Lesson Explainer: Distillation Chemistry

In this explainer, we will learn how to describe and troubleshoot distillation methods and describe their use in liquid separation and purification.

We will describe a couple of distillation methods: simple distillation and fractional distillation.

Simply, to “distill” means to “let drip.”

In the past, liquids could be separated from mixtures by heating, and the glassware was slightly different. A mixture was added to a retort, which was heated, and drops of liquid (for example, plant oils) appeared at the end of the long spout and were collected.

Modern equipment is a little different but relies on the same principles.

The principles are as follows:

  1. Substances have bonding forces between their components (metallic bonds, ionic bonds, intermolecular forces, etc.).
  2. The stronger these forces are, the harder it is to turn the substance into a gas.
  3. The boiling point of a substance is a reflection of how strong these forces are—the higher the boiling point of a substance, the stronger the bonds between its components.
  4. We can separate a mixture of substances by heating them. The substance with the lowest boiling point will boil away first.

If two substances are combined, they will still individually boil away at around their pure boiling points (there are some exceptions, but we will not deal with those here).

There are many types of distillation, but they all share these principles.

Definition: Distillation

It is a method for separating mixtures, using the differing volatilities of the components. Mixtures are heated until the more volatile components turn into gases, which can be condensed and collected elsewhere.

Example 1: Identifying Which Technique Is Appropriate for Separating One Liquid from a Two-Liquid Mixture given Their Boiling Points from a Set of Techniques

Which technique could be used to obtain pure liquid acetaldehyde (boiling point 20C) from a mixture of acetaldehyde and ethanol (boiling point 78C)?

  1. Distillation
  2. Evaporation
  3. Centrifugation
  4. Filtration
  5. Crystallization

Answer

Distillation is a technique for separating substances that have different boiling points. During distillation, substances are turned into gases by heating them up. If substances have different boiling points, the vapors coming off of the mixture will contain more of the substance with the lowest boiling point. These vapors can be further purified and then condensed and collected, or they can be purified further before being condensed, depending on the technique.

Meanwhile, evaporation is a part of distillation, but evaporation alone would produce gaseous acetaldehyde and not liquid acetaldehyde. Centrifugation involves spinning samples at high speeds and is effective for separating materials with different densities—acetaldehyde and ethanol will have very similar densities.

Filtration is generally used to separate solids from solid–liquid mixtures. A mixture of acetaldehyde and ethanol would pass straight through ordinary lab filters like filter paper.

Lastly, crystallization is useful for separating a dissolved substance from a solution. The substance must be much less soluble at a lower temperature for this to work. Acetaldehyde and ethanol are very similar chemicals, in terms of the strength of their intermolecular forces, and they will mix well at all temperatures that are readily obtained in the lab.

The answer is option A, distillation.

Let’s say we have a mixture of substances: substance A boils at 50C, and substance B boils at 100C. On their own, we can easily predict what is going to happen when we heat them up.

Below are the heating curves for pure substance A and for pure substance B.

But, if we have a mixture of the two, there will be two boiling points: the boiling point of substance A, and then the boiling point of substance B.

Below is the heating curve for a 11 mixture of substance A and substance B.

In the middle is a crucial point: when A is a gas and B is a liquid, when they are in different states, we can easily separate one from the other. A gas will escape, leaving the liquid behind.

If we want A, we can cool A(g) elsewhere and collect it. If we want B, we can just let A escape. We can keep both pure liquids, too, if we want.

In practice, a little of B will evaporate while A is boiling, so further purification steps may be needed.

So, how do we separate two liquids in a lab?

If we have two liquids with clearly different boiling points, we can separate them using a simple distillation setup.

Simple distillation can also be used to separate the solvent from a solution containing a dissolved solid (for instance, water can be separated from sea water by distillation).

Simple distillation in the lab is generally appropriate for separating out a molecular substance with a boiling point between 40C and 200C, as long as it has a lower boiling point than any other substance in a mixture. For good separation, a difference in boiling points of at least 50C is usually necessary.

Definition: Simple Distillation

It is a type of distillation where the most volatile component of a mixture is separated from the rest by heating the mixture and directly condensing and collecting the vapor. Simple distillations often involve mixtures of two liquids.

How To: Performing a Simple Distillation to Separate Two Liquids

This is what a simple distillation setup might look like.

The tip of the thermometer should be just in front of the opening to the condenser. This means that the temperature reading will be an accurate reflection of the temperature of the vapor just before it goes into the condenser (and not of the distilling flask, which may be a little hotter).

Once the apparatus is assembled and the mixture is in the distilling flask, we can perform the separation:

  1. Turn on the heat.
    Heating slowly makes for a better separation.
  2. Watch the thermometer and the mixture.
    The thermometer reading will rise initially and then stay steady while a liquid is boiling.
    Check if the temperature is what you expected. It should be very close to the pure boiling point of the liquid with the lowest boiling point (we sometimes call this the first liquid).
    At the boiling point of the first liquid, the liquid in the flask will probably bubble. Keep the heat low enough so that the liquid does not splash up into the rest of the equipment.
    The vapors will move upward from the distilling flask, pass into the condenser, and turn back into a liquid.
  3. Collect the condensed liquid in the receiving flask.
    The separation will be better if it is slower: it will give us drops of liquid rather than a stream. If it is too fast, reduce the heat.
  4. Watch the thermometer and the mixture.
    The mixture will reduce in volume as the first liquid boils away.
    When most of the first liquid is gone, the thermometer reading will start to rise.
    When the thermometer reading rises above the boiling point of the first liquid, most of the first liquid will have moved to the receiving flask—it is time to stop the distillation!
  5. Remove the receiving flask from below the take-off adaptor, and turn off the heat.
    You should put the waste flask under the take-off adaptor to catch any extra drops.

What is in the receiving flask should be the first liquid, almost pure.

What is left in the distilling flask will mostly be the second liquid—if we wanted to purify this further, we could heat it for a little longer.

Generally, we will not have a clean mixture of two liquids for a simple distillation. There may be other contaminants that will stay in the distilling flask that will have to be removed using other techniques.

In the wider world, mixtures are often not as simple as two pure liquids mixed together. In practice, the distillate from a simple distillation will still be a mixture of the volatile components of the mixture. Also, if the difference in the boiling points of the components is less than 50C, simple distillation will not produce good enough separation.

Example 2: Understanding the Reason for the Location of the Thermometer in a Distillation

When performing a distillation, why is the thermometer placed at the top of the column instead of in the solution?

  1. So as to provide an additional surface for the vapor to condense on
  2. So the thermometer does not contaminate the solution
  3. So the thermometer does not break
  4. So the temperature of the vapor is measured
  5. So it does not block the vapor from traveling up the column

Answer

When performing a distillation, we heat a mixture until it gives off vapors.

A boiling liquid will produce vapors that have a temperature around the boiling point.

Therefore, we can use the temperature of the vapor to work out what substances are in the vapor.

If we measure the temperature of the mixture instead, the temperature may not be uniform and it may be higher than the temperature of the vapor, so we will not be able to interpret the temperature properly.

The thermometer is placed at the top of the column, next to the exit to the condenser, to give the most accurate measurement of the vapor that will be condensed and collected.

The thermometer should sit inside the column without blocking it, and the tip should float freely so that it is not cooled by the side of the column or other bits of equipment.

There are reasons for having a thermometer inside the mixture (for instance, so we can monitor the rate of heating), but this is not usually necessary.

The answer is option D, so the temperature of the vapor is measured.

Fractional distillation is appropriate for separating volatile components of a mixture with boiling points that differ by less than 25C.

Definition: Fractional Distillation

It is a type of distillation where the components of a mixture are separated in sequence by heating the mixture, passing the vapors through a fractionating column that improves their purity, and condensing and collecting the vapors. Fractional distillations are usually used on mixtures that yield more than one fraction.

Just like in simple distillation, the temperature of the vapor gives an indication of its composition. The liquid collected over a selected temperature range is called a fraction.

Definition: Fraction

It is the sample of pure substance or mixture that can be, or has been, collected over a temperature range from a fractional distillation. The components of a fraction will have similar boiling points.

While the intention of fractional distillation is to produce fractions that are pure substances, it is not always possible. If there are two volatile components in a mixture that have boiling points within 5C of each other, a lab fractional distillation setup may not be good enough to separate them effectively.

The key component of a fractional distillation setup is the fractionating column. There are many types, but they work in the same way:

  1. Vapors from the boiling mixture enter the column. These vapors will be a mixture of the volatile substances from the mixture but will contain a higher proportion of the more volatile substances.
  2. Vapors condense inside the column, forming droplets of liquid.
  3. When more vapors enter the column, they heat the droplets of liquid, making them evaporate again.
  4. The vapors from the droplets have an even higher proportion of the more volatile components than the vapors from the mixture.
  5. The cycle of condensation and re-evaporation continues up the column.

As we move up the column, the repeated cycles of condensation and evaporation mean that the vapor is purer and purer. With the right equipment (for instance, with a long enough column), we could be confident that the vapors exiting the column will consist of only the most volatile component of the mixture—the vapors will be completely pure.

In the lab, the easiest way to make a column is to add glass beads to a cylinder of glass connected to the distilling flask. The glass beads provide extra surface area for the condensation–evaporation cycles to occur on.

The diagram below shows an idealized version of the progressive purification of vapor rising up a fractionating column.

How To: Performing a Fractional Distillation to Separate Three Liquids

A fractional distillation could be used to separate just two liquids, but here we are going to see how to collect multiple fractions using the same setup.

We will call the liquids liquid 1, 2, and 3.

Liquid 1 has the lowest boiling point, while liquid 3 has the highest boiling point.

This is what a fractional distillation setup might look like.

The tip of the thermometer should be just in front of the opening to the condenser. This means that the temperature reading will be an accurate reflection of the temperature of the vapor just before it goes into the condenser (and not of the distilling flask or the fractionating column, which may be a little hotter).

You will need one receiving flask for each fraction that you are going to condense.

Once the apparatus is assembled and the mixture is in the distilling flask, we can perform the separation:

  1. Turn on the heat.
    Heating slowly makes for a better separation.
    Collect Fraction 1
  2. Watch the thermometer and the mixture.
    The thermometer reading will rise initially and then stay steady while a liquid is boiling.
    Check if the temperature is what you expected. It should be very close to the pure boiling point of the liquid with the lowest boiling point (liquid 1).
    At the boiling point of liquid 1, the liquid in the flask will probably bubble. Keep the heat low enough so that the liquid does not splash up into the rest of the equipment.
    The vapors will move upward from the distilling flask, pass into the fractionating column, and undergo condensation–evaporation cycles, getting purer as they go higher.
    The vapors will then pass into the condenser and turn back into a liquid.
  3. Collect the condensed liquid in the first receiving flask.
    The separation will be better if it is slower: it will give us drops of liquid rather than a stream. If it is too fast, reduce the heat.
  4. Watch the thermometer and the mixture.
    The mixture will reduce in volume as the first fraction boils away.
    When most of the first fraction is gone, the thermometer reading will start to rise.
    When the thermometer reading rises above the boiling point of the first fraction, most of the first fraction will have moved to the receiving flask.
  5. Swap the receiving flask with the waste flask.
    You will now have fraction 1 in your first receiving flask.
    Collect Fraction 2
  6. As before, watch the thermometer and the mixture.
  7. When the temperature of the vapor stabilizes (hopefully close to the boiling point of liquid 2), swap the waste flask with the second receiving flask.
  8. Collect the condensed liquid.
  9. As before, watch the thermometer and the mixture.
  10. When the thermometer reading rises above the boiling point of the second fraction, swap the receiving flask with the waste flask.
    You will now have fraction 2 in your second receiving flask.
  11. Turn off the heat.

Because of the fractionating column, the following should ensue:

  • The liquid in the first receiving flask should be pure liquid 1.
  • The liquid in the second receiving flask should be pure liquid 2.
  • If we keep the distillation running for a little while, the liquid in the distilling flask should be pure liquid 3.

The waste flask will contain a mixture of all three, but hopefully not a lot.

Fractional distillation is used in industry to separate complex mixtures, like air and crude oil. In some applications, it is important that the fractions are pure substances (like nitrogen), and in others, it is ok that they are mixtures of similar substances with similar boiling points (like gasoline). Fractional distillation is also used to separate alcohol from fermentation mixtures.

Example 3: Identifying a Piece of Glassware Used in Fractional Distillation

The image below shows a piece of labware often used in fractional distillation. What name is given to this piece of labware?

  1. Funnel
  2. Adapter
  3. Gas syringe
  4. Condenser
  5. Connector

Answer

In the lab, you would see this piece of glassware near horizontal rather than vertical. On one side, it would be connected to a still head (containing the thermometer), which would be sat on top of the fractionating column. On the other side, it would be connected to a take-off head, which would direct condensed liquid into the receiving flask.

The outer wall is known as a “jacket,” and water would flow in through one opening and out through the other. The vapor from the distillation would flow into the condenser, be cooled by the jacket, and turn back into a liquid.

This condenser would probably not be appropriate for the fractionating column because it does not have a high surface area, so it would not produce the high levels of separation required.

By the way, this particular type of condenser is known as a Liebig condenser.

The answer is option D, condenser.

Example 4: Understanding the Role of Glass Beads in Fractional Distillation

During fractional distillation, why are glass beads placed into the column?

  1. To stop the thermometer from falling into the reaction mixture
  2. To keep the temperature even throughout the column
  3. To strengthen the column
  4. To stop any liquid that condenses from falling back into the reaction mixture
  5. To increase the surface area available for condensing the vapor

Answer

The key difference between simple distillation and fractional distillation is the fractionating column. In the lab, the fractionating column would sit above the distilling flask. During the distillation, vapors come off the distillation mixture, and the fractionating column is designed to make the vapors condense.

The liquid that condenses from the vapor is purer than the original mixture (it contains a higher proportion of the more volatile components). Hot vapor from below will make this liquid evaporate again, producing vapors that are even more pure than before. This cycle continues up the column.

Glass beads in the column provide extra surface area for vapors to condense, helping to produce purer vapor more effectively.

The answer is option E, to increase the surface area available for condensing the vapor.

The main challenges with distillation have to do with timing.

Heating too quickly can reduce the purity of the vapors and the distillate.

If the temperature on the thermometer rises while you are still collecting a fraction, the distillate may contain impurities that should have been in the next fraction.

If you find a distillation is not producing a pure enough product, try the following:

  • heating it more slowly,
  • removing the receiving flask as soon as the temperature on the thermometer begins to rise,
  • and using fractional distillation or, if already doing fractional distillation, using a longer fractionating column.

The other main timing issue is distillations going too slowly. You can help the vapors rise up faster by insulating everything up to the condenser, using cotton wool (be careful not to get the wool too close to the heat source—it is flammable!).

Key Points

  • Distillation is a technique for separating components of mixtures with different boiling points. Mixtures are heated until the more volatile components turn into gases, which can be condensed and collected elsewhere.
  • Simple distillation is a type of distillation where the most volatile component of a mixture is separated from the rest by heating the mixture and directly condensing and collecting the vapor. Simple distillations often involve mixtures of two liquids.
  • Simple distillation is not generally effective for separating substances with boiling points within 50C of each other; for these mixtures, fractional distillation is necessary.
  • Fractional distillation is a type of distillation where the components of a mixture are separated in sequence by heating the mixture, passing the vapors through a fractionating column that improves their purity, and condensing and collecting the vapors. Fractional distillations are usually used on mixtures that yield more than one fraction.
  • How a distillation is performed can have a big impact on the purity of the fractions. Purer fractions can be obtained by:
    • heating more slowly,
    • removing the receiving flask as soon as the thermometer reading begins to rise again,
    • and using fraction distillation rather than simple distillation.

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