State the second law of thermodynamics in terms of entropy.

• A. For any real process, ΔS ≥ 0; entropy of the universe does not decrease
• B. For any real process, ΔS ≤ 0; entropy of the universe decreases
• C. Entropy is always conserved
• D. Entropy can be negative in real processes

Answer: (A)

Energy tends to disperse and disorder tends to increase. The second law tells us that for any real (irreversible) process, the total entropy of the universe—system plus surroundings—does not decrease. In other words, the change in total entropy is zero for an idealized reversible process and positive for a real one: ΔS_universe ≥ 0. This is why the option stating ΔS ≥ 0 and that the entropy of the universe does not decrease is the correct framing.

Think of what happens when heat flows from a hot object to a cold one or when two gases mix: the overall disorder increases, so the total entropy rises. Entropy is not a conserved quantity; it can increase, and it cannot be negative for the universe. The other statements conflict with the fundamental direction of spontaneous processes: they imply zero or negative total entropy changes, or that entropy is always conserved, which isn’t correct.