Final answer:
Positive entropy change accompanies a spontaneous process in an isolated system due to an increase in disorder as dictated by the second law of thermodynamics.
Step-by-step explanation:
A student asked: Why does positive entropy change always accompany a spontaneous process in an isolated enclosure The answer to this question is (b) Increase in disorder. According to the second law of thermodynamics, the total entropy of an isolated system can never decrease over time, and is constant if and only if all processes are reversible. In an isolated system where no energy is exchanged with the surroundings, a spontaneous process leads to an increase in entropy, reflecting the tendency for energy to spread out or disperse rather than to remain concentrated.
For example when an ice cube melts in a room the structured solid water becomes less ordered as it turns into liquid, leading to an increase in entropy. Similarly dissolving salt in water increases the randomness, or entropy, of the system because the ions become dispersed throughout the solvent. These processes result in an increase in entropy because energy becomes more dispersed and less available for doing work.