Which statement best describes the Law of Conservation of Mass in ordinary chemical and physical changes?

• A. The total mass of substances is conserved in chemical and physical changes.
• B. Matter cannot be created or destroyed in any process.
• C. The mass of the products is always greater than the mass of the reactants.
• D. The amount of energy released equals the mass lost.

Answer: (A)

Mass is conserved in ordinary chemical and physical changes: the total amount of matter stays the same before and after a process in a closed system. Atoms are simply rearranged, bonds are broken and formed, but no atoms are created or destroyed, so the sum of the masses on both sides of a reaction remains equal. This is why balancing equations also preserves mass: the same number and type of atoms appear on both sides, yielding the same total mass.

Some statements overstep this idea. Saying matter cannot be created or destroyed in any process is too broad, because in contexts outside ordinary chemistry (like nuclear processes) mass and energy play with each other in more complex ways. The idea that the products must always be heavier contradicts the conservation of atoms and mass. Finally, tying mass loss directly to energy released isn’t how mass conservation is treated in typical chemical changes, where any mass changes are so small they’re effectively negligible and the key point is that mass remains constant when the system is closed.