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During a chemical change, a new substance is formed as a result of the transformation of reactants into products. This process involves rearranging the atoms and bonds in the reactants, leading to the generation of substances that have different chemical properties than those initial reactants.

For instance, when iron rusts, it reacts with oxygen in the presence of water to form iron oxide, which is a new chemical substance with distinct properties compared to the original iron and oxygen. Chemical changes often involve the making or breaking of bonds, which is crucial for the formation of these new substances.

Physical changes, on the other hand, only alter the physical properties of a substance without changing its chemical identity. Additionally, chemical changes typically involve energy changes, such as heat being released or absorbed during the reaction. Therefore, the formation of a new substance is the hallmark of a chemical change, distinguishing it from physical changes and emphasizing the importance of molecular transformations in chemical reactions.