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Chemical reaction, process by which one or more substances may be
transformed into one or more new substances. Energy is released or is
absorbed, but no loss in total molecular weight occurs. When, for example,
water is decomposed, its molecules, each of which consists of one atom of
oxygen and two of hydrogen, are broken down; the hydrogen atoms then
combine in pairs to form hydrogen molecules and the oxygen atoms to form
oxygen molecules. In a chemical reaction, substances lose their
characteristic properties. Water, for example, a liquid which neither
burns nor supports combustion, is decomposed to yield flammable hydrogen
and combustion-supporting oxygen. In some reactions heat is given off
(exothermic reactions), and in others heat is absorbed (endothermic
reactions). Furthermore, the new substances formed differ from the
original substances in the energy they contain. Chemical reactions are
classified according to the kind of change that takes place. When a
compound, which consists of two or more elements or groups of elements, is
broken down into its constituents, the reaction is called simple
decomposition. When two compounds react with one another to form two new
compounds, the reaction is called double decomposition. In so-called
replacement reactions the place of one of the elements in a compound is
taken by another element reacting with the compound. When elements combine
to form a compound, the reaction is termed chemical combination. Oxidation
and reduction reactions are extremely important. Reversible reactions are
those in which the chemical change taking place may be paralleled by
another change back to the original substances. The rates at which
chemical reactions proceed depend upon various factors, e.g., upon
temperature, pressure, and the concentration of the substances involved
and, sometimes, upon the use of a chemical called a catalyst. In some
chemical reactions, such as that of photographic film, light is an
important factor. The changes taking place in a chemical reaction are
represented by a chemical equation. An element's activity, i.e., its
tendency to enter into compounds, varies from one element to another.
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