- [Teacher] 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 point.
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.
We know at that temperature,
if we're below the boiling
point of 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 re-vaporizing.
And as we get higher and higher,
every time you condense
and then re-vaporize,
those re-vaporized 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 do 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 you could imagine,
right at where that condensation ring is,
is roughly initially going
to be at the boiling point
of methyl acetate.
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 is 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 you keep increasing the temperature.
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 gonna
be a mixture of vapors.
You're kind of getting the
last of the methyl acetate,
but you're getting an increased
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 re-vaporization,
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
disproportionately 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.
