Which of the following statements pertaining to entropy are correct ?

A. The entropy of a system reaches its minimum value when it is in a state of equilibrium with its surroundings.

B. Entropy is conserved in all reversible processes.

C. Entropy of a substance is least in solid phase.

D. Entropy of a solid solution is not zero at absolute zero temperature.

Explanation:

• The entropy in the reversible systems is conserved because there is no entropy generation in the reversible process.
• The third law of thermodynamics states that the entropy of a perfect crystal is zero at the absolute zero temperature as it represents the maximum degree of order.
• However, a substance not having a perfect crystalline structure and possessing a degree of randomness such as a solid solution or a glassy solid has a finite value of entropy at absolute zero.
• Since the disorder of molecules is less in the solid phase than in the gaseous or liquid phase, hence the solid phase will have the least entropy.
• The entropy attains its maximum value when the system reaches a stable equilibrium state from a non-equilibrium state. This is the state of maximum disorder.
• The entropy change of the universe in all reversible processes is zero.