Which process involves a change in the chemical identity of a substance?

The process that involves a change in the chemical identity of a substance is a chemical reaction. During a chemical reaction, the atoms within the reactants are rearranged to form new substances called products, which have different chemical properties from the original substances. This transformation alters the arrangement of atoms and often results in the formation of bonds between them in new ways, reflecting the fundamental difference in chemical identity.

In contrast, physical changes, state changes, and phase changes do not result in new substances being created. These processes involve alterations in the physical state or appearance of a substance (like melting, freezing, boiling, or dissolving) without changing its underlying chemical composition. Therefore, the correct choice accurately describes a process that fundamentally transforms the nature of matter at the molecular or atomic level, leading to new substances being formed.