Which law states that entropy in a closed system tends to increase over time and that heat flows from hot to cold?

• A. Zeroth Law of Thermodynamics
• B. Law of Conservation of Energy
• C. Heisenberg Uncertainty Principle
• D. Second Law of Thermodynamics

Answer: (D)

Entropy tends to increase in an isolated (closed) system, and heat tends to flow from hotter to cooler bodies. This combination is described by the Second Law of Thermodynamics.

Think of a hot cup of coffee in a still room—the coffee cools gradually, energy spreads into the surrounding air, and the overall disorder or number of accessible microstates increases as the energy becomes more evenly distributed. That directional tendency is the hallmark of the Second Law: natural processes move toward greater entropy and spontaneous heat transfer goes from hot to cold.

The Zeroth Law is about establishing temperature and when systems are in thermal equilibrium. The Law of Conservation of Energy says energy cannot be created or destroyed, only transformed. The Heisenberg Uncertainty Principle relates to fundamental limits on measuring certain quantities in quantum systems. None of these describe the combined idea of entropy increasing and heat flow from hot to cold like the Second Law does.