Hello, today's lesson is about using distillation techniques to analyse water samples.

This is part of the "Separating Substances" unit.

My name is Mrs. Clegg, and let's begin our lessons.

And here's the outcome for today's lesson.

By the end of it, you should know how to use distillation to purify water samples and how to analyse the distillate to test its purity.

Here are some of our keywords today that I want you to listen out for.

Impurity, pure, sample, distillation, and distillate.

And here are some definitions of those words.

You might like to pause the video and make notes to help you solve through the lesson.

Today's lesson is split into two parts.

So we're going to look at purifying using distillation, first of all, and then we're going to test for purity.

So let's get started with the first part of our lesson.

Water sources are mixtures.

And remember the definition of a mixture is a material containing two or more different substances that can be physically separated.

So we've got three types of water there, groundwater, seawater, and freshwater.

And there might be other things apart from water in them such as sand or grit or salts or gases or dissolved ions.

There might be other things as well.

Potable water, if you remember, is water that's safe to drink.

For example, the tap water.

And that is a mixture.

It doesn't just contain pure water.

Tap water contains things like chloride ions from the sterilisation processes to make it safe to drink, microbes that have been killed by the sterilisation process, and sometimes fluoride irons are added to help prevent tooth decay.

Any substances that are found in water are considered to be impurities.

So our tap water, if you have a look here, if we look at the particle diagram, we've got water molecules, the red and white circles there, and in between you'll see purple and greenish-bluish circles, and those are representing the dissolved ions in the tap water, so we can see it is not pure.

And impurities are a problem in the laboratory because they can contaminate samples.

If water containing impurities is used during a chemical analysis test, for example, you might get unexpected precipitates forming.

So if we add some tap water to our solution that we're testing, you might get a clouded precipitate.

If you use pure water, no precipitate forms. And so you can see how this might lead to an incorrect analysis if unexpected precipitates form.

So it's really important to use the purest possible water when we're doing chemical tests.

The water that we use in the science laboratory must be pure.

And remember, pure substances only contain one type of chemical.

Now we've got the particle diagram again for pure water showing there is only water molecules present.

So any water sources that are going to be used in the laboratory must be purified, i.

e.

made pure, before they're used.

Let's have a quick check.

Tap water can be used for chemical analysis tests.

True or false? Well done if you said false, and what's your justification? Well done if you said B, tap water contains impurities.

Now water samples that contain impurities can be purified using distillation techniques, and distillation is a process that uses both boiling and condensation to achieve that separation between the water molecules and the impurities.

Let's have a look at distillation in a bit more detail.

There are three main steps to use to try and purify a substance.

So the mixture is placed into the conical flask and heated until the water begins to boil.

We have a delivery tube.

And steam would rise from the boiling water and it's collected and moved down the delivery tube to another container.

We've got a test tube in an ice water bath, so it's cold, so the steam is cooled and it condenses back to the liquid states of water.

So you might have seen equipment like this for distillation in your laboratory, but you might also have seen this one.

The same steps occur, but they occur in slightly different parts of the apparatus, so let's have a look at this one.

So the mixture is placed into the round bottom flask here.

It's heated, the water begins to boil.

We can measure the temperature at which the water boils.

And the steam enters a piece of equipment called a condenser where it's cooled and condensed.

You can see that there's a tube in the middle and wrapped around it is another tube.

And in the outer tube, water goes in and water goes out, it's sort of circulating around.

And that cools the steam as it passes through but it doesn't ever come into contact with it.

The steam condenses and passes down and is collected in a beaker there at the bottom and it is now liquid water and we can store it until it's needed.

And this type of evaporators is often used to make the distilled water that's in bottles that we use in the laboratory.

Let's have a quick check.

So which process would produce pure water from potable water containing dissolved solids? Which of those? Well done if you said distillation.

The pure water sample that we collect at the end of distillation is called the distillate.

So if we look at the two setups there, where would the distillate be? Well done if you said here.

So distillation setups that include a condenser are more efficient, and they're more efficient because condensation occurs more quickly, and more steam condenses and less steam is lost to the environment.

In the other piece of equipment, the condensation is happening here.

I think we're ready for Task A now.

So complete the diagram to show how you could distil a water sample and collect pure water.

So the diagram's been started for you there.

Could you finish drawing the equipment and could you label on your equipment the pure water and the water sample? Pause the video and join us when you're ready.

Okay, let's have a look how you've done.

Now remember, any science diagram should be drawn with a pencil and a ruler, and we draw them in 2D.

So we've got the conical flask there.

We've labelled the water sample.

And then we would draw the delivery tube going into a test tube, which is sat in a beaker full of ice.

And there we can label the pure water, the distillate, which is being collected.

We would need heat under the conical flask as well.

So here you're going to use a suitable distillation setup to purify a sample of impure water.

So use the setup that you've got in your school.

Pause the video and come back when you've had a go.

Welcome back, now whatever the setup that you've actually used, you should have seen impurities collecting in the water sample container.

So it might not be quite as easy to see here, but there is a white solid on the walls of the conical flask and around sort of the ring where the water is.

You should have been also been able to collect a pure water sample in your test tube.

Well done, so now you know how to distil a water sample of impure water to get a pure water sample, brilliant.

Let's have a look at question three.

So Alex and Aisha used different distillation setups to purify 50 centimetres cubed of impure water.

They also distilled their samples the same amount of time.

Alex collected a much smaller volume of pure water than Aisha.

Here's the sample that Alex was using, and here's the equipment that Aisha was using.

So explain why Alex collected a smaller volume.

Pause the video, come back when you're ready.

Okay, so why did Alex get less pure water than Aisha? Well, basically, the condensing for his piece of equipment is much slower or less efficient.

He didn't use an ice bath as well, and so less steam was condensing and more of the steam was escaping out top of the test tube.

Hopefully you noticed that.

Let's have a look at the second part of our lesson.

We're going to test for purity now.

So you might have heard the term quality assurance, and quality assurance are a series of analytical tests that we carry out on a sample, and these tests help to determine if the process meets the specific requirements.

In other words, do we create a pure sample at the end of it? And quality assurance tasks can be used in all sorts of areas, but maybe for safety reviews when we're looking at the purity of pharmaceuticals.

And they could be used for identifying possible improvements in chemicals.

For example, to reduce the impact on ecosystems. We can use lots of techniques to find out more about a sample for quality assurance.

Some of these techniques include measurements such as dissolved solid mass, chemical tests to look at precipitation perhaps, and testing properties such as melting point and boiling point.

A pure substance will change its state of matter at a very, very definite temperature.

A pure substance changes its state of matter at a definite temperature.

Now when we're talking about state of matter, we're referring to a substance being a solid, a liquid or a gas.

And when substances are pure, they have a definite temperature at which they melt and at which they boil.

So gold, for example, melts at 1,064 degrees centigrade.

And the chemical ethanol boils at a temperature of 78 degrees centigrade.

So when we've got a substance that we think might be gold and it melts at 1,064 degrees centigrade, we're pretty sure it's gold.

And if we've got a liquid that's boiling at 78 degrees C and we think it's ethanol, it probably is ethanol.

When we've used distillation to purify an impure water sample, we can then collect the distillate and test its boiling point.

And if pure water has been obtained, it should boil at 100 degrees centigrade.

Let's do a check.

Scientists want to determine if a water sample is pure water.

What methods could they use? Well done if you said measure the sample's boiling point.

None of the others would actually tell us if it's pure water.

If the distillation hasn't been done very successfully, the distillate may still contain impurities.

And a sample containing impurities boils over a range of temperatures rather than a specific one.

The more impurities the sample has got, the wider its boiling point range will be.

So pure water will boil at 100 degrees centigrade, as we've said.

But seawater, which has got impurities such as dissolved substances such as salt and gases, it has a boiling point between 102 and 123 degrees centigrade, so there's a big range there.

Let's do a check.

So all water samples boil at 100 degrees centigrade.

Is that true or is that false? Well done if you said false.

Now can you justify your answer? Well done if you said B, only pure water boils at 100 degrees centigrade.

I think we're ready for Task B now.

So in task A, you carried out a distillation and you would've collected some distillate.

Now test the boiling point of that, and I've just put the photograph of the equipment that you might have used to remind you there.

What result would you expect if the distillate was pure or impure? And then comment on the purity of your distillate.

Pause the video and come back when you're ready.

Okay, so what result would you expect if it was pure? It should boil at 100 degrees C.

And if it was impure, it will boil above 100 degrees C and have a range of temperatures.

Remember, seawater had that range from 102 to 123 degrees centigrade.

Part two, comment on the purity.

So it will depend on the distillate you actually collected.

If it boils at 100 degrees centigrade, well done, it's pure.

You did a good job.

And if it boils over a range above 100 degrees centigrade, then it's still got impurities in it.

So we've come to the end of our lesson now.

I hope you feel a lot more confident about being able to use distillation to purify water samples and that you know now how to analyse the distillate to test its purity.

Well done, let's have a quick look at the summary of the main learning points today.

So distillation can be used to purify water samples and we've got a distillation setup there that you should be able to now know how to use and label.

You should know where the mixture is and where the pure water is and that that's called the distillate, and you should also know where the condenser is and what its purpose is.

Boiling point data can be analysed to assess the purity of water samples.

And remember, pure water boils at 100 degrees centigrade.

The boiling point of impure water samples will increase, they'll be above 100 degrees centigrade.

And the more impurities that are present, the larger the temperature range over which the water sample will boil.

So a few things to remember there about distillation and water samples.

Well done and I'll see you next time.