What does the Second Law of Thermodynamics state about entropy?

The Second Law states that entropy of an isolated system never decreases; for any spontaneous process, total entropy (system + surroundings) increases. It establishes the direction of natural processes and limits on energy conversion efficiency. Entropy (S) measures disorder or unavailable energy. Implications: heat flows from hot to cold spontaneously, 100% heat-to-work conversion is impossible, and minimum work required for separation equals the entropy of mixing. It defines the maximum theoretical efficiency of heat engines (Carnot efficiency).