What does the law of conservation of mass state?

• A. Mass is conserved; the total mass of reactants equals the total mass of products in a closed system.
• B. Mass increases during chemical reactions.
• C. Mass is created or destroyed during reactions.
• D. The law only applies to gas-phase reactions.

Answer: (C)

Mass is conserved in chemical reactions. This means that the total mass of the reactants equals the total mass of the products in a closed system, because atoms are merely rearranged during a reaction rather than created or destroyed. You can see this by balancing chemical equations and by weighing everything before and after a reaction in a sealed container—the masses match. The idea that mass is created or destroyed is not correct, and the law applies to all states of matter, not just gases, as long as no mass leaves or enters the system.