According to the third law of thermodynamics, the entropy of a perfect crystal at 0 K is zero. A perfect crystal is treated as having a single, uniquely ordered ground-state arrangement, so there is no uncertainty about its microscopic configuration.
In statistical mechanics, entropy is related to the number of accessible microstates through Boltzmann’s relation, S = kB ln Ω. For one accessible state, Ω = 1, and ln(1) = 0. This gives the crystal an entropy of zero at absolute zero and provides a reference point for calculating absolute entropies at higher temperatures.
The law was developed largely through Walther Nernst’s work between 1906 and 1912 and is also associated with the Nernst heat theorem. A common mix-up is assuming that every material must have zero entropy at 0 K. Real crystals can contain defects, disorder, or multiple equally low-energy arrangements, producing residual entropy. The quiz’s wording is precise: for a perfect crystal with a unique ground state, the canonical answer is zero.