Entropy as Disorder: History of a Misconception

Before reading this essay, go to your kitchen and find a bottle of Italian salad dressing. Get one that’s been sitting still for a while at a fixed temperature—that is, one in thermal equilibrium. You’ll find an oil-rich layer at the top of the bottle and a vinegar-rich layer at the bottom (see Fig. 1). But think for a moment before spreading it over a delicious salad and eating up. That bottle’s in thermal equilibrium, so it’s in a state of maximum entropy. Doesn’t entropy mean “disorder”? No one would call a stack of 50 pennies and 50 dimes disordered if all the dimes were on the top and all the pennies at the bottom. So why is this salad dressing at thermal equilibrium segregated like an ordered stack of coins?