According to the law of conservation of mass, what must always be true for a chemical reaction to occur?

• A. Energy is always absorbed during the reaction.
• B. The mass of the products is greater than the mass of the reactants.
• C. The number of molecules increases during the reaction.
• D. The total mass of the reactants equals the total mass of the products.

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 this means the total mass of all reactants before the reaction must be equal to the total mass of all products after the reaction.
3. Note that energy changes (absorbed or released) do not affect the total mass, so energy absorption is not a requirement for the law of conservation of mass.
4. Understand that the number of molecules can change due to rearrangement of atoms, but this does not affect the total mass.
5. Conclude that the key point is the equality of total mass of reactants and products, which must always hold true for any chemical reaction.