What is the third law of thermodynamics?

The third law of thermodynamics states that the entropy of a perfect crystal approaches zero as the temperature approaches absolute zero (0 K). This implies that at absolute zero, a perfect crystalline structure has only one microstate available, which corresponds to a perfectly ordered state. Since entropy is a measure of the number of possible microscopic configurations of a system (disorder), a perfectly ordered crystal at absolute zero has no disorder, leading to an entropy value of zero.

This concept is crucial because it provides a reference point for the calculation of absolute entropies of substances at higher temperatures. As temperature increases, the entropy of a substance generally increases because more microstates become available. Furthermore, the third law helps in understanding low-temperature phenomena and reinforces the idea that reaching absolute zero is unattainable in practice, further influencing how systems behave at very low temperatures.

Other options do not accurately reflect the principles of the third law. The second option misstates the entropy at absolute zero as one, which contradicts the actual definition. The third option is overly broad and does not capture the relationship between entropy and temperature. Lastly, the notion that entropy is undefined at absolute zero does not align with the established understanding of thermodynamic principles, which clearly define the state of a perfect crystal