Which best explains why the total mass of the product(s) would be less than the total weight of the reactant(s) after a chemical reaction?

• A. A physical change occurred.
• B. Atoms involved in the reaction lost mass.
• C. Precipitates were created in the new solution.
• D. Gases were released to the atmosphere.

Answer: (D)

Mass appears to decrease when gas escapes the system. In a chemical change, mass is conserved if everything stays in a closed container. But if the reaction produces a gas that bubbles out into the surroundings, that gas carries mass away. So the mass you measure for the remaining products inside the container is less than the mass of the original reactants because some of it left as gas.

The other ideas don’t explain a loss of mass in the system. A physical change doesn’t remove matter, and atoms don’t simply lose mass in ordinary chemical reactions. Forming a precipitate changes how the mass is distributed, but the total mass stays the same unless gas escapes.