# 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* ($\pu{0 K}$). The absolute entropy of a pure substance at a given temperature is the sum of all the entropy it would acquire on warming from absolute zero (where $S=0$) to the particular temperature. 

Absolute entropy increases steadily with increasing temperature until the melting point is reached, where it jumps suddenly as the substance undergoes a phase change from a highly ordered solid to a disordered liquid ($\Delta S_{fus}$). The entropy again increases steadily with increasing temperature until the boiling point is reached, where it jumps suddenly as the liquid undergoes a phase change to a highly disordered gas ($\Delta S_{vap}$). As $T$ increases, molecular motion increases, causing an increase in entropy ({numref}`third-law`).

```{figure} assets/third-law.jpg
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A generalized plot of entropy versus temperature for a single substance. Image source: [13.6: The Third Law of Thermodynamics | LibreTexts](https://t.ly/ZMoMe)
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The third law of thermodynamics has two important consequences: it defines the sign of the entropy of any substance at temperatures above absolute zero as positive, and it provides a fixed reference point that allows us to measure the absolute entropy of any substance at any temperature. 

All entropy values of substances are absolute entropies at a given $T$ compared with their corresponding values ($=0$) at absolute zero. Absolute entropies are standard entropies ($S^\circ$) at $\pu{1 atm}$.