 But let's just have a look at a couple of examples to see if we can understand how we might go about predicting changes in the pressure. So the first example is one that we have looked at previously, which is nitrogen dioxide and dinitrogen tetroxide. The most important thing when we're analyzing changes in pressure is to look at the number of gas moles. So on the left side, on the reactant side of this reaction we have two, and on the right hand side we have one. So we're looking at the coefficient in front of each of these substances. So it's a two in front of the nitrogen dioxide, and there is no number, so the assumption then is that it's one in front of the dinitrogen tetroxide. This number of gas moles is very important when we look at pressure. So pressure is related to the collisions between the particles and the walls of the container. So therefore, if you have more particles, you'll have more collisions and therefore more pressure. So one of the things that we look at when we're looking at how systems respond according to Le Chatelier's principle to pressure, if we were to increase the pressure without doing any other analysis, what we would look at is the fact that if our change is to increase the pressure in this particular system, what we might do then is to look at the fact that the system will want to respond to Le Chatelier's principle response. It counters by reducing number of particles. In this case, the right hand side has just one particle where there's a left hand side has two. So if we increase pressure, we're going to push this equilibrium to the right. Therefore, shift right. And at a macroscopic level, you would find that the mixture became a little paler. Some of that brown gas is becoming the colorless gas and therefore the overall color is going to fade a little. This is the way that we try and analyze systems in terms of the changes that occur in the pressure. Obviously, if we were to decrease the pressure, then we're going to potentially make more space. That is going to again change the concentration of each of these gases. But we're also in that extra space. We can now, in order to counteract that drop in pressure, we want more collisions so we make more particles and therefore the equilibrium is going to shift to the right. Now, what happens when we have an equilibrium which involves a gas and a solid? So here is iodine. We know that iodine sublimes and that means it goes straight from a solid into a gas and it can actually set up an equilibrium between the solid and the gas. The gas is more of a purpley color. The solid is a shiny sort of gray black color. What happens if we were to increase the pressure in this system? If we were to increase the pressure in this system, and that's again our change, then our Le Chatelier's response is to counter the change by trying to reduce the particles. Now in this case, if we look at the gas moles, we have one on the left hand side, on the reactant side, we have zero on the right hand side. So because iodine is a solid, that's the formation of the solid, there are no gas particles there. Therefore, obviously one is bigger than zero and therefore we're going to shift right to reduce gas molecules. So an increase in the pressure is going to squeeze that container. It's going to force those little gas molecules closer together in order to counteract that. What they will then need to do is to form more of the solid in order to reduce that pressure counteract that change according to Le Chatelier's principle. So this is just a bit of an introduction at this stage to the idea of changing pressure in systems, in equilibrium systems that involve gases. Again, we'll give you some opportunity in class to practice some examples and thanks for watching.