What defines a chemical reaction?

A chemical reaction is primarily characterized by the breaking and forming of bonds between reactants, leading to the creation of new substances with different properties than those of the original reactants. During a chemical reaction, atoms are rearranged, which results in the transformation of the substances involved.

Choosing the understanding that chemical reactions involve changes in the bonds provides insight into why new substances emerge, as this is a fundamental aspect of chemistry. The changes at the molecular level are crucial in determining the nature and outcome of the reaction.

In contrast, a change in the physical state of matter refers to transformations such as melting or boiling, which do not involve the making or breaking of chemical bonds, and therefore do not result in new substances. Mixing two substances might involve various physical interactions, but it doesn't necessarily indicate a chemical reaction unless a bond-breaking process occurs. Lastly, combining matter without altering properties describes a physical change rather than a chemical one, as the substances retain their original chemical identity.