 All right, so we have an understanding of what a temperature composition diagram looked like for a solution that contains two components. One component in this diagram, the one with the lower boiling point, is the more volatile of the components. I'm calling that one A. Compound B with a higher boiling point is the less volatile component. So if we look carefully at this temperature composition diagram, we can figure out some interesting things about how boiling works in a solution, as opposed to a single phase. So for example, the first thing we can notice is unlike a pure substance, like water, ethanol, and a pure solvent, a mixture of two solvents typically has a range of boiling points rather than just a single boiling point. And what I mean by that and how you can tell that's true, let's say we prepare a solution with some composition, some particular mole fraction of the solvent A. And let's say I prepared a temperature low enough it hasn't yet boiled. If I heat that solution up, raise the temperature, eventually I will get to this line. This is the one we call the bubble point curve. Because at this temperature, when I've heated the temperature up to this value, then we'll see the first bubble form in the solution. The solution will begin to boil. And then what this liquid gas coexistence curve tells us is at that temperature, the liquid with this composition is in coexistence with the gas at this composition. So when I read downward to this axis, this is the mole fraction of the vapor phase. So at this temperature, the liquid has this composition, the vapor has this composition. But notice I can have liquid in coexistence with vapor not just at this temperature, but at a range of different temperatures. I can continue heating the solution. And when I get up to this temperature, it's when I've boiled the last drop wood of the liquid away and converted the entire solution to vapor. At temperatures above this temperature, it's 100% gaseous. Temperatures between these two values, I have a mixture of liquid and gas. So what that means is the solution is boiling. I had liquid coexisting with gas over a range of different temperatures. So there's our first important observation. Unlike pure solvents, which boil at a specific temperature, at a single temperature, a solution will boil over a range of different temperatures. That range might be narrow at some compositions of the solution. It might be relatively broad at other compositions of the solution. But the pure substance will boil at a sharp boiling point. A solution will boil at a range of different boiling points. The other thing we've noticed in using this tie line, as we've talked about before, the composition of the vapor and the composition of the liquid are different, even though those two things are in equilibrium with each other. Another way of describing that is when I take a solution, heat it up until it begins to boil. The vapor that it's in coexistence with, we've talked about how the vapor is more enriched in the more volatile component of the solution. So that more volatile component escapes more readily from the solution and ends up over-represented in the gas phase relative to what it was in the liquid phase. That has an important consequence that we can use in a process called distillation. Distillation is something you may have heard of. You've probably heard of distilled water, for example. We use distillation often as a method of purifying solvents. And the way that works is since we know that when we take this mixture of two solvents, A and B, when we heat it up until it begins to boil, the vapor that boils off the solution is enriched in this more volatile component A. What that means is if we collect that vapor, if we take some of that vapor that has boiled and we collect the vapor, leaving the rest of the liquid phase behind, and condense that vapor down, reconvert it to a liquid phase, it will have this composition. So if I've cooled it back down to this temperature, now I have a solution at this composition, even though it used to be the composition of the vapor phase, now I've converted it back to a liquid. So that's the composition of this condensed vapor phase. Let me show you a picture of what that would look like in case it helps visualize what's going on. So I start out, let's say I start out like in this diagram with a solution that is minority A. So I've got some A molecules in solution and some B molecules. I've got more of the B molecules than the A molecules. I apply some heat to that system. So there's my poor representation of a Bunsen burner. I boil that solution and then into the vapor phase goes some A molecules and some B molecules, but the liquid, I'm sorry, the vapor is enriched in the more volatile component A. So I've got more A molecules in the liquid, in the vapor than I used to have in the liquid. Now let's say I put a lid on that container and I let the vapor molecules get far away from the heat source so they condense back down into the liquid phase. So I can get droplets of liquid condensing into this beaker over here. This is the distillate, the liquid that results from this distillation. And the composition of this vapor will match the composition of the distillate once I've condensed it. So I distill the liquid and what I end up with is not pure A, but purified to some degree in A. The more volatile component is richer in this liquid than it was in this liquid. But there's nothing to stop me from repeating that process again. So once I've condensed the distillate back down to a temperature where it's liquid, I can then proceed to heat it up again. Now I won't be able to heat it up quite as far because it's more concentrated than that volatile component. It's a more volatile solution. When I get to this temperature, I've reached the bubble point curve. I begin to see my first bubble of vapor forming in this liquid. So now I've heated this solution. It also begins to boil if I put a lid and condense the distillate that forms from that solution. This one is now going to be even more concentrated in component A. So in this case, this will be the concentration of A in the vapor phase, which I can then condense back down to the liquid phase. If I repeat that process so I can obtain a new liquid over here, I can repeat this process as many times as I want. Every step of that distillation process works me further and further down into this corner, closer and closer to pure component A. So I'll never get quite to the edge, but this is in a limiting process. I can make the component A as pure as I wish by continuing to distill it. And that's a process called fractional distillation. And that makes sense because I don't distill the entire solvent multiple times. What I do is I take my solution, I distill it, I get a fraction of what I started with. Notice that I can't take the whole solution and recondense it if I converted the entire solution to gas and then recondense it. I just have the same solution I started with. I have to only boil off a fraction of the solution so that the gas phase is still more concentrated in the volatile component. So I only get a fraction of the original solution. When I distill it again, I only get another fraction. So this process, the more steps I take, the smaller amount of distill that I get in each process. So that process is called fractional distillation. It can happen not just in sequential steps, the way I've drawn it here, which is perhaps the easiest way to visualize it on this diagram. But you can, in fact, do this all in one container, all in one flask if you're in a laboratory. Imagine we've got a solution, which again, we're going to boil, containing A and B. If I do that with a fractional distillation column, what that looks like is as the molecules of A and B evaporate or boil out of the solution. Again, the higher up into this column they get, the farther away from the heat source they get, the more they're going to be likely to condense back down and form the liquid phase, depending on what the concentration of that vapor phase is. So I give the liquid a few platforms or ledges, opportunities to condense back down and form liquid, and perhaps, again, condense and distill and drop back down to lower platforms in this distillation column. So essentially, what this fractional distillation column is doing is doing each one of the many steps of a fractional distillation just all in the same beaker. And then when we're done at the top of this container, we can extract a distillate that will be ideally pure A. In practice, it won't be 100% pure, but it will be as pure as this fractional distillation column can make it depending on how many levels of distillation occur in this column. And what amount of it have retransmitted back to the original beaker. So that's also called fractional distillation. Some important things to remember about fractional distillation. First of all, this could be done at the lab scale. We do this at the bench top sometimes when we want to distill a solvent. It's also done at industrial scale with building-sized fractional distillation columns. The other thing to remember is that everything I've described so far is a consequence of this temperature composition diagram, which we developed for the case of an ideal solution. So things do get a little bit more complicated when we talk about non-ideal solutions. So we'll have to consider that next.