The entropy of a pure substance in a perfect crystalline state is zero at absolute zero. This statement is described by which law?

• A. Third Law of Thermodynamics
• B. Zeroth Law
• C. First Law
• D. Second Law

Answer: (A)

Entropy being zero at absolute zero for a perfectly crystalline substance is described by the Third Law of Thermodynamics. This law sets the zero point of the entropy scale at 0 K for a perfectly ordered crystal, because at that point there is only one accessible microstate and no disorder. In other words, with perfect order there’s no uncertainty left, so S = 0. This gives an absolute reference for entropy values, unlike changes in entropy that the other laws address.

The Zeroth Law deals with thermal equilibrium and temperature measurement, not the absolute entropy value. The First Law is about energy conservation, relating heat, work, and internal energy. The Second Law concerns the direction of processes and the general tendency for entropy to increase in spontaneous changes, but it does not fix the entropy’s zero point.

Note that in real substances there can be residual entropy at 0 K due to persistent disorder, but for a theoretical perfect crystal this zero value holds.