In science, we look for patterns to discover nature’s laws. What is the pattern common to all spontaneous changes? To find a pattern, it is often best to start with very simple examples, because then the pattern is likely to be more obvious. So let’s think about two simple spontaneous changes — the cooling of a hot metal and the expansion of a gas — at a molecular level and search for their common feature.
A hot block of metal cools as the energy of its vigorously vibrating atoms spreads into the surroundings. The vigorously moving atoms of the metal collide with the slower atoms and molecules of the surroundings, transferring some of their energy in the collisions. The reverse change is very improbable, because it would require that energy migrate from the surroundings and concentrate in a small block of metal. Such a process would require that collisions of the less vigorously moving atoms of the surroundings with the more vigorously moving atoms of the metal would cause the latter to note even more vigorously. The randomly moving molecules of a has spread out all over their container; it is very unlikely that their random motion will bring them all simultaneously back into one corner. The pattern starting to emerge is that energy and matter tend to disperse in a disorderly fashion.