The statement that entropy change is always greater than zero for natural processes is a generalization based on the second law of thermodynamics. The second law states that the total entropy of an isolated system will always tend to increase over time.
Entropy is a measure of the disorder or randomness of a system. In natural processes, there is typically a tendency for systems to become more disordered or to have a greater number of microstates available to them. This increase in disorder leads to an increase in entropy.
To understand why entropy change is generally greater than zero, consider the following:
Spontaneous processes: Natural processes, also known as spontaneous processes, occur without any external intervention. These processes tend to proceed in the direction that increases the overall entropy of the system and its surroundings. For example, a gas spreading out to fill its container or a solute dissolving in a solvent both lead to an increase in the number of possible microstates and thus an increase in entropy.
Probability and microstates: Entropy can be related to the number of microstates available to a system. Microstates are specific arrangements of particles or energy within a system. A system with a larger number of microstates has higher entropy. As a system evolves, it is more probable to move towards states with a larger number of microstates, increasing the entropy.
Energy dispersal: Energy tends to disperse or spread out in natural processes. When energy is concentrated in a localized region, it has fewer ways to be distributed among particles or modes of motion. As the energy disperses, it becomes distributed over a greater number of particles or modes, increasing the entropy.
It's important to note that while the overall entropy of a system and its surroundings tends to increase, it is possible for localized decreases in entropy to occur. However, these decreases are always compensated by greater increases in entropy elsewhere, leading to a net positive change in entropy for the system as a whole.
In summary, the general trend of entropy change being greater than zero for natural processes arises from the probabilistic nature of systems and their tendency to move towards states with higher disorder or energy dispersal. This behavior is captured by the second law of thermodynamics.