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Chemical reaction, a process in which one or more substances, the reactants, 

are converted to one or more different substances, the products. Substances are either chemical elements or compounds. A chemical reaction rearranges the constituent atoms of the reactants to create different substances as products.This is usually accompanied by easily observed physical effects, such as the emission of heat and light, the formation of a precipitate, the evolution of gas, or a color change.  Absolute confirmation of a chemical change can only be validated by chemical analysis of the products.