# 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 ({numref}`entropy-substance-fig`).

As can be observed from {numref}`entropy-substance-fig`, 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 $1$.