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While a physical property can be observed without altering the identity of a material, a chemical property cannot. Chemical properties are observed when substances are converted to different substances with different properties. Flammability, for example, is a chemical property that can only be observed when a substance reacts with oxygen. If a chemical change has occurred, the products will differ from the reactants. Although one must perform chemical analyses to show that a chemical reaction has taken place, there are certain easily observed changes that generally indicate a chemical reaction has occurred: (1) the production of heat or light; (2) the production of a gas when two or more substances are mixed; (3) the formation of a precipitate when two or more solutions are mixed; and (4) a change in color.