**Considerations regarding the Third Law**

Perfect crystalline solids have zero entropy at absolute zero. Note that materials that are not perfect crystals do not necessarily have zero entropy at absolute zero.

The Third Law provides a zero for temperature that is really a zero. The Fahrenheit and Celsius temperature scales have a zero, but that zero is arbitrarily assigned. In a thermodynamic temperature scale that zero is absolute zero, the coldest temperature possible, or rather a lower bound on the coldest temperature possible.

The Kelvin scale is the International System (SI) standard thermodynamic temperature scale. The unit of the Kelvin scale is the kelvin, not a "degree Kelvin." It is represented by "K."

0 K = -273.15

**°**C

The equality is exact, as the Celsius scale is now defined in terms of the Kelvin scale. The Rankine scale is a less common thermodynamic temperature scale. See my post Converting Units of Temperature for more discussion.

**Sources**

- Atkins, P. W.
*Physical Chemistry*, W. H. Freeman and Company, New York, 3rd edition, 1986 - McQuarrie, Donal d A.,
*Statistical Thermodynamics*, University Science Books, Mill Valley, CA, 1973 - Bromberg, J. Philip,
*Physical Chemistry*, Allan and Bacon, Inc., Boston, 2nd Edition, 1984 - Feynman, Richard P., Leighton Robert B., Sands, Matthew,
*The Feynman Lectures on Physics*, Addison-Wesley, Menlo park, CA, 1965

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