Which quantity is always conserved during all chemical reactions according to the Law of Conservation of Mass?

• A. Volume
• B. Energy
• C. Number of molecules
• D. Mass

Answer: (D)

Verified step by step guidance

1. Understand the Law of Conservation of Mass, which states that mass cannot be created or destroyed in a chemical reaction.
2. Recognize that during a chemical reaction, atoms are rearranged but the total mass of the reactants equals the total mass of the products.
3. Note that volume can change due to gas expansion or contraction, so it is not always conserved.
4. Energy can be absorbed or released in reactions, so it is not always conserved in the form of chemical energy (though total energy including heat and work is conserved according to the Law of Conservation of Energy).
5. The number of molecules can change because molecules can break apart or combine, so it is not always conserved.