Third Law And Absolute Entropy

Question

According to the Third Law of Thermodynamics, the absolute entropy of a perfect crystalline substance at absolute zero temperature is zero. Which of the following statements is the most correct consequence of this law?

RankGuru · Accepted Answer

The Third Law of Thermodynamics provides an absolute reference point for entropy: for a perfectly ordered crystal at 0 K, $S = 0$. This is a profound consequence because it allows absolute entropies (not just entropy differences) to be calculated by integrating $C_p/T$ from 0 K to the desired temperature, accounting for all phase transitions along the way. This is the basis for standard molar entropies $S^\circ$ tabulated in thermodynamic tables. Option A is incorrect because the Third Law applies only to perfect crystals — substances with residual disorder (like ice with hydrogen-bond disorder) have non-zero residual entropy at 0 K. Option C is incorrect — reactions can have $\Delta S \neq 0$ at absolute zero if the products and reactants have different residual entropies. Option D is the opposite of what the Third Law enables: it actually makes low-temperature entropy measurements possible and meaningful. This is an NCERT and JEE Advanced conceptual question on the Third Law. Plausibility check: standard molar entropies are always positive (or zero) for all pure substances, consistent with the Third Law providing a universal zero reference.

Third Law And Absolute Entropy

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