INSTRUCTOR: Phase diagrams are a convenient way to graphically represent the phases of a substance, and the equilibria existing between each phase. As an example, let's take dry ice or solid CO2. I'm going to place this CO2 in this beaker of water that's close to boiling. What will happen when I place this solid CO2 into the water? As we can see, the CO2 goes straight from the solid to the gaseous phase, which is evident by the bubbles that we see here in the water. We can explain what's happening here based on a phase diagram. Phase diagram has been put together to show the transitions between phases of a particular substance. Something that we know is water, right? Water goes from a solid to a liquid to a gas when we heat it up. CO2, on the other hand, has a very, very high triple point. So what happens is we go from a solid to a gas right away. And we can see that here in the phase diagram, which is constructed based on what would happen with the experimental data. As we can see from this demonstration that's still bubbling here, when gaseous CO2 forms, the pressure, if we would have it in a sealed container, would go up dramatically. We have this sealed apparatus here where we have thick tag on tubing with the pressure gauge on one end. I'm going to scoop some solid CO2, put it in here, seal off the end, and see what happens to the phase change as we increase the pressure. So we've added solid CO2 to the inside of the tube here, and clamped off, and sealed the one end. We also have a valve here that's currently open. But if we close the valve, we're going to allow the gas to build up inside the tube, which will increase the pressure. What will happen when the pressure increases inside this tube? So as we can see, inside the tube, we now have a liquid. And notice the pressure gauge is over 60 Psi right now. So this shows that at very high pressure, the CO2 went from a gas to a liquid. If we open up the pressure valve, and let the pressure go back down, we see that it goes from a liquid to a solid. So let's explain what happened here, and see how it relates to this phase diagram. Initially, we performed the experiment under constant pressure, where we dropped the solid CO2 into the warm water. We saw a phase transition from a solid to a gas. But here, as we increased the pressure, we are now moving along this line in the phase diagram. And we have an equilibrium between a solid and a gas. We then reached the triple point, where we have an equilibrium with solid, liquid, and gas all at one point, which we indicate by calling it a triple point, with all three phases present. We then go past the triple point, and then we decided to release the valve, which then came right back down to where we were, and we have an equilibrium between a solid and a gas.