![]() In practice, this is an unattainable ideal. Only a perfectly ordered, crystalline substance at absolute zero would exhibit no molecular motion and have zero entropy. Vibrational, rotational, and translational motions of a carbon dioxide molecule are illustrated here. ![]() Nonetheless, the combination of these two ideals constitutes the basis for the third law of thermodynamics The entropy of any perfectly ordered, crystalline substance at absolute zero is zero.: the entropy of any perfectly ordered, crystalline substance at absolute zero is zero. In practice, absolute zero is an ideal temperature that is unobtainable, and a perfect single crystal is also an ideal that cannot be achieved. Such a state of perfect order (or, conversely, zero disorder) corresponds to zero entropy. The only system that meets this criterion is a perfect crystal at a temperature of absolute zero (0 K), in which each component atom, molecule, or ion is fixed in place within a crystal lattice and exhibits no motion. A perfectly ordered system with only a single microstate available to it would have an entropy of zero. The greater the molecular motion of a system, the greater the number of possible microstates and the higher the entropy. The atoms, molecules, or ions that compose a chemical system can undergo several types of molecular motion, including translation, rotation, and vibration ( Figure 18.13 "Molecular Motions"). To use thermodynamic cycles to calculate changes in entropy.
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