Hello, scientists.

My name is Mr. Wilshire, and in this lesson we're going to be looking at more reversible changes.

The outcome for this lesson is I can demonstrate how mixing and dissolving are reversible changes.

There are some keywords for us to remember as we work our way through this lesson.

The first is state, after that, dissolve, then it's reversible, after that it's recover, and finally, mixture.

Don't worry if you're not too sure what all of these words mean.

The definitions are on the screen for you now, so you can have a recap if you need to and continue the video when you're ready.

The first part of this lesson is called recovering insoluble solids from liquids.

Alex has been learning about changes of state.

Explain to a partner what this means.

Think about those words, changes of state.

What does that mean? Discuss.

Restart the video when you've done that.

So what is a change of state? Is it anything to do with America? No, that's not quite what we're talking about here, is it? Alex says, "A change of state is when one state of matter, like a solid state, a liquid state, or a gas state, changes to another." Oh, so if a solid could become a liquid, or if a liquid could become a gas, or maybe a gas converting back to a liquid again, those are all changes of state, aren't they? Alex has made some lemonade by adding lemon juice, pulp, and rind to water.

There's his lemon juice, his pulp, and his rind there, and they're all being added together to create a mixture.

To make it taste sweeter, he dissolves a tablespoon of sugar into the liquid.

I think he's gonna need to mix it really well here to make sure that all those particles of sugar are dissolved.

Finally, he then freezes the lemonade to make lemonade ice lollies.

He's added a wooden stick, popped it into a small container, and then put it in the freezer to make sure that it can freeze to become an ice lolly.

How delicious.

By freezing it, Alex has cooled the lemonade until it has changed state from liquid state to a solid state.

This has then made lemonade lollies.

Have you ever made anything like this before? Have you ever popped into the freezer in order to freeze it? Alex says, "If I place the ice lollies, as a solid state, somewhere warmer, they will melt and they'll change back to liquid state lemonade." Changing liquid lemonade into solid lemonade lollies is a reversible change.

That means that it's a change that can be undone or reversed.

So a little bit like when you reverse a car, you're going backwards, aren't you? In this case, we are reversing the change that has happened.

We are changing it from being solid and it's going back to a liquid form.

So reversible change includes changes in state, size, and the shape of a substance.

A reversible change does not create a new material however.

It isn't just magic things out of thin air, no.

You're only using what you already have, and in this case, the solid ice lolly is being melted to turn back into a liquid.

Stop and think.

A reversible change is a change that produces a new material, that cannot be undone or reversed, that can be undone or reversed.

What do you think? Pause the video and discuss.

Restart when you've done that.

The correct answer here is c.

A reversible change is a change that can be undone or reversed.

Alex thinks back to his lemonade recipe.

First he added lemon juice, pulp, and rind to the water.

Next, he dissolved a tablespoon of sugar into the liquid.

Are any of these processes used to make Alex's lemonade reversible? Can he get anything back from them? Discuss.

Restart the video when you've done that.

So what did you think? Are any of these processes reversible? Is he able to get all of the ingredients back again? Let's find out more.

Alex looks very closely at the mixture that he has made.

Here you can see Alex's lemonade.

There's lots of bits floating around in there, aren't there? So that means that not all of these items used were things that could dissolve.

He observes that the lemon pulp and the rind have not dissolved.

These are insoluble solids.

He says, "I think that making a mixture is a reversible process because it can be undone or changed back." What do you think here? Is he correct? Can it be undone or changed back? Discuss.

So is making a mixture a reversible process? Is it like that every time? What do you think? A mixture is a material that contains two or more different substances which can be physically separated.

In this case there's lots of substances, aren't there? There's sugar, there's the rind, and there's the pulp, and there's obviously the liquid.

Alex says, "I could pour the lemonade through a filter to recover the lemon pulp and the rind in the solid state from the water and the lemon juice in a liquid state." Do you agree? Is he going to be able to use a filter in this way to separate the mixture again? Discuss.

So what did you think? Is Alex going to be able to do this? I think that using a filter in this way is going to be able to break it all down again, isn't it, and help to separate things out.

I'm not sure if it's going to be exactly the same as it was when it all went in, hmm? Stop and think.

A mixture cannot be physically separated, can be physically separated, or can sometimes be physically separated.

The correct answer here is a mixture can be physically separated.

Task A now.

Demonstrate that making lemonade is a reversible process.

Step one.

Make the lemonade by following these steps.

A, zest one lemon.

B, cut the lemon in half, juice it, and save everything: juice, pulp, and pips.

C, remove the pips from the juice and pulp.

I wonder how you're going to be able to do that.

D, in a jug, mix 140 grammes of sugar with 300 millilitres of warm water and stir until you can no longer see the sugar.

Remember the water has to be warm here to help the sugar start to dissolve quicker.

E, add the zest, juice, and pulp to the jug.

When you've done this, you'll need to move on to part two, so restart the video once you've done that.

How did you get on? Here's some steps that you could have taken.

They say, "We zested one lemon then cut it in half and juiced it." How did they get the pips out of that mixture though? Ah, "We sieved the mixture which trapped the pulp and the pips, and then we removed the pips with a spoon.

In a jug, we mixed the sugar and water and we stirred.

And finally, we added the zest, juice, and the pulp." And there you have the jug of mixture.

In step two, demonstrate that mixing is a reversible change.

Use filtering to recover insoluble solids from a sample of your lemonade.

You should aim to recover the lemon pulp and the rind.

Best of luck with this task.

Restart the video when you've done that.

How did you get on? Were you able to complete that task? Well, let's see.

You might have done something a little bit like this.

It says, "We poured the lemonade through a sieve to recover the lemon pulp and the rind." There you can see the pulp and rind in the sieve and the lemonade on the saucer.

Hmm, I wonder, is there as much liquid left over when you do it in this way? Are you able to recover the majority of the things that were put into the mixture in the first place? What about the sugar? Have you been able to get that out of the mixture? Well done for completing your task and showing that it is possible to recover insoluble solids from a sample of your lemonade.

The next part of this lesson is called recovering soluble solids from liquids.

Hmm, see, I remember that to make the lemonade, they needed to mix lots of different things.

Now, sugar was one of those things, wasn't it? Now, sugar is soluble.

That means that it can dissolve.

They weren't able to get that back, were they, when they put everything through the filter? And Alex says, "To make the lemonade taste sweeter, I dissolved a tablespoon of sugar.

I think that dissolving is a reversible change." Do you think that dissolving is a reversible change? And can you explain why? Hmm, discuss.

Restart the video when you've done that.

So is dissolving a reversible change? Let's find out more.

When a soluble solid is dissolved in a liquid, it does not change state, rather it is broken down to become so tiny that we can no longer see it.

Despite its appearance, the sugar is still there.

You can taste it, don't forget.

Now, it can be recovered from the liquid.

This means that the change can be reversed, so dissolving is a reversible process.

Stop and think.

Which statement about dissolving is correct? When a solid dissolves in a liquid, it is no longer there.

When a solid dissolves in a liquid, it is still there and can be recovered.

When a solid dissolves in a liquid, it is still there but cannot be recovered.

Have a think.

Restart the video when you've done that.

The correct answer here is b.

When a solid dissolves in a liquid, it is still there and it can be recovered.

Alex wants to demonstrate that dissolving is an example of a reversible change.

He plans to recover the sugar dissolved in his lemonade drink.

So what should Alex do to recover the sugar, as a solid state, from the water, as a liquid state? What's he going to need to do here to be able to recover it all again? Have a think and discuss.

Restart when you've done that.

What did you think? What is Alex going to need to do? Let's find out more.

Alex says, "I know that filtering doesn't separate a soluble solid from a liquid, so I need to evaporate the water from the lemonade to leave the sugar behind." Ah, yes.

Now, a solid can't be evaporated.

It can't just magically turn into a gas, can it, whereas the liquid can.

So by evaporating all of the liquid here, he's going to be left over with the solid sugar.

Evaporation is a change of state where a material in a liquid state changes to be in a gas state.

But remember, the sugar isn't in a liquid state, is it? It's just that the particles have been broken down so much that they are now very, very microscopically tiny, so that means that you can't really see them very well.

Explain to a partner what Alex means by evaporate.

What's this process? What do you need to do in order to make something evaporate? Discuss.

How do you get on with your explanation? How is Alex going to evaporate things? Let's find out more.

Alex asks an adult to help him to work safely.

He demonstrates a reversible change to recover the sugar from his lemonade drink by evaporating the water from the solution.

An oil burner is the perfect kind of thing to use here to evaporate some liquid, but this is why you need an adult because you need to light a candle underneath, and that's going to heat the little dish in the top there, and that can start to evaporate the liquid.

Heating something is one way to speed up the evaporation process.

Heating something is just one way that we can speed up the evaporation process, and this is a great example, and how we can recover something solid that's inside a liquid.

Which process can you use to recover dissolved solid from a liquid? Have a look at those statements and decide which one answers this question.

The correct answer here is evaporation.

So here is Task B.

You need to demonstrate that dissolving is a reversible change.

Use evaporation here to recover soluble solids from a sample of your lemonade.

You should aim to recover the sugar.

Remember, the sugar is stuck in a solid state, but it has been dissolved.

It's now tiny, tiny particles mixing round in the liquid, so to recover it, you'll need to evaporate.

Have a go at evaporating, and remember to be very careful when you're doing this experiment.

Best of luck.

Restart the video when you've done that.

How did you get on? Now, you might have done something a little similar to this.

Here's lemonade on a saucer, and there's solid sugar left over on a saucer.

You need to look very closely to be able to see it.

There's not a lot of sugar there, is there? So they said that they put a small sample of lemonade on a saucer and left it in the sun to evaporate the water and recover the sugar.

Of course, there's lots of ways that we can evaporate things.

We don't just need to use a candle.

The sun is a perfect example of doing that, especially if it's a hot sunny day.

So hopefully you've been able to see that you can reverse most changes of states in this way.

By evaporating the liquid, you're able to recover the solid.

The only problem that I can see is that we weren't able to get all of the sugar back that we put in there.

Some of it is broken down into such small parts that we now can't see it anymore, but you can see that there is still some sugar left over.

You could also try this experiment using some salt as well.

Let's summarise our lesson.

Reversible changes are where the original material can be recovered after the change.

A mixture is material that contains two or more different substances which can be physically separated.

When a soluble solid dissolves, it's broken down to become so tiny that we can no longer see it, but it is still there.

Dissolving is a reversible process.

Evaporation can be used to recover soluble solids from a solution.

So just because there is something that we can no longer see, we may be able to detect traces of it.

It could be its smell, or in this case it could be the taste.

You could still taste that the sugar was in the water even though you could no longer see it.

By mixing something together in this way and dissolving all of the particles, you're breaking it down even more into such small amounts that you can no longer see it with the naked eye.

The reverse process of that obviously is evaporation where you can recover some of the sugar back, but not all of it.

Hopefully you have a better understanding now about how those different changes of state work in comparison to the other, and you've been able to have a go at dissolving and at evaporating in order to recover the mixtures.

I've been Mr. Wilshire.

Thank you very much for listening.