 In addition to chemical changes, we can also look at physical changes, and most physical changes are reversible. This includes melting and freezing, evaporation and condensation, and dissolving and precipitation. I want to look at an example of dissolving and precipitation. This FET simulation shows, at a molecular level, crystals of sodium chloride being dropped into water. The red balls are sodium ions, and the green ones are chloride ions. The individual water molecules aren't shown to avoid the picture becoming too cluttered. They just use a simple blue background. Note that, at first, the sodium chloride crystals dissolve, as you'd expect. This is what happens when you put salt into water. The ions separate, and they diffuse through the water. However, at some point we've added so much salt that the water has become saturated with salt. This means that the aqueous salt solution has reached a maximum concentration of sodium chloride, and no more can dissolve. This can be observed by the fact that when you add more crystals, they don't dissolve. And also, by noticing that the number of bound ions, that's the one stuck in solid form in the crystals, no longer changes. However, watch the saturated solution closely. You'll see that, in fact, ions are dissolving from the crystals, but at the same time other ions are precipitating out from solution onto the surface of the crystal. So the crystal is both dissolving and precipitating. And right now the rate at which they're precipitating equals the rate at which they're dissolving. What's the effect of that? Well, the crystals do change shape as their edges are dissolved and rebuilt, but they don't change size. The number of ions that are stuck in the crystals stays the same. Equally, the number of ions that are dissolved in solution stays the same, which means that the salt concentration in the water remains constant. This means the system is in dynamic equilibrium. And we have two key points that should now be familiar. On average the system seems unchanging. This is apparent if we look at the number of dissolved and bound ions. If you were watching this in the lab or your kitchen from a macroscopic viewpoint, you'd just see a clear solution with salt crystals sitting at the bottom. It would appear as if nothing were happening. But on a molecular level it's obvious that the separate processes of dissolving and precipitation are continuing with great rapidity. It's just that the two processes are happening at the same rate, so the effect of one cancels out the effect of the other. When these two things are true for a system, the system is in dynamic equilibrium. The processes could be melting and freezing, condensation and evaporation, dissolving and precipitating, or the forward and reverse directions of a chemical equilibrium.