What type of change do compounds involve?

Compounds involve a chemical change because they consist of two or more elements that chemically combine in fixed proportions to form a new substance with different properties from the original elements. During this process, the chemical bonds between atoms are formed or broken, resulting in a transformation at the molecular level. This fundamental change means that the original elements can no longer be identified in the compound, and such transformations are characteristic of chemical reactions.

In contrast, physical changes do not alter the chemical structure of a substance. For example, changes in state (such as melting or freezing) are physical changes because the substance remains the same at the molecular level, merely transitioning between solid, liquid, or gas. Biological changes usually refer to processes related to living organisms, such as metabolism or growth, and thermal changes pertain to changes arising from temperature variations. Neither of these categories directly relates to the formation or alteration of compounds in the way that chemical changes do.