What does a chemical reaction involve?

A chemical reaction fundamentally involves the breaking and forming of chemical bonds between atoms within substances. During a reaction, the reactant molecules undergo a transformation where their existing bonds are disrupted, allowing the atoms to rearrange and create new products with different chemical identities. This process is key, as the characteristic properties of the products often differ significantly from those of the reactants, indicating that a chemical change has occurred.

For example, in the combustion of methane, the bonds between carbon and hydrogen in methane (CH₄) are broken, and new bonds are formed to create carbon dioxide (CO₂) and water (H₂O). This not only changes the materials involved but also releases energy, often in the form of heat and light.

In contrast, other options point to different concepts. The first choice refers only to physical changes, which do not alter the chemical identities of the substances involved. The third choice discusses changes in temperature without modifying substances, which can happen in both physical and chemical processes but does not define a chemical reaction. Finally, the last choice suggests that nothing changes in terms of chemical identities, which directly contradicts the principle of chemical reactions where one set of substances is transformed into another with different properties.