Final answer:
A change in composition indicates a chemical change, where the substance is transformed into a different material at the molecular level, unlike a physical change which only alters the appearance without affecting the structure.
Step-by-step explanation:
A change in composition typically indicates a chemical change. Unlike a physical change, which alters the physical properties of a substance without affecting its inherent chemical structure, a chemical change transforms one substance into another at the molecular level. This type of change is often evidenced by signs such as a change in color, temperature variation, emission of light, formation of bubbles or gas, and the production of a precipitate.
Physical changes might include processes such as melting or evaporating, where no new substance is created, and the original material can usually be recovered in its original form. In contrast, during a chemical change, the original substance is converted into one or more different substances, which signifies a change in its composition, thus indicating that a chemical reaction has occurred.