According to the third law of thermodynamics, entropy is zero for which condition?

• A. Perfect crystalline substances at absolute zero.
• B. All gases at standard conditions.
• C. Liquids at cryogenic temperatures.
• D. Any pure substance at any temperature.

Answer: (A)

The third law says that the absolute entropy of a perfect crystal goes to zero as temperature reaches absolute zero. This happens because a perfect crystal has only one accessible microstate—the ground state—at 0 K, so S = k ln(1) = 0. Real crystals may have tiny residual entropy due to defects, but in the ideal case the entropy is zero at absolute zero.

All other scenarios involve multiple accessible microstates at low or finite temperatures, so their entropy is not zero. Gases, liquids, and even pure substances at finite temperatures possess disorder and energy that create many microstates, giving S > 0.