For an isolated system, the entropy:
A. Either increases or remains constant
B. Either decreases or remains constant
C. Can never decrease
D. Can never increase

Hint: Entropy is the measure of a systemic thermal energy per unit temperature that is available for doing useful work. The amount of entropy is a measure of the molecular disorder, or in other words, randomness of a system.
- For this question, we’ll have to use the second law of thermodynamics.

Complete Solution :
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.
- For example: Let us consider a system made up of two closed chambers separated by a partition. We assume that both the chambers consist of different gases. Now, if we remove the partition that separates these chambers, the gases of both chambers will mix with each other. If we place the partition back in its position, the gases will not ‘un-mix’ or in other words, will not return to its initial condition. Therefore, we can say that entropy (measurement of disorder in a system) always increases and never decreases.
- This leaves us with problem (C) as the correct answer after option (A), (B) & (D) are eliminated for being partially correct or completely wrong.
So, the correct answer is “Option C”.

Note: Isolated systems spontaneously evolve towards thermodynamic equilibrium which corresponds to maximum entropy. In an isolated system, for example with non-uniform temperature, heat might irreversibly flow and the temperature becomes more uniform thus increasing the entropy.
Entropy of solids (or particles that are closely packed) is more in comparison to that in gases (in which particles are free to move).
- In a spontaneous process, the entropy of the process increases.
- The SI unit of entropy (S) is Joule per Kelvin .
- The more positive value of entropy means a reaction is more spontaneous.
- Entropy is one of the few quantities in the physical sciences that require a particular direction of time, sometimes called an arrow of time.