In a chemical reaction explain 2 things that are always the same on the reactant and product side and one thing that might change in the course of the reaction.
Two things that are always the same on the reactant and product side of a chemical reaction are the number and types of atoms involved. This is due to the principle of conservation of mass, which states that matter cannot be created or destroyed during a chemical reaction. The reactant side and the product side will have the same types and numbers of atoms, although they may be rearranged into different molecular configurations.
One thing that might change in the course of the reaction is the chemical energy. Chemical reactions involve the breaking and formation of chemical bonds, and this process results in a release or absorption of energy. The total energy of the reactants may differ from the total energy of the products due to this exchange of energy. The reaction can either be exothermic, where energy is released, or endothermic, where energy is absorbed.