What is the name of the law that states that the total entropy of an isolated system can never decrease?

The law stating that the total entropy of an isolated system can never decrease is the second law of thermodynamics. In mathematical form, the entropy change of an isolated system is greater than or equal to zero: ΔS ≥ 0.

The law describes the preferred direction of natural processes. Heat flows spontaneously from a hotter body to a colder one, gases spread through available space, and friction converts organized mechanical energy into less useful thermal energy. These processes can sometimes be reversed locally, but reversing them requires changes elsewhere that increase the total entropy.

The second law emerged from the study of heat engines. Sadi Carnot identified limits on engine efficiency in 1824, and Rudolf Clausius later gave the law its entropy-based formulation in the 1850s. A common mix-up is treating entropy as simply “disorder.” It is more precisely related to the number and distribution of microscopic states. Also, entropy can decrease in a subsystem—such as during refrigeration—provided the surroundings gain enough entropy to keep the isolated total from decreasing.