A substance undergoes a change in composition, making it a different substance. This is classified as a:

A chemical change occurs when a substance transforms into a different substance through a reaction that alters its chemical composition. This may involve the breaking of bonds between atoms and the formation of new bonds, leading to entirely different substances. For example, when iron rusts, it reacts with oxygen in the presence of moisture, resulting in iron oxide, which has distinct properties from iron.

In contrast, a physical change involves a change in physical properties without altering the chemical composition of the substance. Examples include changes in state, such as ice melting to water or water boiling to steam, where the chemical structure (H2O) remains intact.

Heterogeneous change refers to a mixture composed of different substances that remain distinct, and is not directly related to a change in composition. Mechanical change refers to changes involving the physical movement or manipulation of objects without altering their chemical identity, like breaking a piece of glass.

Therefore, the classification of a change in composition that results in the formation of a new substance is accurately described as a chemical change.