Lesson video

Hello, my name's Mrs. Niven, and today we're going to be talking about potable water as part of our unit on separating substances.

Now, you may have some experience of what we talk about today from your previous learning, but what we do today will help us to answer that big question of how can we explain how substances behave, but also how they're made and changed.

By the end of today's lesson, you should feel more comfortable being able to explain what potable water is, but also describing how it can be obtained from groundwater and seawater.

I'll be referring to some keywords throughout today's lesson, and they include pure, potable, sterilisation, desalination and reverse osmosis.

Now, the definitions for these keywords are given in sentence form on the next slide and you may wish to pause the video here so you can read through those definitions and I would recommend making a quick note of what each means so that you can refer back to these definitions later on in this lesson or later on in your learning.

Today's lesson is broken into three parts.

We'll look at different types of water before moving on to discuss how potable water can be formed from both groundwater and seawater.

So let's get started by looking at what we mean by different types of water.

Now, you may recall that a pure substance from a scientific standpoint is a substance that contains only one type of chemical.

Now, water that's used in the science laboratory must be pure.

So what we're talking about is a substance that contains only water molecules.

Now, the reason that pure water needs to be used is because impurities in water, so things like dissolved ions, could contaminate a laboratory sample.

For instance, if water that contained dissolved ions, things like tap water, was used in a chemical analysis test, it might produce an unexpected precipitate.

Now, at first glance, that might not seem like a big deal, but actually, an unexpected precipitate could hide the correct result from what we were trying to find out using this water as part of our chemical analysis test, or it could lead to an incorrect conclusion for that test.

Let's stop here for a quick check.

True or false, tap water can be used when analysing samples in a laboratory.

Well done if you said false, but which of these statements best justifies that answer? Well done if you said B, tap water contains dissolved ions and because of that, it should not be used when analysing samples in the laboratory.

So very well done if you manage to get those correct.

A fantastic start to the lesson, guys, keep it up.

Now, besides water being used regularly in the laboratory, it's actually essential for life as well.

In fact, the average person in the UK uses about 150 litres of water per day and for a variety of things, including staying hydrated, perhaps for cooking, cleaning in a variety of ways, so washing the dishes, washing clothes, washing your body, and even when you're flushing toilets.

Now, all of these examples are the ways that water is used in a domestic way, so in the home.

But water can also be used for agriculture, for instance watering plants, and that could be on an allotment, it could be in a greenhouse or it could even be watering plants in a field.

Water can also be used for hydrating your animals on the farm or in a field as well.

Let's stop for another quick check.

Which of the following is not a domestic use of water? Well done if you said B.

Hydrating cows would actually be listed as an agricultural use of water.

So very well done if you managed to get that correct.

Now, water that's used for domestic or agricultural purposes must be potable and by potable we mean water that is safe to drink.

Now, it may contain safe levels of dissolved substances.

It may even contain some microbes, hopefully none, but if they do contain microbes, there's a safe level of them.

So something that say your stomach acid could deal with quite easily.

Now, potable water by definition then is a solution as are all the sources from which potable water can actually be produced.

Now, one possible source from which potable water can be obtained is freshwater.

So examples of freshwater would be things like water found in lakes, rivers, reservoirs, ice caps and even glaciers.

We could also obtain potable water from things like groundwater.

So that's water that has seeped through porous rocks and through soil and is now found underground.

Or we could also use water from an aquifer, which is kind of a trapped reservoir underground and we can tap into these using wells.

Some other sources of water that we could use to make potable water is seawater, so that's in the oceans and the seas, but also wastewater, which is essentially used water.

So when we're talking about wastewater, we're talking about water that could come from homes, from factories and even in agriculture.

Now, all water sources have some varying degrees of dissolved substances or microorganisms in them.

So if we were to compare the different types of water here, we know that pure water, the definition, scientific definition, is it contains no dissolved substances and no microorganisms, because as pure water, it will only contain water molecules.

Freshwater and groundwater are very similar in that they have low amounts of dissolved substances and low amounts of microorganisms, whereas seawater and wastewater are more similar to each other in that they have high amounts of dissolved substances and they would also contain some microorganisms. Potable water, then that's water that's safe for us to use, could contain small amounts of dissolved substances and either no or safe amounts of microorganisms in it.

Let's stop for another quick check then.

Which of the following contains low amounts of dissolved substances? Well done if you said A and D, both potable water and groundwater might contain low amounts of dissolved substances.

So well done if you managed to get one of those correct and very well done if you got both of them.

Great job, guys.

It's now time for the first task in today's lesson.

So what I'd like you to do for this first part is to match each description to the correct type of water and then match each type of water to the correct example.

So you may wish to pause the video here and come back when you're ready to check your work.

Okay, let's see how you got on then.

So the description that we had first of all was found in lakes, rivers, reservoirs or glaciers, and that is an example of freshwater which would match the third picture, 'cause that looks a little bit like an ice cap or some glaciers.

The second description was used water from homes, factories, and agriculture.

Now, that's wastewater and I would match that up then to the drain, which was the bottom picture.

Something that's composed of only H2O molecules would be pure water.

So that would match the diagram that is the second picture down, 'cause that's only showing us H2O molecules, which means then that something that contains high amounts of dissolved substances, the best description would be seawater, and that would be the picture at the top showing a bay.

So very well done if you managed to match those up correctly, guys.

Great job.

For the second part, we're gonna help Sam.

Now, Sam is preparing to analyse a sample in the laboratory by first dissolving it in water.

Now, they reckon that tap water will be okay to use because it's potable.

What I'd like you to consider is, is Sam correct? And I'd like you to explain your answer.

So I'm looking for a because clause.

You may wish to pause the video here so you can discuss your ideas with your neighbours before jotting down your answer and then come back when you're ready to check your work.

Okay, let's see how you got on.

Now, Sam is actually incorrect.

Tap water is potable, but it contains small amounts of dissolved ions in it, which may form an unexpected precipitate.

Now, that could lead to an incorrect analysis of the sample.

So Sam should really be using pure water instead of tap water here.

So well done if you managed to say that Sam was incorrect, but very well done if you included that because clause and incredibly well done if you also suggested an alternative to what Sam should be using.

It wasn't asked for, but well done if you managed to include that, 'cause what that shows is you're just extending your answer just that little bit.

Well done, guys, great job.

Now that we're feeling more comfortable discussing the different types of water, let's move on to look at how potable water can be made from groundwater.

The thing to remember is that much of the potable water in the UK is obtained from rainwater and that's usually been collected in either groundwater or freshwater sources.

So places like lakes, rivers, reservoirs, underground streams and those aquifers that we can tap into using a well.

Now, groundwater and freshwater sources may contain varying amounts of several different things, including large insoluble objects, like twigs or leaves, small insoluble objects, grit, silt, maybe even some sand, soluble substances like salts or fertilisers and pesticides, some harmful microorganisms, also known as microbes, and that each of these impurities in the water needs to be removed to a safe level for that water source then to be considered potable.

Now, there are a few main steps that are needed in order to create potable water from groundwater or freshwater, and the first step is known as screening.

In this step, a grid acts like a sieve removing those large insoluble objects.

So as the groundwater or freshwater flows through this grid, that twigs and those leaves are caught, left behind and the rest of the water is able to flow to the next step.

In this next step sedimentation occurs.

So those small insoluble particles settle out of the screened water source, and this can take a little bit of time, 'cause it's a very slow process as that water flows from one end of the tank to the other.

But as it does so, those smaller particles start to settle to the bottom and it creates something known as sediment.

So we can see that there are now fewer particles in that water to then come out as your clearer water moving onto then that next step.

This clearer water then moves now into a filtration tank where very small insoluble particles are filtered out of the water through these increasingly fine layers within the tank.

So as the water then moves through these different layers, as the water flows towards the bottom of this tank, then we start to see that more and more of those particles are caught in those different layers and we have those filtered out very small insoluble particles until we can see on that very bottom layer, there's far fewer particles than we had at the start of this filtration process, and we now have filtered watered out to that next step.

So the last step in this potable water process is sterilisation.

Microbes, things like bacteria, are killed using either chlorine, UV light or ozone.

So that filtered water goes into the tank, a steriliser then is applying to this tank where that bacteria then is killed and we are left then with sterilised water.

This sterilised water now would be considered potable, safe to drink.

Let's stop here for a quick check.

Which of the following is not a step to make potable water from groundwater or freshwater? Well done if you said B.

Aerating is not a step that we discussed that is used to make potable water from groundwater or freshwater.

So very well done if you manage to get that correct.

Let's try another one.

True or false.

Chlorine removes bacteria from groundwater.

Well done if you said false, but which of these statements best justifies the answer? Well done if you said A.

Bacteria is still in the water but it's been killed and because of that, it can no longer make you ill.

So that water, even though it contains bacteria that is killed, is safe to use.

So the chlorine kills the bacteria, but the bacteria remains in that water.

Now overall, obtaining potable water from freshwater or groundwater has actually quite low energy demands.

Now, these low demands on energy then means that the process is relatively an inexpensive one.

Time for the next task in today's lesson.

For this first part, I'd like you to match each description to the correct step name.

So pause the video and come back when you're ready to check your work.

Let's see how you got on.

Filtration should be matched to removes very small insoluble particles.

Screening uses a grid that acts like a sieve, removing large insoluble objects.

Sterilisation kills bacteria and other microbes while sedimentation removes small insoluble particles as they settle out of that water source.

So very well done if you managed to correctly match up each of those descriptions to the step name.

Well done.

For this next part, I'd like you to put each step then into the correct order to form potable water from groundwater.

So the steps are matched up to the correct descriptions, all you need to do is correctly put in which step would happen first, second, third and then fourth.

So pause the video and come back when you're ready to check your work.

So if you've done this correctly, you should have had screening as the first step, then sedimentation, thirdly is filtration and fourth is sterilisation.

Very well done if you managed to put those in the correct order.

For this next task, I want you to consider what Izzy's trying to do.

She's written some instructions for how dirty water could be cleaned using common laboratory equipment.

So I'd like you to read through Izzy's apparatus and her method and correct any mistakes you might find.

Now, there's a variety of ways that you could do this, so I might recommend that you pause the video whilst you discuss your ideas with the people nearest you and jot down your corrections to any mistakes you find and then come back when you're ready to check your work.

Okay, let's see how you got on.

Now, as I said, there were a variety of things that you could have done here.

One thing that you might have suggested doing is actually pouring dirty water through a sieve, 'cause that would simulate essentially the screening process.

And in fact, she had sieve in the apparatus, but it wasn't actually used in her method, so this would show her using the equipment.

Filtering the water would also require extra equipment.

She didn't mention using a conical flask or even the filter paper.

So even though you have the filter funnel, that's supporting the filter paper and it's the filter paper that actually does the filtering.

So that would be a really important addition to the apparatus.

You could repeat the process if needed, but it's not always necessary and it's a definite no-no to drink that water afterwards, because at no point has this water been sterilised.

That's an essential step when cleaning dirty water.

So very well done if you said no to drinking the water.

That was not an easy task, guys, so very well done if you managed to correct any of these mistakes and very well done if you managed to correct more than one or two of them.

Great job, guys.

Now that we know how potable water can be made from groundwater, let's move on to look at how potable water can be made from seawater.

Now, you might be wondering why do we even consider making potable water from seawater when making potable water from groundwater is such an easy, relatively low demand and inexpensive process? Well, when you take a closer look at the total global water supply that is on Earth, a very, very small proportion of it is actually freshwater.

The vast majority of water that we can find on Earth is in the oceans or other salty water.

Now, that means that actually many countries don't actually have enough access to freshwater to make their potable water.

They have to use seawater.

Now, the concentration of dissolved substances in seawater, if you remember, is very high.

So to make seawater potable, it needs to undergo a process known as desalination.

But what is desalination? Let's break that word apart to help us come up with a definition.

De simply means to remove, salina comes from saline, which means containing salt, and tion simply refers to a process.

So when we combine all those ideas, desalination is simply a process of removing dissolved substances, usually salt, from a solution.

Now, desalination of seawater can take place in several different ways, but to make potable water, we tend to use either simple distillation or a process known as reverse osmosis.

So simple distillation is a process used to desalinate seawater on an industrial scale, but the process that's used is very similar to how simple distillation takes place in a laboratory.

And if you recall, there are three main steps essentially.

The first step is that that seawater solution is heated until the water in the solution boils.

That water then is cooled until it condenses back into the liquid state.

That pure water then that forms is collected and stored until it's ready for use.

Reverse osmosis then is a process used to desalinate seawater on a smaller scale.

What happens is, you start with your seawater, which remember is water with some dissolved salts in it, and pressure then is put on that seawater.

As it flows through a partially permeable membrane, that membrane prevents the movement of the dissolved salts from one side of the membrane to the other and what you end up with then is pure water on one side of the membrane and the dissolved salts caught on the other side.

Now, you've gotta be really careful with this particular process, because the very salty wastewater that forms as a result of this process needs to be disposed of very carefully, otherwise you risk damaging the environment that it's released into.

Let's stop for a quick check here.

Which of the following is a process used to obtain potable water from seawater? Well done if you said A and C, both distillation and reverse osmosis can be used to obtain potable water from seawater.

So very well done if you got at least one of those and incredibly well done if you managed to select both of them.

Great job, guys.

The process of desalinating seawater to produce potable water can be a very expensive one and that's because of these high energy needs.

A lot of energy is needed to boil the seawater during distillation or to produce that pressure that's required for reverse osmosis.

Additionally, depending on the fuel that's used, some of these processes could actually contribute to global warming as well.

Desalination processes then take place in very specific locations, and these include where freshwater is rather scarce, but there's a lot of seawater and where energy resources are plentiful and or rather inexpensive.

And a good example of that would be the United Arab Emirates.

It is in the Middle East where there is a lack of freshwater, but it is on the coast and so it has access to quite a lot of seawater.

Also being in the Middle East, it tends to have rather easier access to energy resources that are required to boil that water for distillation or produce the pressure necessary for reverse osmosis.

Let's stop for another quick check.

True or false.

Desalination is an expensive way to obtain potable water.

Well done if you said true, but which of these statements best justifies that answer? Well done if you said B.

Desalination processes require energy to either boil that seawater or create and maintain the pressure needed for reverse osmosis.

So very well done if you managed to get that correct, guys, great job.

Time for the last task in today's lesson.

In this first part, I'd like you to place appropriate statements from the side onto this Venn diagram about distillation and reverse osmosis.

So pause the video whilst you complete the task and come back when you're ready to check your work.

Okay, let's see how you got on.

In distillation, I would've put the words temperature, boil and condensation.

The overlap region should contain seawater, desalination and pure water, and reverse osmosis should have pressure and partially permeable membrane.

The phrases and statements that you should not have included was filtration and freshwater.

So well done if you managed to choose the correct words and very well done if you put them in the correct places on your Venn diagram.

Great job, guys.

For this next part, we wanna remember that many factors are considered when determining which method should be used to obtain potable water from a particular source.

So I'd like you to consider what are some advantages and disadvantages of using distillation to obtain drinking water from seawater, and also why is distillation not used to provide drinking water in the UK? So pause the video while you maybe discuss your ideas with the people nearest to you and jot down your answers and then come back when you're ready to check your work.

Okay, let's see how you got on.

You were first asked to consider the advantages and disadvantages of using distillation to obtain drinking water, that's potable water, from seawater.

So some advantages might be that it actually produces pure water, that it's useful where freshwater supplies are low and where energy costs are also low.

But there are some disadvantages and those include that it will require more energy, which can be expensive, and depending on that energy source, it could produce some greenhouse gases.

You might have also mentioned that if that waste water is disposed of badly, it could damage the environment, so you do need to be careful with that as well.

So very well done if you managed to suggest one or two advantages and disadvantages of using distillation.

Very well done if you've managed to suggest more than two advantages and disadvantages.

That's not an easy task, guys.

I'm really pleased with the effort you've put in.

Great job.

Next, you were asked to suggest why distillation is not used to provide drinking water in the UK, and one thing to remember is that although the UK is essentially an island with a very large coastline surrounded by seawater, we do actually have quite a large supply of freshwater and groundwater.

And producing potable water from those water sources is far less expensive than distilling seawater because of those lower energy demands.

So we're trying to form that safe drinking water, that potable water in the cheapest way possible.

So distillation is not used because of that excellent supply of freshwater and groundwater.

Well done with this task, guys.

It wasn't an easy one and you've had a great go.

Fantastic work.

Wow, we have gone through a lot in today's lesson, so let's just take a moment to summarise what we've learned.

Well, we've learned that potable and pure water are not the same substance.

Water that's used in analysis must not contain any dissolved ions and therefore must be pure.

And the method that's used to obtain potable water, that's water safe to drink, depends on the water source.

So filtration and sterilisation are used to obtain potable water from groundwater or freshwater, whereas distillation or reverse osmosis is used to obtain potable water from seawater, and those processes are really expensive because of the high energy demands to boil that water in distillation or to create and maintain that pressure needed for reverse osmosis.

Whew, it's a lot of stuff that we've gone through.

I mean, I've had a great time learning with you today and I hope you've had a good time learning with me and I hope to see you again soon.

Bye for now.