AQA GCSE Chemistry Paper 1 Practice Exam 2026 – Complete Study Resource

Mass is conserved during a chemical reaction because no atoms are created or destroyed in the process. This principle is rooted in the law of conservation of mass, which states that in a closed system, the total mass of reactants must equal the total mass of products. During a chemical reaction, the atoms that make up the reactants rearrange to form new substances, but the total number of each type of atom remains constant.

For instance, if you start with a certain number of hydrogen and oxygen atoms reacting to form water, all of those atoms will still be present in the new substance, just in a different arrangement. Therefore, the mass before the reaction will equal the mass after the reaction, affirming that mass is conserved.

Other options do not accurately reflect the principle of mass conservation. Changes in temperature do not affect the number of atoms involved in a reaction, and while reactants do have a weight, it is not a reason for mass being conserved. Lastly, the assertion that products always weigh less than the reactants contradicts the principle of conservation of mass, as in some reactions, the mass of products can be equal to or even greater than that of the reactants, depending on the reactants involved.