui-bian I 383
comes into contact, and it can dissolve many elements in significant quan¬ tities. Chemical hydrology is concerned with the processes involved and thus includes study of phenomena such as the transport of salts from land to sea (by erosion of rocks and surface runoff) and from sea to land (by evaporation, cloud formation, and precipitation) and the age and origin of groundwater in desert regions and of ice sheets and glaciers.
chemical reaction Any chemical process in which substances are changed into different ones, with different properties, as distinct from changing position or form (phase). Chemical reactions involve the rupture or rearrangement of the bonds holding atoms together (see bonding), never atomic nuclei. The total mass and number of atoms of all reactants equals those of all products, and energy is almost always consumed or liberated (see heat of reaction). The speed of reactions varies (see reaction rate). Understanding their mechanisms lets chemists alter reaction conditions to optimize the rate or the amount of a given product; the reversibility of the reaction and the presence of competing reactions and intermediate prod¬ ucts complicate these studies. Reactions can be syntheses, decomposi¬ tions, or rearrangements, or they can be additions, eliminations, or substitutions. Examples include oxidation-reduction, polymerization, ion¬ ization (see ion), combustion (burning), hydrolysis, and acid-base reactions.
chemical symbol Notation of one or two letters derived from the sci¬ entific names of the chemical elements (e.g., S for sulfur, Cl for chlorine, Zn for zinc). Some hark back to Latin names: Au (durum) for gold, Pb ( plumbum ) for lead. Others are named for people or places (e.g. einstein¬ ium, Es, for Einstein). The present symbols express the system set out by the atomic theory of matter. John Dalton first used symbols to designate single atoms of elements, not indefinite amounts, and Jons Jacob Berzelius gave many of the current names. Chemical formulas of compounds are written as combinations of the elements’ symbols, with numbers indicat¬ ing their atomic proportions, using various conventions for ordering and grouping. Thus, sodium chloride is written as NaCl and sulfuric acid as H 2 S0 4 .
chemical warfare Use of lethal or incapacitating chemical weapons in war, and the methods of combating such agents. Chemical weapons include choking agents such as the chlorine and phosgene gas employed first by the Germans and later by the Allies in World War I; blood agents such as hydrogen cyanide or cyanogen gas, which block red blood cells from taking up oxygen; blister agents such as sulfur gas and Lewisite, also dispensed as a gas, which burn and blister the skin; and nerve agents such as Tabun, Sarin, Soman, and VX, which block the transmission of nerve impulses to the muscles, heart, and diaphragm. The horrific casu¬ alties suffered in World War I led to the 1925 Geneva Protocol, which made it illegal to employ chemical weapons but did not ban their pro¬ duction. Chemical weapons were used a number of times afterward, most notably by Italy in Ethiopia (1935-36), by Japan in China (1938^-2), by Egypt in Yemen (1966-67), and by Iran and Iraq against each other (1984- 88). During the Cold War the Soviet Union and U.S. built up enormous chemical arsenals; these were dismantled under the terms of the 1993 Chemical Weapons Convention, which prohibits all development, pro¬ duction, acquisition, stockpiling, or transfer of such weapons. Not all countries have signed the convention, and many are suspected of pursu¬ ing clandestine chemical programs. Many military forces have adopted various defensive measures, including chemical sensors, protective gar¬ ments and gas masks, decontaminants, and injectable antidotes, and some have reserved the option of retaliating in kind to any chemical attack. In 1995 a religious cult killed 12 civilians and injured thousands more with Sarin gas in Tokyo; this pointed out the power of chemical agents as weapons of terror as well as the difficulty of protecting civilian popula¬ tions. See also biological warfare.