Lesson video

Hello, everyone.

How are you doing today? I hope you're feeling good.

My name is Ms. Afzal.

And I'll be a teacher for this lesson.

I'm feeling good about that because I'm really interested in today's topic.

We're looking at seawater and we're looking at how we can separate salt from seawater.

The title of our lesson is Removing Salt From Seawater, and it comes from the unit of work Properties, Changes, and Separating Materials.

So I wonder, have you come across seawater before, salty seawater? And whether you have or you haven't, don't worry, we're going to be exploring how to remove salt from seawater.

It's going to be a very practical lesson.

So I hope you're feeling ready for this.

You've got some energy, focus, and enthusiasm.

And if you have, we'll begin.

The outcome for today's lesson is I can investigate ways of making seawater safe to drink.

I hope that sounds interesting to you.

We have some keywords in our lesson today.

I'd like us to go through them one at a time.

My turn, your turn.

Are you ready? Seawater.

Solution.

Evaporation.

Condensation.

Desalination.

It is great to hear those keywords.

We have seawater, solution, evaporation, condensation, and desalination.

What do all these words mean? Well, seawater is water from a sea or ocean.

Solution.

Some solids dissolve in liquids to form a solution.

Evaporation is the change of state from liquid to a gas.

Condensation is the change of state from a gas to a liquid.

And desalination is the process of removing salt from seawater.

These are our keywords.

Let's be on the lookout for them.

Let's listen out for them.

They'll be coming up in our lesson today.

Today's lesson is called Removing Salt From Seawater, and it has two learning cycles.

First of all, evaporation and condensation, and next, removing salt from seawater.

We are getting practical.

But let's begin with exploring evaporation and condensation.

Here's Aisha.

"I think people who live near the coast must have all the drinking water they need." Oh my goodness, can you see that coastal town in that image? I want to be there.

Do you agree with Aisha? Do you think people in a coastal town have all the drinking water they need? Pause here and share with someone what you think.

Thanks for sharing your ideas.

The fact is we cannot drink seawater.

So just because they're near the coast, it doesn't mean they have all the drinking water they need.

And do you know why? Why is it that we cannot drink seawater? Pause here and share with someone.

Thanks for sharing your ideas.

We're gonna be exploring more about why it is that we cannot drink seawater in the next few slides.

This water in this lake is freshwater.

Freshwater is the type of water that is made safe to drink and comes through our taps.

Oh my goodness, how beautiful is that image of the freshwater lake.

The water in our seas and oceans is different to freshwater found in raindrops, lakes, and rivers.

The water in our seas and oceans contains salt.

Look at those waves crashing in.

The salt not only makes the water taste bad, but the salt in it would also make us ill if we drank a lot of it.

Is it possible to make seawater suitable for drinking? I'd like you to ponder this question.

Pause here and share with someone.

Can we make seawater suitable for drinking? Thanks for sharing your ideas.

Let's have a check for understanding.

True or false.

Seawater is safe for drinking.

Pause here and share with someone, is this statement true or false? Well done if you selected false.

And now I'd like you to say a little more about your answer.

Pause here while you do this.

Perhaps you said something like this.

The water in our seas and oceans contain salt, and so it's not safe for drinking.

Salt is soluble in water.

When salt dissolves in water, it makes a salt solution.

Here's Aisha.

"In school, we've been learning how to separate salt from a salt solution." Can you remember which process you can use to do this? Pause here and share with someone.

What's the name of this process to separate salt from a salt solution? Thanks for sharing.

Evaporation can be used to remove the water from a salt solution.

Aisha knows that the rate of evaporation increases as the temperature increases.

To make the water evaporate quickly, Aisha could heat a sample of seawater, a salt solution.

And now we can see some salt on a saucer.

Let's have a check for understanding.

What is the name of the process used to remove water from a salt solution? Is it a, filtering, b, evaporating, or c, melting? Pause here while you decide.

Well done if you selected answer b.

Indeed, the process used to remove water from a salt solution is evaporating.

And we can see in this image a candle to provide heat, the oil burner, a salt solution in a well, and water vapor.

Alex knows that during evaporation, the liquid in a solution evaporates and the original solid is left behind.

"When we evaporated the water from our salt solution, we were left with the salt.

This time, we want to collect the water in a salt solution to investigate if it could be safe to drink." What could the children do? Pause here and shower with someone.

Thanks for sharing your ideas.

When a substance changes from a liquid state to a gas state, often it cannot be seen, but it still exists as invisible vapor in the air.

We can collect this gas and cool it down so that it changes state back into a liquid.

Do you know the name of this process? Pause here and share with someone.

It's called condensation.

That's one of our keywords.

Let's say it again, my turn, your turn.

Condensation.

Good to hear that.

Condensation occurs when water vapor, which is warm, hits a colder surface, like a window in winter or a cold bathroom wall.

So there we can see the water droplets on a window, perhaps on a cold day.

And we can see water droplets on the bathroom wall.

After someone has taken a hot shower, water droplets formed on the cold bathroom wall.

Pause here and share with someone, have you ever seen condensation before and where have you seen condensation? Thanks for sharing.

Let's have a check for understanding.

Which of these describes condensation? Is it a, water in the liquid state is heated and changes to water in the gas state, or b, water in the gas state is cooled and changes to water in the liquid state, or c, water in the liquid state is heated and changes to water in the solid state.

Pause here while you decide which of these describes condensation.

Well done if you chose answer b.

Water in the gas state is cooled and changes to water in the liquid state.

This describes condensation.

And now it's time for your first task.

I would like you to take a closer look at evaporation and condensation.

So number one, ask an adult to half-fill a mug with hot water.

And please make sure you're careful around hot water.

Two, take a cold plate and place it on top of the mug.

Three, wait for a few minutes, then carefully remove the plate from the mug.

Four, explain to a partner what you observe on the underside of the plate.

So pause here while you have a go at this task, taking a closer look at evaporation and condensation.

I'll see you when you're finished.

It's great to be back with you.

How did you get on with your task of taking a closer look at evaporation and condensation? Let's read a response.

Some of the hot water evaporated and became water in the gas state, water vapor.

The water vapor hit the cold plate and it was suddenly cooled.

This made the water vapor change back to the liquid state.

This liquid was observed as tiny droplets of water on the underside of the plate.

Well done for having a go at this task.

And now we're onto our next learning cycle, removing salt from seawater.

Scientists and engineers investigate ways of removing salt from seawater using evaporation and condensation.

This process is called desalination, which means removing salt.

One way to do this is to heat seawater.

Water evaporates from the salt solution.

The water vapor rises and moves away into a cooling tube.

In the cooling tube, it condenses back into water and is collected.

Maria Telkes was a Hungarian engineer born in 1900.

There we can see a photo of her.

She was interested in developing technology which needed sunlight to work.

During World War II, Telkes developed the first solar-powered water desalination system.

It was designed to help soldiers produce drinking water in difficult locations.

This was called a solar still.

And this is how it works.

Number one, unclean, salty water is held within a shallow bowl covered by a sloped roof.

Two, sunlight heats the water causing it to evaporate.

Three, the water vapor then condenses on a plastic roof and flows down into a collection container.

And there we have the clean water in the collection container.

Many boats still carry solar stills to allow them to produce drinking water in emergencies.

Let's have a check for understanding.

Which professionals would investigate ways of removing salt from seawater using evaporation and condensation? Is it a, electricians and electrical engineers, b, teachers and teaching assistants, c, scientists and engineers, or d, doctors and nurses.

Pause here while you decide which professionals would investigate ways of removing salt from seawater using evaporation and condensation.

Well done if you selected answer c.

Indeed, its scientists and engineers who would investigate in this way.

Desalination is an important area of scientific research and engineering.

This is especially true where populations are growing and there is limited freshwater.

Here's Aisha.

"There are desalination plants all around the world.

They are factories where salt is removed from seawater to make the water suitable for drinking.

Some modern desalination systems use processes other than evaporation and condensation to remove the salt from seawater.

Scientists are working to reduce the amount of greenhouse gases produced during desalination and make the process more sustainable.

And we can see a scientist deep in research there.

After seawater has undergone desalination, microorganisms must be removed from it.

Microorganisms are very tiny living things which are so small that we cannot see them without using microscopes.

Oh my, take a look at these microorganisms. If we drink water containing microorganisms, we can become ill.

Let's have a check for understanding.

True or false.

Microorganisms are not usually removed from water to make it safe to drink.

Pause here while you decide, is this statement true or false? Well done if you selected false.

And now I'd like you to justify your answer by choosing from one of these two statements.

A, microorganisms must be removed from water to make it safe to drink, or b, microorganisms do not affect the safety of drinking water so they can be left in it.

Pause here while you decide which of these two statements justifies your answer.

Well done if you chose statement a.

Indeed, microorganisms must be removed from water to make it safe to drink.

And now it's time for your next task.

As a class, follow the instructions on the worksheet to make your own mini-desalination plant.

Use the sun, or if it is cloudy, use a desk lamp to represent the sun.

After 24 hours, observe what has happened.

So pause here while you follow the instructions on the worksheet and make your own mini-desalination plant.

Enjoy your task, scientists, and I'll see you when you're finished.

It's good to be back with you.

So how did you get on as a class making your own mini-desalination plant? Let's find out.

One, we poured 200 milliliters of salt solution into a big bowl, then placed a smaller empty bowl into the big bowl.

Two, we covered the top of the big bowl with a clear recycled plastic bag and held it on with an elastic band.

Three, we placed a small weight, we used a pile of coins, on the middle of the plastic bag to make sure that the lowest part of the plastic was above the collection bowl.

As it was winter, we used a desk lamp to represent the sun.

We left it for 12 to 24 hours then observed what had happened.

So what were the results? Your results may have been like this.

After 24 hours, there was a pool of water in the small bowl and the level of the salt solution had gone down.

We observed lots of drops of condensation on the plastic bag.

So there we can see the freshwater in the small bowl and less saltwater in the large bowl.

Oh my, you have created your own mini-desalination plant.

Well done, scientists and engineers.

In our lesson, Removing Salt From Seawater, we've covered the following.

The water in our seas and oceans contain salt.

This water is not safe for drinking.

Evaporation can be used to remove the water from a salt solution.

Condensation is when a water in the gas state, water vapor, is cooled so that it changes to water in the liquid state.

Scientists and engineers investigate ways of making water safe to drink using evaporation and condensation.

Microorganisms must also be removed from saltwater to make it safe to drink.

Well done, everyone, for joining in with this lesson.

It was so great to explore this topic of water, of seawater containing salt, how we can remove it, what is condensation, and evaporation, and desalination.

So many things we've investigated.

And crucially, you became engineers and scientists and created your own mini-desalination plant.

I hope that was satisfying to create that freshwater for yourselves.

And now you know what to do if you are out somewhere surrounded by seawater and you want to make a little freshwater drink for yourself.

I hope you've enjoyed this lesson.

I've really enjoyed teaching you.

And I'm looking forward to seeing you at another lesson soon.

Until then, stay curious.