Entropy

Entropy, like internal energy, is a state function: when a system goes from one state to another, the change in entropy does not depend on the path, only on the initial and final states. For a reversible process at constant temperature, ΔS = Q/T, where Q is the heat exchanged and T is the temperature in kelvin; a system that absorbs heat gains entropy. An object that gives off heat can lose entropy, but once whatever receives that heat is counted, the entropy of the universe still increases in an irreversible process.

The link between entropy and disorder is a simplified picture; the full argument belongs to statistical physics. The familiar illustration: a new deck of cards arranged by suit and number is highly ordered; shuffling randomizes it and increases its entropy. Put this way, the second law says that in any irreversible process the universe becomes more disordered. A gas expanding freely into a larger volume has more possible arrangements of its atoms, so its entropy goes up.