One of the consequences of the second law and the studies by people
such as Kelvin and Clausius was the invention of a new and useful
function, the entropy, S. As originally defined, entropy was strictly
a function of heat and temperature.
With enough experimental ingenuity and patience, the entropy of
any substance at a given temperature can be calculated from calorimetric
measurements. Because of the third law of thermodynamics, which
states that the entropy of every pure, crystalline substance at
absolute zero is zero, these values calculated from calorimetric
measurements are called third-law entropies.
They are tabulated as S0298 for elements
and compounds in their standard states at 298ēK, alongside heats
of formation, in the appendix
(and in any standard chemical handbook).