 All right, so let's look at azotropes a little more closely. Azotropes being for systems that exhibit strong deviations from ideality, either strongly negative deviations from Raoult's law, which we can see on a pressure composition diagram, or strongly positive deviations, also on a pressure composition diagram. These phase coexistence regions undergo a minimum or a maximum. Then the points at which the minimum and maximum occur correspond to the azeotropic compositions of these mixtures. So an azeotrope is a mixture with a concentration such that the phase diagram looks like this above that concentration. It's more interesting, more useful usually, to talk about these phase diagrams, not in terms of pressure and composition, but in terms of temperature and composition. So if we redraw these phase diagrams now as a temperature composition phase diagram, so plotting not the vapor pressure of the solution, but the boiling point of the solution, in this case, continuing with my pattern of drawing the more volatile component as component A, that more volatile component will have a lower boiling point than the other component. So the Raoult's law behavior, the ideal solution behavior, would have relatively smooth curves connecting these two positions. So system with strong negative deviations from Raoult's law. Remember the negative deviations describe deviations in the pressure. When the pressure deviates in the negative direction, decreasing the vapor pressure, we have to heat that system up higher in order to boil it in order to get the pressure up to atmospheric pressure. So it's going to exhibit positive deviations in the temperature. Pressure and temperature act inversely in this sense. So at that same azeotropic composition, instead of having a minimum in the vapor pressure of the solution, we're going to have a maximum in the boiling point. So you can describe these systems, these azeotropic forming systems, either as being strong negative deviations from Raoult's law, in which case you're describing the pressure behavior, or you can describe the system as having a max boiling azeotrope, meaning the azeotrope in this diagram is the highest boiling point of any composition of the system. This maximum corresponds to the largest possible boiling point we can achieve by mixing these two solvents together. And again, it's a phase diagram. So at low temperatures, we have a liquid. High temperatures, we have a gas. And then we have these tie lines in the phase coexistence region, describing the liquid gas coexistence regions. Either for A rich solutions consisting mostly of A, or for B rich solutions consisting mostly of B. Everything is reversed, of course, over in the case where we have strong positive deviations from Raoult's law. If I draw a temperature composition phase diagram, my more volatile component A will have the lower boiling point. And instead of Raoult's law type behavior, the positive deviations in pressure that result in a maximum in the vapor pressure will be, again, at the same concentration, a minimum in the boiling points. And again, with the bubble point and dew point curves, both meeting at that azeotropic composition. Low temperature liquid, high temperature gas, phase coexistence between the two. So if the strongly negative deviations case results in a max boiling azeotrope, if we have positive deviations strong enough to form an azeotrope, then we describe that as a minimum boiling azeotrope. So again, the azeotrope describes the particular solution with this special concentration. That azeotrope will either have the largest possible boiling point or the smallest possible boiling point. So now let's consider what actually happens when we boil a system or when we distill a system that does have an azeotrope. If we start with this case, let's say I prepare a solution not at the azeotropic concentration, but at some particular concentration. I make a 50-50 mixture of A and B or whatever concentration I want, prepare it in the liquid phase, and now I heat that system up until it boils. So I'm going to raise the temperature until I get to the bubble point. When I get to the bubble point, the first bubble in the solution forms. The concentration of that bubble is enriched in the more volatile component, component A. That's good. If I want to continue distilling, if I condense, boil, condense, re-boil, and so on, I can eventually purify component A by this process of fractional distillation as we've talked about before. So far, that seems just like what we've talked about previously, except notice what happens if the concentration of the solution is very B-rich if I'm over on this portion of the temperature composition phase diagram. If I take this system, heat it until it boils, the composition of the vapor reading across the timeline has become enriched in component B. It's more dilute in component A. It's enriched in component B. So the enrichment happens in the opposite direction. I can, again, perform fractional distillation to purify the system. But now what I'm getting out of my fractional distillation column is purified solvent B. So in this aziotrope, max boiling aziotrope system, it turns out I can purify either solvent A or solvent B, depending on which side of this aziotrope I start on. So that may be good news. In a non-aziotrope performing system, you always get the more volatile component out. In an aziotropic system, you can get either component out as a result of distillation. Turns out this actually doesn't violate our rule of thumb that when you distill, what you get out is a purified version of the more volatile component. If we think of the aziotrope as a separate component, pure solvent A has this boiling point. Pure solvent B has this boiling point. The aziotrope has this boiling point. So instead of thinking of it as a mixture of A and B, if I think of this B-rich solution as a mixture of B with the aziotrope, I'm somewhere between the pure B and the aziotropic mixture. Out of those two choices, the aziotrope and B, B is the one with the lower boiling point, and therefore the more volatile of the two components. So when I distill this mixture, it enriches in the two options that have the one of the two options that has the lower boiling point. The more volatile component is B. On this side, I'm enriching in the one that has the more volatile component out of either A or the aziotrope. As you might expect, things are similar but flipped around when I consider a minimum boiling aziotrope. So let's see if we can understand what happens if I distill this system. Let's say I start with a B-rich liquid at this composition in the liquid phase and I heat it up until I get to the bubble point. The vapor that boils off that solution has this concentration. If I re-destill it, sorry, recondense it, boil it again, re-destill, condense boil again, I can zig my, zag my way down. Notice in this case where I end up, where these two curves meet, is at the aziotropic concentration. So I can't distill my way all the way to pure A like I could previously. What I'm distilling towards is the aziotropic concentration. Likewise, if I start with an A-rich solution, heat it up until the bubble point, collect the vapor, condense, boil again, and so on, I'm zigzagging my way down toward the aziotropic concentration. So that's actually bad news if what you're interested in doing is purifying a solvent. If you have a mixture of two solvents, if you have an impure solvent mixed with another component that forms a minimum boiling aziotrope, when you distill that solution, you're always making it move towards the aziotropic concentration away from either pure B or pure A. So, and again, you can think of that as always moving towards the more volatile component. In this case, the aziotrope, the minimum boiling aziotrope, the lowest boiling point, any mixture of these two solvents can have is the aziotropic concentration. So if I have a mixture of pure B and the aziotrope somewhere in this range, then when I distill, I move toward the more volatile aziotrope. If I have something that I can think of as a mixture of pure A and the aziotrope, somewhere between those two concentrations, when I distill, I move toward the aziotrope. So, aziotrope is the endpoint of all distillations if you have a minimum boiling aziotrope. And again, that's bad news for a couple reasons. Bad news, because usually we want to purify solvents rather than complicate them. And also because, in fact, most pairs of solvents form, exhibit positive deviations from Ralph's law and have minimum boiling aziotropes rather than the more rare case of forming a maximum boiling aziotrope. So, another important thing to say about aziotropes, in addition to what we've just said, that if we have a minimum boiling aziotrope, all distillations will move towards that aziotrope in composition. Another important observation is that the aziotropes itself, unlike other concentrations of these solutions, has a sharp boiling point. In each of these cases that I've drawn so far, when we heat the system up to the bubble point, if we had continued heating, there's a range of temperatures between the bubble point and the dew point where the system can exhibit coexistence between the liquid and the gas phases. So, because this phase coexistence region has some width to it, it's a finite range of boiling points, a range of different boiling points that each of these concentrations can exhibit. For the pure solvents, A and B, as well as for the aziotrope, the curves meet at a single point, which means just like pure A boils only at this specific temperature, pure B boils only at this specific temperature, an aziotrope will also boil at a sharp boiling point, at a single boiling point. So, another important thing to remember is that aziotropic mixtures of solvents will boil at a single temperature, a sharp boiling point, rather than a range of temperatures.