Main content
Course: Chemistry library > Unit 9
Lesson 4: Separating mixtures and solutionsDistillation curves
The progress of a distillation can be represented using a distillation curve, which plots temperature versus volume of distillate collected. In this video, we'll learn how to interpret the various regions of a distillation curve for the fractional distillation of a 1:1 mixture of methyl and propyl acetate. Created by Sal Khan.
Want to join the conversation?
I didn't really comprehend the notion of the condensation ring. I understood that at the bottom, there would be vapors, and at the top, there would be condensation, but I didn't understand the point at2:13where he says that there is not much volume until the distillation ring gets to the bulb of the thermometer. I understand that there is not too much volume before the boiling point of methyl acetate, but why is it that the condensation ring needs to be at the bulb? Why does that mean that methyl acetate specifically is the substance being condensed, and not propyl acetate? Furthermore, what happens to the condensation ring during the vaporization of propyl acetate?(6 votes)
So all the condensation ring is is the farthest height the vapors of both chemicals can reach before they condense on the column and fall back down to the flask.
We're trying to evaporate the vapors from the first round-bottom flask and collect them in the flask at the ice bath. For that to happen the vapors must be able to reach the tube leading to the condensation tube. So that means the condensation ring must be at least reach the height of the opening of the tube junction because only then can the vapors below that ring enter the condensation tube.
At lower temperatures most of the vapors constituting the condensation ring are methyl acetate since it has a lower boiling point and is easier to evaporate than propyl acetate. Since most of the vapors at lower temperatures are methyl acetate, you'll mostly separate out the methyl acetate first in the ice bath flask. You'll still collect a small portion of propyl acetate at this low temperature since some of the vapors are propyl acetate too. Once all the methyl acetate has been separated at the lower temperature, the condensation ring will disappear since the vapors constituting it have condensed in the ice bath. We would then increase the temperature to create a new condensation ring formed primarily of propyl acetate and be able to condense it the same way.
Hope that helps.(9 votes)
Why is "Temperature" on the y-axis when it is the independent/controlling variable. And why is "Volume of Distillate" on the x-axis when it is the dependent/responding variable?(5 votes)
Volume of distillate is the controlling variable. "The progress of a distillation can be represented using a distillation curve, which plots temperature versus volume of distillate collected"(1 vote)
- The distillation curves at the end look like fourier transform(1 vote)
2:13Video transcript
- [Instructor] In this video, we're gonna dig a little bit
deeper into distillation, and in particular, we're
gonna learn how to construct and interpret distillation curves. So let's say we're trying to
distill roughly 50 milliliters. That is 50% methyl acetate
and 50% propyl acetate. If you don't know what methyl
acetate or propyl acetate are not a big deal, but what's interesting is the difference in their boiling points. The boiling point of methyl
acetate is 56.9 degrees Celsius. While the boiling point of
propyl acetate is a lot higher, 101.5 degrees Celsius. And so you can imagine, as we have this fractional
distillation setup, we're going to be able to
separate these quite readily because of those differences
in boiling point. So what you do, you have the solution, you begin to heat it up. Now you're going to have to heat it up to the boiling point of the mixture, which is going to be someplace in between these two boiling points. It's probably going to be
70-something degrees Celsius. Now once you get this thing boiling, you're going to start having
a lot more vapors form. Now those vapors are
going to be a combination of methyl acetate and propyl acetate. Because methyl acetate
has a lower boiling point than propyl acetate, a disproportionate number of those vapors are gonna be methyl acetate. And then, because this is
fractional distillation, that mixture of vapors is
going to have multiple cycles of condensing and then revaporizing. And as we get higher and higher, every time you condense
and then revaporize, those revaporized vapors are going to have even a higher proportion
of methyl acetate. And what you see as you gradually increase
the temperature here, you're going to have
this condensation ring, and you can view that as the boundary between where it's hot enough
for enough vapors to form and where it's not hot enough for enough vapors to
form and they condense. And so when this temperature
is having fairly low readings, maybe right at the beginning, you're not going to be
generating a lot of distillate. You're not really gonna be
generating a lot of distillate until this condensation
ring gets pretty close to where the bulb of this thermometer is, and those vapors can start entering into this condensation tube. And now you can imagine by the time that the ring gets up here,
that means that the vapors, the mixture of vapors, have had multiple cycles of
condensation and vaporization, which is equivalent to multiple
simple distillation cycles. So by the time the
vapor ring gets up here, we're going to have mainly
methyl acetate vapors. And the temperature here, the boiling point of the vapors up here, are gonna be roughly the
boiling point of methyl acetate. But that's the point at which you start seeing
a lot of those vapors being able to enter into
the condensation ring and get condensed, and we're starting to
produce a lot of distillate. So right at around 56.9
degrees Celsius, roughly, we now start producing
a lot of distillate. Now you can imagine the
whole time we're doing this, you have to slowly increase
the temperature of the mixture. Why is that? Well, as we start losing more and more methyl acetate vapors, then the boiling point of the
solution is going to increase. As we said, this original
solution's about 50 milliliters. So once you get about
20-something milliliters of your methyl acetate distilled, you are not going to be able to produce a lot more distillate at that temperature. You're gonna have to keep
increasing that temperature until the bulb here
gets to the temperature of the boiling point of propyl acetate. So the temperature of the
bulb keeps increasing. You're not seeing a lot of new distillate, maybe a little bit more form, and you are getting more and
more of the propyl acetate. So this especially is going
to be a mixture of vapors. You're kind of getting the
last of the methyl acetate, but you're getting an increase
amount of propyl acetate. But once you're hot enough at the boiling point of propyl acetate, so about 101 degrees Celsius, then you start producing a
lot more of your distillate. And this is essentially
the condensing vapors of propyl acetate. And so if you wanted
to separate these two, what you would do is, in this first phase, when you see this plateau and you're seeing a lot
of this volume happen at the boiling point of methyl acetate, you could have one little
tube here or one little jar, and then you can swap them in this phase. Before you raise the temperature too much, you can then collect the condensation of the propyl acetate. Now you can also go the other way around. People will oftentimes present
to you a distillation curve that looks something like that. And they might say, okay, what were the boiling points
of the constituents there? And you'd say, okay, well the
lower boiling point substance, I can see that right over here. We were able to get a lot of distillate when the bulb right over
here was at that temperature, so that must be the boiling
point of substance one. And you might even be able to identify it if you know the boiling point
of different substances. And then you would identify this plateau as the boiling point of
substance number two. So some of you all might be thinking, this is all good and well
for fractional distillation, but what would simple distillation
curve actually look like? Well, remember, in simple distillation, you don't have multiple cycles of condensation and revaporization, which is equivalent to multiple cycles of simple distillation. So when you're hot enough for
the vapors to get up here, it's gonna be less pure methyl acetate. It's going to be a combination of the two. It still will be
disproportionally methyl acetate, but the boiling point of
that combination of mixtures is going to be higher. So what happens is that you start getting
significant distillate at a higher temperature and the whole notion is just
a little bit more gradual when you're doing simple
distillation versus fractional. And you could imagine if
someone were to present a simple distillation curve like this, it's much harder to pick
out the boiling points of the various substances,
because at any given point, you have more of a mixture of vapors than you do when you're doing fractional.