What principle states that mass is unchanged during a chemical reaction?

• A. Conservation of mass
• B. Law of definite proportions
• C. Boyle's law
• D. Charles's law

Answer: (A)

Mass stays the same in a chemical change. Atoms are just rearranged during a reaction, so the total amount of matter is preserved overall. In a closed system, the mass of the reactants equals the mass of the products because no atoms enter or leave the setup. For example, burning a fuel in a sealed container still yields the same total mass when you measure the reactants and the products together, even though substances transform.

The other ideas describe different ideas: the law of definite proportions says a compound has fixed relative masses of its elements, not that mass is conserved in a reaction; and Boyle’s and Charles’s laws describe how pressure, volume, and temperature relate for gases, not how mass changes during a reaction.