What occurs when a substance undergoes a chemical change?

When a substance undergoes a chemical change, a new substance is formed. This transformation involves a rearrangement of atoms and bonds, which results in different chemical properties from those of the original substance. Chemical changes are often accompanied by observable signs such as color changes, gas production, or the formation of a precipitate, all indicating that new substances have been created as a result of the chemical reactions.

In contrast, a physical change, which is not applicable here, would involve changes that do not alter the substance's chemical identity—things like changes in state (solid to liquid), changes in shape or size, or alterations in appearance, without changing the composition of the matter. The physical state remaining the same or only the appearance changing does not capture the essence of a chemical change, as these processes do not result in new substances being created.

Additionally, energy is often involved in chemical changes, either absorbed or released, such as in exothermic and endothermic reactions, which further distinguishes chemical changes from mere physical alterations. Thus, the key aspect of a chemical change is the formation of new substances due to the transformation of chemical bonds and composition.