Hello scientists, my name is Mr. Wilshire.

And in this lesson we're going to be looking at reversible changes of state.

The outcome for this lesson is I know that changes of state are reversible changes.

There are some key words for us to consider as we work our way through this lesson.

The first is state.

Next is substance.

After that is change of state.

And finally, reversible.

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

The definitions have appeared on the screen for you now, so you can have a recap and a reconsider before moving on.

Pause the video now and continue when you're ready.

The first part of this lesson is called Reversible changes: solid state and liquid state.

Aisha is eating an ice cream on a warm day.

Here's Aisha and here's Aisha's ice cream.

Oh, that looks yummy.

Some of it is started to run down the cone and onto her hand.

Hmm.

She says the ice cream is melted.

It's changed into a liquid.

This means that the ice cream has changed its state, hasn't it? It's melted and turned into a liquid form.

So Alex says, what does that mean? What does state mean? What do you think? Pause the video here and discuss.

Restart when you've done that.

So what does change of states mean? Hmm, let's find out more.

Aisha says the state of a substance is the form in which it can exist.

She says that there are three common states, known as states of matter.

I wonder if you know what they are.

Have a really good think here.

We've heard one of them already in this lesson.

Discuss and restart the video when you've done that.

So what types of change of states can you remember? There are three common states that we know of.

The three common states of matter are solid state, liquid state, and gas state.

We see water in all three states in our daily lives.

What do we call water when it's at each state? So what do we call water when it's a solid? What do we call water when it's a liquid? And what do we call it when it's a gas? Have a good think here and discuss.

Restart the video when you've done that.

So can you remember what we call each state? Water in a solid state is known as ice.

It's very hard, isn't it? But you can melt it.

Water in a liquid state is known as water.

It's runny, it fills a container, doesn't it? And water in a gas state is known as water vapour.

You may see this when you've boiled a kettle or you may have seen it when it's a very, very hot day and it's rained and the water starts to evaporate.

That's when you can see the water vapour.

I wonder if you got all of those right.

Did they come to mind when you were thinking about solid water, liquid water, and gas water? Stop and think.

Which of these are common states of matter? A runny state, solid state, a rigid state, and a gas state.

Which of those are a common states of matter? The correct answers here is solid and gas.

They are the two common states of matter.

Aisha's ice cream changed from a solid state, a solid ice cream to a liquid state, therefore a liquid ice cream.

It started melting and running down her hand, didn't it? A change of state is when one state of matter changes to another.

Here you can see the solid.

Solid doesn't just mean that you can knock it onto things or that it's very, very hard.

Solid means something that could be manipulated or moved around in your hand.

When ice cream changes from solid state to liquid state this is called melting.

And although you can move the liquid around in your hand, it's very, very hard to hold it 'cause liquid fills a container.

There you can see the liquid ice cream running down the side of the cone.

Can you remember the names of the processes for any other changes of state here? Hmm.

Have a think and discuss.

Restart when you've done that.

So what were the names of the processes for any other changes of state? When you heat a material in its solid state it melts to be in a liquid state.

There you can see a solid in a solid state and a liquid in a liquid state.

When you melt the solid, it turns into a liquid.

When you cool a material in a liquid state, it can freeze to become a solid state.

So one turns to the other and then the other turns back to the one.

Stop and think.

What is changing state? Is it when you get married, when one state of matter changes to another, when one type of material matter is mixed with another, or when you move between countries? The correct answer here is when one state of matter changes to another that is what changing state means.

Changes of state are reversible changes.

When we reverse in a car, this means that we are going backwards in the opposite direction to where we came from, says Aisha.

A reversible change in science means the material that has been changed can return or go back to its original state.

Melting and freezing are examples of reversible changes of state.

Aisha says if we freeze water to make ice, we can get the liquid water back by melting the ice.

So if you melt the ice, it turns into a water, and you get that by heating.

If you freeze that water again, it then turns back into ice, but possibly not in the same sort of shape.

So you cool the liquid down to turn it back into a solid.

Stop and think.

Which of these processes are used to demonstrate reversible changes of state? Is it melting, a liquid state, freezing, or a solid state? The correct answer here is melting and freezing.

Both of these are reversible changes of state.

So here is task A.

Demonstrate a change of state.

Step one is to follow the instructions on the next slides to make ice cream.

Explain to a partner what change of state has taken place in this case.

You need to be very careful with cold temperatures here.

Ask an adult to help you.

Demonstrate freezing by making some ice cream.

So step A here is to put 10 ice cubes into a large sealable bag.

Step B is add three tablespoons of salt to the ice cubes.

And step C is pour 50 millilitres of milk into the small bag and then seal it.

After this, you need to place a small bag inside the large bag and seal it all up.

Step E is to wrap the bag in a tea towel and spend five minutes shaking and squeezing and squashing it.

Step F is to take the small bag out of the large one and observe.

Here is part two of this first task.

You need to demonstrate how changes of state are reversible changes.

So place a teaspoon of ice cream on a saucer or in a bowl and observe what happens over time.

Explain to a partner what change of state has taken place and how this is a reversible change.

So demonstrate how changes of state are reversible or irreversible.

Best of luck with your task.

Restart the video When you've done that.

How did you get on? Well, after squeezing the bag of milk inside the bag of ice and salt for five minutes, the milk in liquid state has frozen and changed states to become ice cream in the solid state.

I wonder if you found the same when you were working with the ice cream here.

Were you able to turn the ice cream into a solid state? Part two then, we placed the ice cream in a bowl and left it for 20 minutes.

Slowly, the ice cream in solid state melted and changed state to become milk again in the liquid state.

This is an example of a reversible change because the material that has been changed has returned to its original state.

So it's gone all the way back to the beginning again in like a great big cycle of reversible and irreversible changes.

The second part of this lesson is called Reversible changes: liquid state and gas state.

Aisha says, changing state is when one state of matter changes to another.

Can materials in liquid states change to a gas state? Alex says, I think it's impossible for a liquid to become a gas because gas is invisible and liquid is not.

What do you think? Pause the video here.

Restart when you've done that.

What did you think? Is it impossible for a liquid to become a gas? Let's find out more.

Water in the liquid state is being heated in a pan on a stove.

Large bubbles appear, which then become water vapour in the gas state.

When a substance has changed state from a liquid to a gas, often it can't be seen, but it still exists as invisible vapour in the air.

Here you can see water being heated.

And you have to be very careful here because the bubbles can rise to the surface and pop, and that can sometimes send very hot liquid flying everywhere.

When water changes from liquid state to gas state, this is called evaporation.

Aisha says, is there a reverse of evaporation where water in the gas state changes to liquid state? What do you think? Is there a reverse of this operation? Discuss and restart the video when you've done that.

Well, what did you think? Well when you heat a material in a liquid state, it evaporates to be in a gas state.

Here you can see an example of a liquid state and a gas state.

One side is evaporating and when you cool a material in a gas state, it condenses to be in a liquid state.

In this case, both boiling and evaporation can both be used to turn a liquid into a gas.

Boiling is a process that takes place from a fixed heat source that happens all throughout the liquid that eventually turns it into a gas.

Evaporation on the other hand, is a very slow process and can take place from the surface of the water.

So boiling is very quick, evaporation is very slow, but they're both ways that we can turn liquid into a gas.

And then the opposite of boiling and evaporating there is condensing.

When you are condensing a gas, you are turning it back into a liquid.

True or false? Changes of state are reversible.

The correct answer here is true.

Can we justify our answer here? Which of these statements best fits? Correct answer here is A, changes in state are reversible because the material that was changed can return to its original state.

Evaporation and condensation are examples of reversible changes of state.

Aisha says, if we evaporate water to make water vapour, we can get the liquid water back by condensing the water vapour.

So you evaporate the liquid, it turns into gas.

So you do that by heating it up and then you condense it, you cool it down to get it back to being a liquid again.

This lid has been covering a pan of boiling water.

Can you observe what's happened? Pause the video here and observe.

Restart the video when you've done that.

Condensation has occurred where the water vapour has touched the underside of the lid.

Water vapour has condensed on the lid to form droplets of water.

Condensation is a reversible change of state.

So this is an example of condensation, water forming on the underside of the pan lid here as it cools down from when it was boiling.

Scientists can use special equipment to demonstrate reversible changes of state.

They collect water vapour and cool it down so that it changes back into a liquid state again.

Here is an example of some very old fashioned equipment to collect condensed gases.

Water can change from a liquid state to a gas state and back again to a liquid state.

Stop and think.

Which of these processes are used to demonstrate reversible changes of state? Is it water vapour, heating, evaporation, or condensation? The correct answer here is evaporation and condensation.

Both of these processes can be used to demonstrate reversible changes of state.

So here is task B.

The first part is to draw a diagram showing the three states of water and name the processes used to change its state.

Add the following details to your diagram.

The name of water in each state, the name of the state, the arrows between states, and the name of the process between each state and whether that process is caused by heating or cooling.

So if it is evaporating, for example, that process can be done through heating.

Pause the video here and have a go at that task.

Restart the video when you've done that.

How did you get on? Were you able to draw a diagram? Well, it might have looked a little bit like this.

You can heat up the solid and that will turn it into a liquid.

You can then heat up the liquid and it will evaporate so it turned into a gas.

You can collect that gas by cooling it down and helping it to condensate.

And then that will turn into liquid, which you can cool and freeze it to turn it back into a solid.

Did you get that right as well? Well done.

The second part of this task is to complete the table to show how daily actions change the state of water.

So you've got this full table here.

You've got water at each of the different states that you need to input the data for.

The actions are boiling in a kettle, putting in a freezer, and leaving it room temperature twice there.

So you decide how are you going to complete this table.

Best of luck with your task.

Restart when you've done that.

How did you get on? Let's have a look through some of these results.

Yours could have been similar or a little bit different.

So water in a liquid state needs to be boiled, that's heating it.

It evaporates and turns into water vapour or gas, and the reverse of that is called condensation.

The water in a liquid state can be put in a freezer.

It cools and freezes to turn into ice, which is a solid state.

That can then be melted as a reverse process.

The ice in a solid state can be left at room temperature.

That's heating it up, isn't it? It's gonna be hotter than it was in the freezer.

It will then start to melt and turn into water, which is the liquid state.

The reverse of this is called freezing.

And finally, the water vapour in a gas state can be left at room temperature again, that's gonna cool it down.

It's going to condensate turn into water, which is the liquid state.

And the reverse process of this is evaporation.

Whew, that's a lot of observing and writing.

Well done for completing that task.

Let's summarise our lesson.

Solids, liquids, and gases are three common states of matter in which substances can exist.

Changing state is when one state of matter changes to another.

Changes of state are reversible changes whereby the material that was changed can return to its original state.

Melting, freezing, evaporation, and condensation are examples of reversible changes of state.

Hopefully you've got a much clearer idea as to the three common types of state, solid, liquid, and gas.

Hopefully you can go on to use some of this information in your future observations and some future experiments too.

I'm Mr. Wilshire, thank you very much for listening.