What type of change occurs when a substance undergoes a transformation resulting in a new composition?

A chemical change is characterized by a transformation that alters the composition of a substance, resulting in the formation of new substances. This process involves the breaking and forming of chemical bonds, which can lead to changes in properties such as color, temperature, and state. For instance, when iron rusts, it undergoes a chemical change as it reacts with oxygen and moisture in the air to produce iron oxide, which has different properties and composition compared to its original form.

In contrast, a mechanical change does not alter the chemical composition of a substance; it simply changes its physical form, such as breaking a piece of wood into smaller pieces. Similarly, a physical change refers to transformations that affect physical properties (like melting, freezing, or dissolving) without changing the substance's chemical identity. Homogeneous change typically refers to processes that do not result in visible separation of different components, often used within the context of mixtures, rather than indicating a change in composition.