Q.State the Third Law of Thermodynamics.

At absolute zero, a perfectly ordered crystalline substance has zero entropy — this is the Third Law, and it gives a natural reference point for calculating absolute entropies.

Statement: The entropy of a perfectly crystalline substance (one with perfect internal order, no defects or disorder) is taken as zero at the absolute zero of temperature (T = 0 K).

Explanation: Entropy (S) is a measure of the disorder/randomness of a system. At 0 K, all thermal motion of particles theoretically ceases, and in a perfect crystal every atom/molecule occupies one fixed, ordered position in the lattice with no positional or configurational disorder. Hence there is only one possible microscopic arrangement (W = 1), and by Boltzmann's relation S = k ln W = k ln 1 = 0.

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