He points out two related and common misconceptions about entropy:
- mixing is always irreversible (and so must involve an increase in entropy)
- entropy is related to disorder
This is illustrated with the three processes of mixing shown below. All involve mixing, but only the first is irreversible.
What determines the entropy change is not the mixing (or amount of disorder) but the change in volume of each gas. That can be related to information (or ignorance) about the state of the gas molecules.