Video: Distillation

In this video, we will learn how to describe and troubleshoot distillationmethods and describe their use in liquid separation and purification.

16:17

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.

Nagwa uses cookies to ensure you get the best experience on our website. Learn more about our Privacy Policy.