The Third Law of Thermodynamics states that the absolute entropy (S) of a perfectly crystalline substance at 0 K is zero.
Thus it is possible to calculate the absolute entropy of a substance by adding the entropy changes that occur on heating the substance from 0 K to the temperature specified. This involves
- calculating ΔS for the steps in the list that are required to reach the specified temperature
- heating solid to melting temperature
- melting then
- heating liquid to boiling point then
- boiling then
- heating gas
- summing the calculated entropy changes.