Q.State the 3rd law of thermodynamics.

Third law: a perfectly ordered crystal at 0 K has zero entropy.

Statement

The entropy of a perfectly crystalline substance (a substance with a completely ordered, defect-free crystal lattice) is taken as zero at 0 K (absolute zero).

Significance

At absolute zero, there is no thermal motion and the crystal exists in a single, unique, most ordered microscopic arrangement, so its entropy (a measure of disorder/randomness) is zero. This law provides an absolute reference point (S = 0 at T = 0 K) from which the absolute entropy of a …