What aspect of a chemical reaction is preserved according to the law of conservation of mass?

The law of conservation of mass states that in a closed system, the total mass of reactants must equal the total mass of products during a chemical reaction. This fundamental principle implies that matter cannot be created or destroyed in the course of a reaction; instead, atoms are rearranged to form new substances. Therefore, although the arrangement of atoms may change and new bonds may form or break, the overall mass remains constant.

While other aspects listed, such as the number of bonds formed or the energy released, can vary during a reaction, they do not have the same preservation as mass. Similarly, temperature can change based on the nature of the reaction (endothermic or exothermic), but it is not conserved in the same manner as mass. Thus, it is the total mass of reactants and products that is preserved according to the law of conservation of mass.