According to the law of conservation of mass, which of the following must be the same before and after a chemical reaction?

• A. The physical state of all reactants
• B. The number of molecules of each substance
• C. The total volume of the system
• D. The total mass of all substances

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; it remains constant throughout the process.
2. Recognize that while the physical state of reactants, the number of molecules, and the total volume can change during a reaction, the total mass of all substances must remain the same before and after the reaction.
3. Recall that molecules can rearrange, and gases can expand or contract, affecting volume and molecular counts, but these changes do not affect the total mass.
4. Therefore, identify that the quantity that must be conserved according to the law is the total mass of all substances involved in the reaction.
5. Conclude that the correct answer is the total mass of all substances, as it is the only property guaranteed to be the same before and after the chemical reaction.