The Third Law of Thermodynamics

The Third Law of Thermodynamics states that it is impossible to reach absolute zero in a finite number of steps. A shortcut way to remember the law is "you cannot leave the game."  An alternate shortcut to remember the law is "let's keep score" which is based on the fact that the third law provides the foundation for thermodynamic temperature scales.

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