 Okay, so this simulation shows what happens when you add a salt to water. So we've got a salt shaker here, and when I shake it, some crystals come out and they fall into the water. And you can see each of the crystals is represented as a little sort of crystalline cluster of ions. This is table salt, sodium chloride, so the green ones are chloroides and the red ones are sodium ions. You can see that when they go into the water, they break apart from their crystal structure and dissolve in the water and then are able to move freely through the water. Notice that the volume in this simulation, the sort of pretend volume of water in our container, is tiny, 10 to the minus 23 litres. The reason for this is that it then means that this particular number of ions that we've got dissolved in this volume of water actually gives a physically reasonable concentration. So it's as if you're really scaling down a normal size system, like shaking some actual salt crystals into a glass of water, scaling it down to how small it would be if you could see the individual ions. And you could in fact calculate a concentration from this. So you can see over on the right hand side here, we've got a little tally of the sodium and chloride ions. We've got 92 dissolved sodium ions, 92 dissolved chloride ions. If you were to turn that number of ions into moles, using Abogadro's number, and then divide it by the volume, 5 times 10 to the minus 23 litres, you would arrive at, like I say, a physically reasonable concentration. Okay, so I'm going to add some more salt. And if you watch the tally over here, you can see that the effect is to increase the number of dissolved ions. The bound row here refers to ions that are stuck in crystals. At the moment we have none of those. It's pretty much as soon as they go into the water they dissolve. So we've got now 162 sodium ions and chloride ions in the same volume of water. So that means I've increased the concentration of the salt. But at the moment we have no equilibrium. There's dissolving going on, but there's no reverse reaction happening. There's no precipitation. So we're going to keep adding some more salt. And eventually we get to a point, as you can see now, where not all of the crystals dissolve. So we have a lump of crystals sitting at the bottom of the container. And you can see now that you look at our tally of ions. Our dissolved ions, about 180 each of sodium and chloride. But now we have a certain number of bound ions. That means the ones that are fixed in crystals, about 87 of each. Now watch the crystals closely and have a look at the edges of them. If you watch for a little while you will see that what's going on is that every now and then an ion will detach from the crystal and head off into solution. That's the forward reaction, that's the dissolving. But equally every now and then an ion will come from solution and attach itself to the crystal. That's precipitation, that's the reverse reaction. And the effect of that, if you watch for a while, the effect of that is actually to change the shape of the crystal, as some parts dissolve and other parts precipitate. But overall, if you keep watching the tally of bound ions, overall the size of the crystal is not changing at all. If we increase the amount of crystals, we're going to add a few more. Watch the tally over here. There we go. Before we had about 180 sodium and chloride ions, we've added more crystals and we still only have about 180 sodium and chloride ions that are dissolved in solution. The only effect of adding more crystals from the shaker was to increase the number of bound ions, the undissolved ions in the solution, the crystals that are sitting at the bottom. So it doesn't matter how much more salt we add at this point, you can add even more, you will see that the number of dissolved ions just does not change. You've reached in fact a saturated solution. And this is the point at which an equilibrium has been established when the concentration of the dissolved ions is no longer changing. So the equilibrium for this process was the sodium chloride solid in equilibrium with the sodium ions and the chloride ions that were both aqueous. So when we write the equilibrium expression for this, it's the Ksp, the solubility product, equals the concentration of sodium ions times the concentration of chloride ions. So you can see that the solid sodium chloride doesn't appear in this expression, and that's due to the fact that the concentrations we're interested in and the concentration of a solid doesn't change. Its amount might change, but its concentration never changes. Therefore it doesn't affect the rates of the forward and reverse reactions, and therefore it doesn't affect the position of the equilibrium. So when we were adding more crystals in the simulation, we weren't affecting the concentration of the sodium ions or chloride ions that were dissolved in the water at all. Now it's interesting to note that the value for the Ksp, now it's interesting to note that the value for the Ksp for sodium chloride is about 36. If you think about this, it means that the concentrations of the sodium and chloride ions must be quite high because when you multiply them together, you get a large number like 36. And this reflects the fact that sodium chloride is very soluble.