What type of change occurs when a substance undergoes a chemical reaction?

When a substance undergoes a chemical reaction, a permanent change occurs because the chemical structure of the substance is altered, resulting in the formation of new substances. This transformation often involves the breaking and forming of chemical bonds, leading to changes in properties such as color, temperature, and state. Unlike physical changes, which may alter only the form of the substance without changing its chemical identity, a chemical change results in substances that are fundamentally different from the original material. This is indicative of a permanent change, as the new substances cannot easily revert to their original form without another chemical reaction. The other options do not capture the essence of what happens in a chemical reaction; for example, physical changes, while related, do not involve changing the chemical identity of the substances involved.