Video Transcript
In this video, we will learn about
distillation, an experimental technique that’s used to separate and purify
liquids. We’ll learn how this technique
works, how to set up distillation experiments, and some common ways distillation is
used in the chemistry lab and in industry.
Distillation is a technique that’s
used to separate liquids by their boiling point. Say, for example, that we had a
mixture of water and ethanol. As we know, water’s boiling point
is 100 degrees Celsius and ethanol’s is a bit lower at 78 degrees Celsius. If we heat this mixture, eventually
it will come to a boil. Recall that boiling is the process
of a substance turning from a liquid to a gas. If we could look at this process at
the atomic level, what we would see is that as we heat the liquid, some of the
molecules of the liquid gain enough energy to escape the liquid and turn into a
gas.
Since the boiling point of a
substance is a reflection of how much energy molecules of that substance need to
escape the liquid and become a gas, when we have a mixture of liquids, like we do
with our water and ethanol mixture, when our mixture boils, the molecules that are
more likely to escape the liquid first and become a gas will be the ones with the
lower boiling point, in this case, ethanol. This is how distillation is able to
separate liquids by their boiling point. The component of our mixture with
the lowest boiling point will become a gas first. And therefore, we’ll be able to
separate it from the rest of the components in our mixture. So this is the theory behind
distillation experiments. Now let’s see how we might go about
setting a distillation experiment up.
In this video, we’ll look at two
experimental setups for distillation. The first is simple distillation,
and the second will be fractional distillation. Simple distillation works best when
the components of our mixture have large differences in their boiling points on the
order of about 50 degrees Celsius or greater. It’s also good for purifying a
liquid that has a solid dissolved in it. This is, for example, how we create
distilled water. We’re able to obtain pure H2O
without any of the salts and minerals that normal tap water has in it.
The experimental setup for
distillation has quite a few pieces of glassware, so let’s go through them all piece
by piece. The first piece of glassware is a
distillation flask. This will contain the sample that
we want to distill as well as some boiling chips so that the sample will boil calmly
and there won’t be any violent eruption of bubbles. The next piece of glassware is a
three-way adaptor. On one end of the adaptor we’ll
have a thermometer and a stopper to hold the thermometer in place. And on the other end of the adaptor
we’ll attach a piece of glassware called a condenser that looks like a long glass
tube with a smaller glass tube inside of it.
On the other end of the condenser,
we’ll attach another adapter. And then we have our final piece of
glassware, another flask that we’ll use to collect our sample once it’s finished
distilling. Once all that’s set up, we’ll
attach two rubber hoses to the condenser, one where water is flowing into the
condenser and the other where water is flowing out of the condenser. This will circulate cool water
inside the condenser. So when the vapor we’re distilling
reaches the condenser, it will condense back into a liquid so that we can collect
it. Finally, we’ll need a heat
source. For distillation experiments, this
is typically a sand bath, an oil bath, or a heat gun. It’s not very common to use Bunsen
burners in distillation experiments because we’re often distilling organic liquids
that are quite flammable.
When we’re ready to begin
distilling, we can turn on the heat source. After a while, our sample will come
to a boil. We might see some vapor condensing
around the sides of the distillation flask, but we won’t see an increase in
temperature yet. This is because the thermometer is
not measuring the solution temperature during this experiment. It’s going to be measuring the
temperature of the vapor that reaches the thermometer, which is why we’ve positioned
the thermometer bulb near the condenser.
As the solution continues to boil,
we’ll see condensation creep up the sides of the adapter, which will cause the
temperature to increase. Then we’ll start to see liquid
condensing inside the condenser, which will eventually fall into the collecting
flask. We want to make sure that we record
the temperature range where we see drops falling into the collection flask since
this will be the boiling point of the liquid that we’re collecting. We’ll also want to monitor the rate
that we’re collecting our liquid. If we’re collecting too fast, that
means that we’re not going to get good separation. A speed of about one drop per
second is appropriate.
We can stop distilling when we
observe a change in temperature. A temperature decrease would
indicate that we’ve finished distilling the component that we were collecting and
vapor is no longer reaching the thermometer. A temperature increase would
indicate that we’ve started collecting the next component of the mixture that has a
higher boiling point. We’ll also want to stop distilling
if the level of our liquid gets too low since boiling dry can cause dangerous side
reactions to occur.
The next setup we’ll look at is
fractional distillation. Fractional distillation is capable
of separating components in a mixture that have boiling points that are closer
together. For example, fractional
distillation could be used to separate the mixture of ethanol and water that we
discussed at the beginning of the video. The setup for fractional
distillation is identical to the setup that we saw for simple distillation. The only difference is the presence
of an additional piece of glassware called a fractionating column. This column can be a glass tube
with some bumps in it or it can be packed with glass beads or steel wool. But what the column’s packed with
isn’t so important. What is important is that packing
the column increases the surface area.
So when our solution boils, the
increased surface area inside the fractionating column has the effect of causing the
vapor to recondense inside the column. The fact that the vapor recondenses
is why fractional distillation has better separation than simple distillation
does. After our vapor recondenses, it
will be easier for the components of our mixture that have lower boiling points to
turn back into a vapor and then be collected in the collection flask. Because fractional distillation is
better at separating components of a mixture, it’s often used to separate out more
than one component. Each component that’s separated is
called a fraction.
We can tell when we’re collecting a
new fraction by closely monitoring the thermometer during the experiment. When we finish distilling a
fraction or component, we should see the temperature decrease when the vapor of that
component stops reaching the thermometer. And then the temperature will
increase when we start collecting a new fraction or component that has a higher
boiling point. But we won’t always see a
temperature decrease before the temperature increases, so we should be very
observant of the temperature during the course of the experiment.
Fractional distillation is used all
the time in chemistry labs before and after experiments are performed. It can be used to create pure
reagents. And it can be used to obtain a
desired product, separating it out from the solvent that the reaction was performed
in. But fractional distillation is also
used all the time in industry. Let’s take a look at a few
examples.
An extremely common use of
fractional distillation in industry is in the separation of crude oil. Crude oil is composed of a mixture
of different hydrocarbons, which are compounds that contain both hydrogen and
carbon. We can separate this mixture of
hydrocarbons into the different kinds of hydrocarbons that make it up, obtaining
things like gasoline, diesel, kerosene, lubricants, and many other chemicals.
Fractional distillation can also be
used to separate air into pure oxygen and nitrogen gases. To accomplish this, air is first
compressed into a liquid. And then all the other gases and
impurities are filtered out. Then what you end up with is a
mixture of liquid oxygen and liquid nitrogen. This mixture is then pumped into a
tank. Since oxygen boils at negative 183
degrees Celsius and nitrogen has a lower boiling point at negative 196 degrees
Celsius, when the mixture of oxygen and nitrogen is pumped into the tank, nitrogen
will become a gas first. The pure nitrogen gas can then be
removed from the tank. And the remaining pure liquid
oxygen can then be removed and heated up until it’s a gas again.
The final example of fractional
distillation in industry that we’ll look at is the production of ethanol. This process is used to create pure
ethanol for use in chemistry labs. It’s used to create biofuels. And it’s used in the hard alcohol
industry to produce alcohols like vodka and whiskey. Ethanol is produced by a
single-celled organism called yeast. Yeast likes to eat sugar. And when it does, it produces
carbon dioxide and ethanol. If yeast continues to eat sugar
producing carbon dioxide and ethanol, the concentration of ethanol will increase,
which kills the yeast, which will leave you with a solution that’s about 14 to 18
percent ethanol.
So if we want a solution that has a
higher concentration of ethanol than that, we need to perform distillation on the
solution. This can be easily accomplished by
fractional distillation since we’d be separating ethanol from water, which is what
the yeast and sugar would’ve been dissolved in.
So that’s all we need to know about
the theory behind distillation, how to set up a distillation experiment, and the
uses of fractional distillation in industry. So let’s take a look at some
problems before we finish this video.
The given table shows the boiling
points of four liquids that are mixed together. In what order would each liquid be
collected if fractional distillation was used to separate them?
Distillation is a technique that
can be used to separate liquids by their boiling point. Fractional distillation, in
particular, is great at separating liquids whose boiling points are somewhat close
together, like we see in this problem. The boiling point of a substance is
a reflection of how easy it is for molecules of that substance to leave the liquid
and become a gas. That means that the liquids in our
mixture will be collected in order of increasing boiling point since the liquids
that have a lower boiling point are more likely to become a gas first.
So that means to answer this part
of the problem, we simply need to put our liquids in order of increasing boiling
point. The lowest boiling point of our
list is 65 degrees Celsius, which corresponds to methanol. The next lowest is 77 degrees
Celsius, which corresponds to ethyl ethanoate. Next is 98 degrees Celsius, which
is the boiling point of 2-butanol. And then our highest boiling point
is 118 degrees Celsius, the boiling point of ethanoic acid. So if we distill this mixture of
liquids, the order that we collect them in would be methanol, ethyl ethanoate,
2-butanol, and finally ethanoic acid.
Ethanol has a boiling point of 78
degrees Celsius. Why would fractional distillation
not separate ethanol from the mixture?
Though fractional distillation is
quite good at separating a mixture of liquids whose boiling points are close
together, it’s not perfect. The boiling point of ethanol, 78
degrees Celsius, is quite close to the boiling point of another liquid in our
mixture, ethyl ethanoate, which has a boiling point of 77 degrees Celsius. Since these two liquids have a
boiling point that’s so close together, they would boil at about the same time,
which means that when we perform distillation with a mixture of these two liquids,
we would be collecting them at about the same time as well.
Fractional distillation is simply
not capable of separating liquids whose boiling points are only one degree Celsius
apart. So the reason that fractional
distillation could not separate ethanol from the mixture is because ethanol has a
very similar boiling point to ethyl ethanoate.
When performing a distillation, why
is the thermometer placed at the top of the column instead of in the solution?
Here we have two similar
distillation setups. There’s a distillation flask that
contains the sample, a fractionating column, an adapter, a condenser, another
adapter, a second flask to collect the liquid that’s been distilled, and finally a
heat source. The only difference between these
two setups is that on the one on the left, the thermometer is placed at the top of
the column and on the one on the right, the thermometer is placed in the
solution. To figure out why it would be
preferable to place the thermometer at the top of the column instead of in the
solution, let’s think about what the thermometer would be measuring for each
case.
If the thermometer is placed in the
solution, it’s going to be measuring the temperature of the solution. Since the temperature of a solution
doesn’t change while it’s boiling, this will ultimately be constant during the
experiment once the solution comes to a boil. If we have the thermometer placed
at the top of the column, there’s no way that it could be measuring the temperature
of the solution. It’s simply too far away. Instead, it will be able to measure
the temperature of the vapor after the liquid begins boiling, becomes a gas, and
makes its way up through the column.
So what do we want to measure
during a distillation experiment? The temperature of the vapor or the
temperature of the solution? As we’ve already said, the solution
temperature will be constant during the experiment. But if we have the thermometer
located at the top of the column, the temperature reading will change throughout the
course of the experiment. The temperature will increase once
vapor reaches the top of the column and we begin collecting liquid. If we then see a decrease in
temperature, that means the component that we were collecting has finished
distilling and its vapor is no longer reaching the thermometer.
And if we see an increase in
temperature, that indicates that a new component of the mixture that has a higher
boiling point has started distilling. So by measuring the temperature of
the vapor instead of the solution, we’re able to get much more information
throughout the course of the experiment. So we place the thermometer at the
top of the column instead of in the solution when we’re performing a distillation
experiment so that the temperature of the vapor is measured.
Now, let’s conclude this video with
the key points for this topic. Distillation is a technique that
separates liquids by their boiling point. When we perform a distillation
experiment to separate liquids, the components of the mixture are collected in order
of increasing boiling point. So the liquids in our mixtures that
have the lowest boiling point will come out first.
Simple distillation can be used to
separate liquids that have large differences in their boiling points. It can also be used to purify a
liquid that has a solid dissolved in it. Fractional distillation can be used
to separate liquids that have smaller differences in their boiling points. Fractional distillation is also
used in industry to separate crude oil, to separate gases such as separating air
into pure oxygen and pure nitrogen gas, and to produce ethanol.
