Which law of thermodynamics states that the entropy of an isolated system always increases?

The second law of thermodynamics states that the entropy of an isolated system always increases over time. Entropy is a measure of disorder or randomness in a system, and this law implies that natural processes tend to move toward a state of greater disorder. In practical terms, it means that energy transformations are not 100% efficient; some energy is always lost as heat or disorder, leading to increased entropy.

As time progresses, systems will evolve toward thermal equilibrium, where entropy is maximized. This principle is fundamental to understanding various processes in physics, chemistry, and engineering, as it accounts for the direction of spontaneous processes and informs energy efficiency considerations.

Other principles, such as the first law of thermodynamics, focus on the conservation of energy within a closed system, but do not address the concept of entropy. The third law of thermodynamics deals with the behavior of systems as they approach absolute zero, and the law of conservation of energy refers to the constancy of energy in closed systems, rather than the directionality of energy transformations and entropy changes. Thus, the second law's emphasis on increasing entropy is what defines it distinctly.