What is the term for a process that measures the disorder or randomness of a system? The answer is entropy. Entropy is a thermodynamic state variable associated with the number and probability of microscopic arrangements, or microstates, compatible with a system’s observable condition. It is commonly introduced as a measure of disorder or randomness, although that wording is only an approximation.
In statistical mechanics, Ludwig Boltzmann expressed entropy with the relation S = k ln W, where k is the Boltzmann constant and W represents the number of accessible microstates. More possible arrangements generally mean higher entropy. In classical thermodynamics, entropy changes can also be calculated from reversible heat transfer using dS = δQ_rev/T.
Entropy is central to the second law of thermodynamics. For an isolated system, the total entropy cannot decrease during a spontaneous process; it stays constant in an ideal reversible process and increases in an irreversible one. Entropy should not be confused with energy itself: energy is conserved, while entropy tracks energy dispersal and the limits on converting heat into useful work. Enthalpy, internal energy, and free energy are different thermodynamic quantities.