What distinguishes a physical change from a chemical change?

The distinction between a physical change and a chemical change is fundamentally based on whether the substance's chemical composition is altered. A physical change involves alterations in the form or appearance of a substance—such as phase changes (like melting, freezing, or boiling)—without affecting the underlying chemical identity. For example, when ice melts into water, the molecules remain H2O, merely transitioning from one state to another.

In contrast, a chemical change involves a transformation that results in the formation of one or more new substances, which have different chemical properties. An example of this is when iron rusts; it reacts with oxygen to form iron oxide, which has distinct properties compared to the original metal.

Understanding this distinction is crucial, as it helps in identifying processes taking place in various scientific fields, from chemistry to materials science. Therefore, recognizing that physical changes affect form but not composition, while chemical changes create entirely new substances, clarifies why the correct statement is centered on the alteration of substance identity.