The diagram at left depicts a more random or disordered configuration, but the key point is that there is a vast number of ways that such configurations could be achieved. If it occurred, it would be for a brief instant and then the molecules would move to some other configuration. At ordinary temperatures, the internal energy of a gas would give the molecules high velocities, and it is evident that this orderly arrangement would be very rare because there are only a few ways to do it. The top diagram depicts time's arrow as pointing from order to disorder, but one must admit that the apparent tendency to move from order to disorder is not the most fundamental way to look at the top diagram. The very fact of differences of opinion on the use of order and disorder can itself be instructive. Chemists, on the other hand, often protest this approach because in chemical applications order vs disorder doesn't communicate the needed ideas on the molecular level and can indeed be misleading. It is typical for physicists to use this kind of introduction because it quickly introduces the concept of multiplicity in a visual, physical way with analogies in our common experience. The diagrams above have generated a lively discussion, partly because of the use of order vs disorder in the conceptual introduction of entropy. In teaching thermodynamics, I have always tried to keep in mind the words of one of the great physics teachers of the mid-20th century, Mark Zemansky: 'Teaching thermal physics There are some subtleties in the nature of entropy and other thermodynamic quantities, subtleties that we try to put into word pictures and sketches which sometimes oversimplify. For an isolated system, entropy always increases or remains the same, so if you compare states of different entropy, the one with the greater entropy will be later in time. Using Newton's laws to describe the motion of the molecules would not tell you which came first. This tells us that the right hand box of molecules happened before the left. One of the ideas involved in the concept of entropy is that nature tends from order to disorder in isolated systems.

Law of entropy