2.5 The Third Law

This law is an application of the second law. It states that the entropy of a perfect crystalline substance is zero at absolute zero (\(\pu{0 K}\)). As \(T\) increases, molecular motion increases causing an increase in entropy (Fig. 32).

As can be observed from Fig. 32, all entropy values of substances are absolute entropies at a given \(T\) compared with their corresponding values (\(=0\)) at absolute zero. Absolute entropies are referred to as standard entropies (\(S^\circ\)) at \(1\).