What type of change occurs when a substance's composition changes so that it is no longer the same substance?

The correct answer is a chemical change. In a chemical change, the composition of a substance is altered at a molecular level, resulting in the formation of one or more new substances. This process involves the breaking and forming of chemical bonds, which transforms the original material into something different that has its own unique properties and characteristics. Examples include rusting iron, burning wood, or digestion of food, where substances undergo reactions that fundamentally change their structure and composition.

The other types of changes listed do not result in a new substance being formed. A physical change, for instance, involves alterations that do not affect the substance’s internal structure, such as changes in state (like ice melting to water) or changes in shape. Heterogeneous change typically refers to physical mixtures where the components retain their individual properties and identities. Physical properties describe characteristics of substances that can be observed or measured without changing the substance itself, such as color, boiling point, and density. Thus, the distinction between chemical and physical changes is crucial in understanding material transformations.