Q.What is the third law of thermodynamics?

Entropy of a perfect crystal is exactly zero at 0 K — this law is what allows absolute entropies of substances to be measured and tabulated.

The Third Law of Thermodynamics states: the entropy of a perfectly crystalline substance (one with perfect order, no positional or orientational disorder) is taken as zero at the absolute zero of temperature (T = 0 K).

This is reasonable physically: at absolute zero, all thermal motion ceases and a perfect crystal has only one possible microscopic arrangement (perfect order), so its entropy (a measure of disorder/number of accessible microstates) is minimum, i.e. zero.

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