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Introduction to Distillation

Lecture Slides are screen-captured images of important points in the lecture. Students can download and print out these lecture slide images to do practice problems as well as take notes while watching the lecture.

  • Intro 0:00
  • Distillation: Purify a Liquid 0:04
    • Simple Distillation
    • Fractional Distillation
  • Theory of Distillation 1:04
    • Theory of Distillation
  • Vapor Pressure and Volatility 1:52
    • Vapor Pressure
    • Volatile Liquid
    • Less Volatile Liquid
  • Vapor Pressure vs. Boiling Point 4:03
    • Vapor Pressure vs. Boiling Point
    • Increasing Vapor Pressure
  • The Purpose of Boiling Chips 6:46
    • The Purpose of Boiling Chips
  • Homogeneous Mixtures of Liquids 9:24
    • Dalton's Law
    • Raoult's Law
  • Distilling a Mixture of Two Liquids 11:41
    • Distilling a Mixture of Two Liquids
  • Simple Distillation: Changing Vapor Composition 12:06
    • Vapor & Liquid
    • Simple Distillation: Changing Vapor Composition
    • Azeotrope
  • Fractional Distillation: Constant Vapor Composition 19:42
    • Fractional Distillation: Constant Vapor Composition

Transcription: Introduction to Distillation

Hi, welcome back to

Today, we are going to be talking about the process of distillation.0002

Distillation is a technique used to purify a liquid.0006

Now, depending on what the contaminant is in that liquid, you have a couple of different choices for your distillation.0010

If your contaminant is a nonvolatile contaminant -- something that cannot boil itself.0016

Let us say if you have a solution of water with the food coloring in it.0023

That food coloring dye is some kind of solid organic compound component that is dissolved in it.0029

Then, all you need is a simple distillation to separate those two0034

because the liquid component is going to distill over and0039

the nonvolatile component is going to stay behind in the distillation flask.0042

Simple distillation is also an OK way to purify liquid.0046

If you have two liquids with very large differences in boiling points.0050

For the most part, if you have a mixture of two or more liquids,0056

the only way you can separate them is by using a fractional distillation.0061

What does a distillation apparatus look like?0066

Essentially, we have this set up here, this is some kind of heat source.0069

Sorry, either heating mantle or Bunsen burner, or something like that.0074

We are going to boil this liquid.0077

The vapors are going to travel up here.0079

We are going to record the temperature of those vapors.0082

And then, they are going to go into this water condenser which is cool.0084

That is going to cause vapors to condense into a liquid.0088

That liquid is going to be collected into a new flask, new container.0093

The theory is that we have this clear liquid here, mixture of liquids or liquid plus other nonvolatile components.0098

But the vapor is going to be purer.0105

Once that condenses and collects, what we are going to end up is a pure liquid.0107

Let us talk a little bit about vapor pressure and volatility in boiling points.0115

Because all these are important concepts, to understand how a distillation works.0119

Every liquid that we have is always in equilibrium with its vapor.0124

What that means, at the same rate, we have the liquid evaporating, entering into a gaseous phase.0129

We have gas molecules condensing into the liquid phase, that is in equilibrium.0136

All the vapor molecules that we have above the liquid are exerting some kind of a pressure.0142

Let us imagine we have a volatile liquid.0149

A volatile liquid is something with a low boiling point, something like diethyl ether, it has a quite low boiling point at 35°.0151

If you have a low boiling point that means you have a high vapor pressure.0158

Ether is something that has lots of vapor molecules above the gas phase, because it is quite volatile.0162

Ether has a very strong smell.0175

When you open up a can of that, it has a very strong smell because there is so much of it in the gaseous phase.0178

It travels really far because it is exerting pressure and it is going to be moving out.0183

We compare that to a less volatile liquid.0190

Here we have lots of vapor phase molecules, for a very volatile liquid.0194

But if we look at something like water which has a much higher boiling point of 100°.0202

This is going to have a very low vapor pressure.0207

Just relatively few water molecules above the liquid water.0210

Still in equilibrium, the water vapor is condensing, it is the same rate that the liquid water molecules are evaporating.0216

But that is how we kind of start thinking about the relationship between volatility, 0225

liquid's ability to volatilize, turn it to gaseous phase vs. its boiling point.0230

Low boiling point means high vapor pressure.0237

High boiling point means a low vapor pressure.0241

What does it mean to boil a liquid?0246

Your vapor pressure, your liquid has some amount of vapor pressure that is exerted by the gaseous molecules.0248

We always have some atmospheric pressure acting in the opposite direction.0259

Suppressing down on that liquid, pushing down this atmospheric pressure.0264

If you heat a liquid, let us say we have our low volatile, something like water,0267

which does not have many water molecules, gaseous water molecules above.0274

If we heat the molecule, if we heat the liquid, excuse me, then that increases your vapor pressure.0278

All of a sudden, we need to see a few water molecules here.0286

Now we have more and more, as we boil.0291

As we start to heat the liquid, we see little gaseous bubbles forming down here and being released.0295

We see steam rising, we see all sorts of evidence of increasing presence of vapor molecules.0300

If something has an odor, if it is very cold, you do not smell it very much.0308

But as you heat it up to room temperature, you heat it up even warmer, suddenly you can smell it a lot more.0312

Again, evidence of having that increased number of vapor molecules, that is going to increase the vapor pressure.0317

And at some point, when these two are equal, when your vapor pressure increases and increases and 0325

finally meets the atmospheric pressure, that is when we describe that as the boiling point of the liquid.0332

That is where rapid conversion from the liquid form to the vapor form occurs.0340

We see the boiling water is rapidly bubbling because so much of that gas is being released.0346

What if you were to affect the atmospheric pressure, 0352

you are going to affect the vapor pressure by heating the liquid or cooling a liquid.0354

How could you affect the atmospheric pressure?0358

If you ever go to a high altitude that has a lower vapor pressure, 0360

which means if this number is smaller then this number needs to be smaller, in order to reach boiling.0365

If you boil water in Denver, or at some mile above sea level, it is going to boil at a lower temperature.0372

It is easier for water to boil at high elevations because0380

it does not have to fight as much against the lower atmospheric pressure.0384

It affects things, like if you are cooking, you are trying to boil water, it is going to be at lower temperature.0389

You need to cook things longer.0395

Sometimes if you look at the back of a box for a cake mixture, it might have some high altitude cooking directions on there.0396

You have to modify your cooking, in order to get it to come out right.0404

Anytime we are boiling a liquid, we always want to have a boiling chip present.0409

You may have seen some videos where they put in a beaker,0413

a mug of water into the microwave then put it on for 2 minutes.0417

And then when they take it out, it explodes all over the place, scalding water goes all over the place.0421

It is a huge danger.0427

That can in fact happen, it would be very difficult for that to happen.0429

But what is happening there is, sometimes when you heat a liquid, what you get is a superheated solution.0432

You are heating and heating it and you manage to heat it above the boiling point, 0438

which means it is a very high energy situation.0444

It has the energy needed to go boiling but for some reason, you have not had that nucleation site,0449

that place where the liquid can convert itself to a gas.0457

That is the purpose of a boiling chip.0463

When you put in a boiling chip, it simply is some kind of porous material.0465

It has a lot of a surface area and that is a place0469

where a liquid can very easily convert from one phase, undergo that phase change.0471

It promotes even boiling, prevents bumping.0478

The reason that we typically do not have our water explode on us,0482

when we take something out from heating in the microwave is0487

because most coffee mugs that we have are highly imperfect surfaces.0489

They have lots of mixing grooves and cuts and bits.0495

It is very easy for water to boil in such a surface.0498

You would have to have a really unique situation, in order to create a superheated solution in a microwave.0501

But because that can happen in a lab, we want to make sure that we always have a boiling chip in there0510

and we never have a superheated solution.0515

Another alternative is to stir the solution.0517

If you are constantly stirring it, that promotes even heating and 0520

you would never have these little patches of superheated liquid in there.0523

We can do that too with a stir bar and a magnetic stir plate.0527

Sometimes we start a reaction and forget to add a boiling chip.0534

You never want to start heating your solution or your reaction mixture,0536

and then throw in your boiling chip because now you may have something that is kind of close to boiling.0541

And now when you throw that chip in, it says I really want to start boiling.0547

It can fall and it can bump over.0551

It can spill over your reaction vessel.0552

Make sure you always add that the boiling chip, when your solution is cool so that it is there as you are starting to heat.0555

Let us consider now a mixture of liquids, if we have two miscible liquids which I have shown here as red and blue liquids.0566

Each of those liquids is going to be exerting a pressure.0576

We have two laws that govern this.0583

Dalton’s law states that the total pressure that is over these miscible liquids,0585

the total pressure comes from both the partial pressure of A and B.0593

Notice we are showing that the blue atoms are coming from a more volatile liquid.0600

There is a greater pressure for that.0606

That is the one with the lower boiling point.0608

The red molecules represent something with a higher boiling point, it is not as volatile.0611

There is fewer of those.0614

A is exerting some smaller pressure and B is exerting a larger pressure.0617

But they combine to give us the total pressure of the liquid.0621

Raoult’s law states that the pressure that is exerted by A and B not only have to do with their volatility,0627

their boiling points but also their mole fraction.0635

If you have an equal mixture of A and B, then they have the same number of liquid particles that are contributing.0638

And then, now we have a difference in the vapor particles, based on the volatility, based on the boiling point.0648

As your molar fraction changes, is if this red liquid has a very low vapor pressure.0655

But if this is predominantly A, if that is a larger mole fraction0665

then you are going to have a much larger proportion of the A molecules than we would here than if it was a 50-50.0668

We are concerned with both the differences in boiling point and we are concerned with the differences in the mole fraction.0675

What we want to recognize is that when we have miscible liquids...0682

That is what I am describing as a homogeneous mixture liquid.0686

When we combine these two liquids, they form just one layer.0690

When we have miscible liquids, these we can add those partial pressure together to get a combined partial pressure.0692

I’m sorry, a combined total pressure of the liquid.0699

What if we have a mixture and we want to distill them?0703

Let us assume we have a 1 to 1 mixture, that is what we are starting with, of A and B.0707

A is our lower boiling component, B is our higher boiling component.0712

That means A has the higher vapor pressure, that is more volatile.0718

The blue is the more volatile, in this case.0722

If I have some colors coming up.0725

Let us consider what the boiling point of the mixture is going to be and what the vapor composition is going to look like.0728

On the bottom here, I have the mole fraction.0735

On the far left, what if we have 100% mixture of A?0738

A has a boiling point of 40.0744

We would expect that liquid to boil at 40°.0746

Over on the far right, we have the other extreme where we have 100% only B, no mixture.0751

B has a boiling point of 70°.0760

We will expect that to boil at 70°.0762

What if we had some mixture in between?0765

What if we had some A and some B?0767

As you add in some B to the mixture, it is going to add to the partial pressure.0770

B has a higher boiling point.0775

What happens is the boiling point of the mixture is going to be just a little higher than 40.0777

If we continue to add more B and add more B, we start to slowly increase the homogeneous liquid mixture,0783

is going to have a higher boiling point.0793

A boiling point somewhere in between the lower boiling component and the higher boiling component.0796

It is going be a direct reflection of how much of each that you have.0802

What we are going to end up getting, it is not a direct straight line but it is close,0808

it curves just the slightest bit going from one to the other.0814

What we are going to get is a steadily increasing boiling temperature, as we change the mol fraction of A and B.0819

This is the liquid temperature, as we are boiling these mixtures.0828

But it turns out that the vapor phase is different.0836

The composition of the vapor phase is different.0839

The boiling temperature of the vapor phase is different.0842

It looks like this, it actually goes up a little bit and then comes back down.0844

If you are distilling or boiling pure liquid B, we expect it to boil at 70°.0850

We expect the vapor temperature to also be 70°,0858

exactly 70° because the vapor temperature is the same as liquid temperature.0864

They are both pure B.0868

This is the vapor curve that is showing the temperature of the vapors throughout the distillation.0871

Let us imagine having a mixture of A and B.0881

We start to do our distillation.0884

What are we going to observe?0885

Let us say we have a 50-50 mixture, just as a good starting point.0890

If we have a 50-50 mixture, do we expect the vapor if 50-50 liquid, do we expect the vapor composition also to be 50-50?0898

We do not because A is more volatile, has a lower boiling point.0910

It exerts a greater vapor pressure.0916

We expect there to be more A in the vapor phase, not 50-50.0919

More higher proportion in A than we had in the liquid.0925

What we see is this 50-50 mixture is going to boil at some temperature.0929

Where is that, this is 40 and that is 70, this is maybe 50°, 55°, something like that.0937

At 55°, let us say this is 55, that is the boiling point of this mixture.0943

At 55°, your vapor composition is right here which is not 50-50.0952

It is maybe 75, maybe 80%, and 20% B.0960

We can use this phase diagram to help explain at a given temperature,0968

what is that liquid looking like and what is that vapor looking like.0976

It is coming out the way we would intuitively guess is that the vapor that we are collecting is going to be enriched in A.0979

It is going to be mostly A, because A has the lower boiling point.0989

It is easy for that to volatilize.0994

As we start to do the distillation, the distillation is collecting more A than B.0998

Our mol faction is going to be changing.1003

Our mol fraction of the liquid phase is going to be moving in this direction.1006

If we keep drawing off more and more A, if the vapors are enriched in A than B, 1010

the distillation flask is going to be more and more B.1016

If I selectively pull out all of this, the red molecules, we are going to have more and more blue molecules in the distillation flask,1020

when I use to start with 50-50.1027

What is going to be happening to our temperature?1029

As we get more and more, the greater distillate has more and more B because that is what is left in the flask.1032

The more and more B we get, the higher the temperature gets up closer to the boiling point of pure B.1040

In a simple distillation, the temperature of our vapors steadily increases.1048

It steadily increases because it changes continuously, we start out with a mixture of A and B.1053

The whole time we have a mixture of A and B.1062

But we are gradually adding more and more B to that vapor composition.1064

Therefore, the boiling point is creeping up and creeping up.1068

If we were to graph, for example, if we are to do a simple distillation and1072

we are to graph what is the temperature look like throughout the distillation, it would look something like this, steadily increasing.1075

That tells us if we are observing that in a distillation, it is telling us that we are collecting a mixture of compounds.1082

If the temperature is changing that means the composition is changing as well.1093

If you were to do a distillation of just pure A and your vapors are pure A, what would your temperature look like?1098

40°, 40°, it would be a steady temperature.1104

Our distillation at a steady temperature is an indication that we are likely to be distilling a pure liquid.1107

Where if we have a steadily rising temperature, that is an indication that we are continuously changing mixture of liquid.1114

There is one exception to that.1122

When we have an azeotrope, this is a mixture, it is somewhat interesting.1123

Just random mixtures of various liquids that distills at a steady temperature and a fixed composition.1132

We would not get the same kind of graph for an azeotrope.1148

We would have a point where the vapor and the liquid lines meet1151

because your liquid composition is exactly the same as the vapor composition.1154

A common example of that is ethanol and water, if you were to distill off ethanol.1158

Once it gets to the 95% ethanol, it will always draw a 5% of water with it.1164

It is impossible to distill off ethanol at anything higher than a 95-5 mixture.1171

That is the highest purity you can get by doing a distillation process.1177

Let us take a look at a fractional distillation.1183

In a fractional distillation, what we are going to do is we are going to introduce... I did not have a picture of this.1185

With our round bottom flask, instead of coming off and just immediately collecting the vapors,1194

we are going to add a fractionating column.1199

This is something that has a packing material in it, glass beads or metal turnings, or even paper clips.1206

You can make your own.1214

Some kind of surface in here.1215

Then, we are going to have our steel head up here, or thermometer to record our temperature.1217

This is going to be our condenser that we are going to add water in to cool it.1223

And then, we are going to collect our vapors out here.1230

This is getting heated.1232

Our simple distillation that we just looked at, does not have this fractionating column.1238

We simply heat the vapors and collect them in a receiving flask.1242

By introducing this fractionating column, what happens is, 1247

our liquid boils and then it rises up the column and it hits a surface and cools, and condenses.1250

But then, we continue to heat this and eventually that is going to now re-vaporize and continue up,1257

and then re-condense and vaporize, condense.1264

Slowly but surely, it slowly works its way up the column.1268

What happens every time we do that evaporation, every time we do an evaporation, 1273

the vapor composition is going to be more heavily enriched in the lower boiling component than the liquid composition was.1278

If we started out 50-50 here and then after the first time it evaporated, we went up to 60%.1288

Then the next time we started at 60.1294

The next time it goes up now it is 70% and 80%, and so on.1296

Eventually, your initial distillate is going to make its way to the top of the column.1300

It is going be the lower boiling component in its pure form.1305

It is like we are doing several simple distillations in succession, until finally we get just pure A.1310

What that looks like on the graph here.1319

Again, if we look at a 50-50 mixture or 0.5-0.5 mol fraction of A and B, this is going to distill at some temperature.1321

And then, those vapors are going to have a different composition.1336

We look at that and we recognize the vapors are going to be more closer to A, more enriched in A than B.1343

But now we are going to take that vapor, instead of just taking it off and collecting it in a receiving flask.1349

We are going to condense them back to a liquid.1354

And then, we are going to let that liquid evaporate again.1359

We call this process reflux.1362

Reflux is when you vaporize and then condense, vaporize and condense.1366

We have this process of reflux working its way slowly at the column.1371

This liquid is going to vaporize again.1375

Maybe we started with 2/3 A but when it evaporates, now we are up to 80% A.1379

And then, we let it condense and we still have not collected it.1387

It is still in the column.1391

It is going to evaporate again.1392

Now we go from 70% to 90%, and so on, until we end up being pure A.1394

That process of reflux will end up enriching until we are finally at 100% A.1403

Our vapor composition is going to remain constant.1411

We are going to distill off A, A, A, A.1414

What is our boiling temperature going to look?1417

Our temperature re-vapors, it is going to remain steady because it should match the boiling point of A,1420

assuming our thermometers are calibrated.1427

And then, let us say we had a 50-50 mixture, let us say we had a 10 ml each of A and B.1430

We do our distillation and A distills and distills.1437

Finally, when the 10 ml of A have been collected, 1442

this is going to continue heating up until now the B is already in this column, in the distillation flask.1446

It is already in this column but now we are going to be able to heat it at a higher temperature, 1458

where now the B is going to move its way slowly up the column, at the boiling point of B.1462

And then we can collect B.1468

In an ideal situation, in a fractional distillation, you distill off at a steady temperature of your lower boiling component.1469

That distillation kind of drops off, it either cools down a little bit.1477

Either your thermometer cools down a little bit where you start to see a spike in your temperature.1481

At that point, what you could do is you can switch flasks, switch your receiving flasks.1485

Now when the second liquid comes over, that is going to be your higher component B.1490

With a fractional distillation, you should be able to analyze an azeotrope mixture.1495

You should successfully be able to isolate pure A, your lower boiling component, and pure B, the higher boiling component.1500

I think that wraps it up for the lecture here.1512

They are a lot of tips and tricks to discuss setting up a fractional distillation 1515

or a simple distillation apparatus, is quite a detailed process.1519

There is a lot of parts, there is a lot of clamping going on and making sure everything is fitting together properly.1524

It takes a little experience to get good at that.1530

There is a lot of hints and tips and tricks to make sure you have a successful distillation.1533

I have saved that for the video portion because there are so many things to point out and1539

you really need to see it, see the glassware as we are going along.1542

This wraps up the theory of distillation.1547

I hope it helps you run successful distillations in the future, thank you.1549