Nernst heat theorem

The principle that the entropy change of an isothermal process approaches zero as temperature approaches absolute zero. It is an early formulation of the third law of thermodynamics.

Entropy: A thermodynamic state function that quantifies energy dispersal and the number of microscopic arrangements compatible with a state. The theorem is specifically about how entropy differences behave near absolute zero.
Walther Nernst: A German physical chemist whose work on chemical equilibria and low-temperature thermodynamics earned him the 1920 Nobel Prize in Chemistry. He formulated the heat theorem from studies of reactions near absolute zero.
Heat capacity: The amount of heat required to raise a system's temperature by one degree under specified conditions. Its low-temperature behavior determines how entropy accumulates as a substance is cooled.
Absolute entropy: The entropy of a substance measured relative to a zero reference established by the third law. Nernst's principle helped make absolute entropy differences experimentally accessible.
Isothermal process: A thermodynamic process that occurs at constant temperature. The theorem constrains entropy changes along processes held at fixed temperature.
Third law of thermodynamics: A thermodynamic principle that fixes entropy behavior as temperature approaches absolute zero. The Nernst heat theorem is a foundational historical formulation of this broader law.
Thermodynamic integration: A method for calculating state-function changes by integrating measurable thermodynamic quantities along a path. Entropy changes can be obtained by integrating heat-capacity data from low temperatures.
Unattainability principle: The principle that no finite sequence of physical processes can cool a system to absolute zero. The third-law framework also constrains how closely practical processes can approach zero kelvins.
Absolute zero: The lowest possible temperature, defined as zero kelvins. The theorem describes the limiting behavior of entropy as temperature approaches this boundary.
Planck's formulation of the third law: A formulation stating that the entropy of a perfect crystal approaches zero at absolute zero. It sharpens Nernst's claim about entropy changes into a statement about absolute entropy.

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Debye T³ lawRelated: The T³ heat capacity is consistent with entropy vanishing as a solid cools toward absolute zero.