In a chemical reaction, what does the Law of Conservation of Mass state?

• A. The mass of the reactants equals the mass of the products
• B. Mass is created in the reaction
• C. Mass is destroyed in the reaction
• D. Mass changes during the reaction

Answer: (A)

Matter can’t be created or destroyed in ordinary chemical reactions; it’s just rearranged into different substances. The Law of Conservation of Mass states that the total mass entering a reaction equals the total mass leaving it. This happens because atoms—the building blocks of all substances—are conserved and simply find new bonds and arrangements in the products. So, in a closed system, the mass of reactants matches the mass of products.

If you see something like mass seeming to change, it’s usually because some material escaped or was added from the surroundings, not because mass was created or destroyed within the reaction itself. That’s why the statement that mass of reactants equals mass of products is the best description of what the law means.