What defines a chemical reaction?

A chemical reaction is defined by the formation of new substances through the breaking and forming of chemical bonds. During a chemical reaction, reactants interact with one another to create products that have different properties from the original substances. This transformation often involves a change at the molecular level, where the atoms rearrange to form new compounds or elements.

The other options describe processes that do not involve a true chemical change. For instance, the combination of existing substances without change does not lead to new substances being formed and represents a physical change rather than a chemical reaction. Similarly, the physical mixing of different substances is again a physical process where no new bonds are created, and no new substances emerge. Observing a color change may occur during some chemical reactions, but color change alone is not definitive proof of a chemical reaction taking place, as it may also occur during physical changes or mixing processes. Thus, the correct definition of a chemical reaction hinges on the formation of new substances, which is encapsulated in the first choice.