Third law of thermodynamics

The third law of thermodynamics states that the entropy of a perfect crystal approaches zero as its temperature approaches absolute zero. It implies that absolute zero cannot be reached through a finite sequence of physical processes.

Absolute zero: The temperature at which a system has its minimum possible thermal energy. The law describes crystal entropy as temperature approaches this limiting temperature.
Entropy: A thermodynamic state function measuring energy dispersal and the number of microscopic configurations compatible with a state. The third law sets entropy's limiting value for a perfect crystal.
Walther Nernst: A German physical chemist whose heat theorem laid the foundation for the third law of thermodynamics. His theorem first stated the low-temperature entropy behavior underlying the law.
Third-law entropy: The absolute entropy of a substance determined from heat-capacity measurements and phase-transition data. The law supplies the zero-temperature reference needed to calculate it.
Zeroth law of thermodynamics: The law establishing that thermal equilibrium is a transitive relation and thereby defining temperature. It concerns temperature's meaning, whereas the third law concerns entropy near zero temperature.
Residual entropy: Entropy that remains at zero temperature because a system retains configurational disorder. Disordered solids can retain entropy where the law's perfect-crystal condition does not apply.
Thermodynamic temperature: An absolute temperature defined through thermodynamic relations and measured in kelvins. The law concerns the limit of entropy as thermodynamic temperature tends to zero.
Max Planck: A German physicist who developed quantum theory and formulated a zero-entropy version of the third law. His formulation explicitly assigns zero entropy to a perfect crystal at absolute zero.
Standard molar entropy: The entropy of one mole of a substance in its standard state at a specified temperature. Third-law data let chemists determine absolute standard entropies rather than relative values.
First law of thermodynamics: The principle that energy is conserved and transferred as heat or work. Energy conservation does not determine the entropy reference fixed by the third law.

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