What does the law of conservation of mass state?

• A. Mass is created during chemical changes.
• B. Mass is destroyed during physical changes.
• C. In a closed system, mass is conserved; it is not created or destroyed during physical or chemical changes.
• D. Mass changes form but not amount.

Answer: (C)

Mass is conserved in a closed system; the total amount of matter stays the same before and after physical or chemical changes. In a sealed container, atoms are just rearranged during reactions, so the mass of the reactants equals the mass of the products. This is why chemical equations are balanced—to reflect that the same number of each type of atom is present on both sides. For example, burning wood with oxygen in a sealed space yields ash, gases, and heat, but the total mass remains the same as the wood plus the consumed oxygen. The statement specifies a closed system because if matter can enter or leave, the total mass can change. The other ideas describe creation or destruction of mass or omit the closed-system condition, which doesn’t fit the law.