What is the third law?

What is the third law?

• A. No system can reach absolute zero.
• B. Absolute zero can be achieved in practice.
• C. Entropy is zero at all temperatures.
• D. Energy is conserved at absolute zero.

Answer: (A)

The third law concerns how entropy behaves as temperature approaches absolute zero. For a perfectly crystalline solid, the number of accessible microstates becomes one at zero temperature, so the entropy goes to zero (S → 0 as T → 0). In other words, the ground state is perfectly ordered with no configurational disorder. It’s worth noting that absolute zero cannot be reached in a finite number of steps—that practical unattainability is a related idea but not the law’s exact statement. In real materials some have residual entropy due to ground-state degeneracy, which means they can retain entropy at absolute zero. The other statements don’t capture this behavior: entropy isn’t zero at all temperatures, and the law is not about energy conservation.

The third law concerns how entropy behaves as temperature approaches absolute zero. For a perfectly crystalline solid, the number of accessible microstates becomes one at zero temperature, so the entropy goes to zero (S → 0 as T → 0). In other words, the ground state is perfectly ordered with no configurational disorder. It’s worth noting that absolute zero cannot be reached in a finite number of steps—that practical unattainability is a related idea but not the law’s exact statement. In real materials some have residual entropy due to ground-state degeneracy, which means they can retain entropy at absolute zero. The other statements don’t capture this behavior: entropy isn’t zero at all temperatures, and the law is not about energy conservation.