Which statement describes the total amount of matter in a closed system staying constant?

• A. Law of Conservation of Energy
• B. Kinetic Theory
• C. Law of Entropy
• D. Law of Conservation of Mass

Answer: (D)

The key idea is that in a closed system matter cannot be created or destroyed; it can only change form, so the total mass stays the same. This is the Law of Conservation of Mass, which explains why, in a chemical reaction or physical change happening in a sealed container, the combined mass of all substances before equals the combined mass after.

This differs from other concepts: Conservation of Energy is about energy, not matter—energy can shift between kinetic, potential, heat, etc., but the total energy stays the same. Kinetic Theory explains how particle motion relates to temperature and pressure, not how much matter is present. Entropy concerns disorder and tends to increase in isolated systems, not mass. In ordinary chemistry, mass conservation is the appropriate description of what stays constant.