How can we resolve this seeming paradox? The answer lies in the fact that for any system the entropy may indeed decrease - water freezing is an example of this phenomenon. The problem with this is that we are all well aware of changes where the entropy apparently decreases. The Second Law of Thermodynamics tells us that for every change that occurs, the entropy of the universe must increase. How can it be that a change in which the entropy of the system decreases (for example freezing of ice) can occur? Are we forced to conclude that things we know to happen are impossible according to the Second Law of Thermodynamics? We can calculate how entropies change for materials as they go from gas to liquid to solid, and as we have predicted they decrease. How do we think about entropy in these systems? Doesn’t a substance become more ordered as we move it from gas to liquid to solid? Clearly the entropy of a solid is lower than that of a liquid (and the entropy of a liquid is lower than that of a gas).
Now let us return to the situation with solids, liquids, and gases.
