Which law states that the mass of substances stays the same throughout chemical reactions?

• A. Law of Definite Proportions
• B. Avogadro's Law
• C. Charles' Law
• D. Law of Conservation of Mass

Answer: (D)

Matter cannot be created or destroyed in a chemical reaction; atoms are simply rearranged. Because of this, the total mass of all substances before a reaction equals the total mass after the reaction. This principle is described by the Law of Conservation of Mass. In a sealed system, when reactants combine to form products, the mass you measure remains the same because all the atoms—carbon, hydrogen, oxygen, etc.—are still present, just in new arrangements. For example, burning a hydrocarbon in a closed container converts it to other substances, but the total mass doesn’t change. The other statements describe different ideas: a compound has a fixed ratio of elements by mass, which is about composition; or the behavior of gases with temperature or amount, which are not about mass being conserved.