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 oxidation and reduction, complementary chemical reactions characterized by the loss or gain, respectively, of one or more electrons by an atom or molecule. Originally the term oxidation
..... Click the link for more information. 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 catalyst, substance that can cause a change in the rate of a chemical reaction without itself being consumed in the reaction; the changing of the reaction rate by use of a catalyst is called catalysis.
..... Click the link for more information. . 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 chemical equation, group of symbols representing a chemical reaction .

Basic Notation Used in Equations

The chemical equation 2H₂+O₂→2H₂O represents the reaction of hydrogen and oxygen to form water.
..... Click the link for more information. . An element's activity, i.e., its tendency to enter into compounds, varies from one element to another.

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 products complicate these studies. Reactions can be syntheses, decompositions, or rearrangements, or they can be additions, eliminations, or substitutions. Examples include oxidation-reduction, polymerization, ionization (see ion), combustion (burning), hydrolysis, and acid-base reactions.