What characterizes a chemical reaction as opposed to a physical change?

A chemical reaction is characterized by the formation of new compounds, which distinguishes it from a physical change. During a chemical reaction, the atoms involved rearrange to create new substances with different properties and compositions. This transformation often involves breaking and forming chemical bonds, leading to new molecular structures.

In contrast, physical changes, such as melting, freezing, or dissolving, do not alter the fundamental chemical identity of the substances involved; rather, they only change the state or appearance of the matter without forming new compounds. Hence, the formation of new compounds is a definitive feature of chemical reactions, making it the correct answer in this context.