Third law of thermodynamics – entropy at absolute zero The statement “At absolute zero temperature, the entropy of every perfectly crystalline substance becomes zero” follows from which thermodynamic principle?

Introduction / Context:
The low-temperature limit of entropy is fundamental to thermodynamics, materials science, and cryogenics. It provides an absolute reference for entropy and underpins calculations of absolute entropies from calorimetric data.

Given Data / Assumptions:

• A “perfectly crystalline” substance has a single microstate at 0 K (no configurational disorder).
• Absolute zero is the limit T → 0 K.
• The quoted statement is Planck’s formulation of the third law.

Concept / Approach:
The third law of thermodynamics (Planck statement) asserts that the entropy of a perfect crystal approaches zero as temperature approaches zero. Nernst’s heat theorem is closely related, stating that the entropy change for any isothermal process approaches zero as T → 0 K. While the two are connected, the exact wording given corresponds to the third law’s Planck form, not the general second law or the mathematical Maxwell relations.

Step-by-Step Solution:

Verification / Alternative check:
Entropy tables set S°(0 K) = 0 for perfect crystals, consistent with third-law calorimetry practices.

Why Other Options Are Wrong:

• Second law: Governs entropy increase and reversibility, not the absolute zero limit.
• Nernst heat theorem: Related but not the exact quoted statement.
• Maxwell’s relations: Mathematical identities from exact differentials; unrelated to the 0 K entropy axiom.

Common Pitfalls:
Equating Nernst’s theorem with the entire third law; while related, the wording here is specifically the third law.

Final Answer:
third law of thermodynamics