What does the Third Law of thermodynamics state about perfect crystalline substances at absolute zero?

The Third Law of Thermodynamics states that the entropy of a perfect crystalline substance approaches zero as the temperature approaches absolute zero (0 K). This means that at absolute zero, a perfectly ordered crystal would have only one microstate: the arrangement of its particles would be in a completely fixed position, with no randomness or disorder.

As temperature decreases, the thermal energy of the system decreases, and thus the possible configurations or microstates of the system reduce, leading to a reduction in entropy. Therefore, at absolute zero, the only possible state is the ground state corresponding to perfect order, resulting in zero entropy. This theoretical condition is important as it establishes an absolute reference point for the determination of entropy values for other substances at higher temperatures.