Lesson video

I'm Mrs. Mytum-Smithson, and I'm pleased that you've chosen to learn about melting points and freezing points with me today.

Don't worry if you don't know anything about melting and freezing, we are going to go through it bit by bit.

By the end of the lesson, you will have all the knowledge you will need to explain about melting and freezing points.

This lesson is part of the unit, Solid, Liquid, Gas States and Changes of State.

The keywords for today's lesson are liquid state, solid state, melting point, freezing point.

On the next slide, there's a description of these keywords.

You may pause this slide if you want and have a read of them.

Don't worry if you've not come across these keywords before, by the end of the lesson you'll be very familiar with them.

Here's today's lesson.

We're going to look at freezing, freezing point, comparing melting and freezing.

We're gonna start with freezing, so let's get going.

What I'd like you to do is I'd like you to have a little think about things that you know from everyday life that have melted.

Perhaps an ice lolly, perhaps something else.

I'll give you a couple of seconds just to have a little think about that.

I hope that you were thinking that when a substance is in the solid state and it melts, it changes to be the liquid state.

You can see from this particle diagram that the particles start off in a fixed position in a regular arrangement and end up able to move over each other as they are melted.

Now freezing is the opposite of melting.

So when the substance in the liquid state is cooled it then changes into the solid state.

And you can see from the animation here that we start off with the particles in a random arrangement and eventually, as they are cooled, it then becomes in a regular arrangement.

So when a substance in the liquid state is cooled, it will change into the solid state.

This is called freezing.

What I'd like you to do is identify the two substances that have frozen.

So we've got a snowball, a lip balm, and some wax.

Well done if you identified that A and B have frozen.

So the snowball, you're probably familiar with snow, that is definitely frozen water, and the lip balm once was a liquid and now it's cooled down and it's now in the solid state.

So that means it has frozen.

So here's your first task.

I want you to explain why the substance is frozen or not frozen for each substance.

All you've got to do is look at the pictures and then write frozen or not frozen for each substance.

Then what I want you to do is just do a little sentence, a short sentence about how you know that it's not frozen.

So we've got a picture of a unlit candle.

That's number one.

Number two, we've got orange juice.

Number three, we've got an ice lolly.

Number four, we've got some fudge.

Number five, we've got some plastic building blocks.

And number six, we've got some black treacle.

Pause the video now, then press play when you're ready for the answers.

Here are the answers.

So we've got an unlit candle.

That is frozen because that wax is in the solid state.

It's not flowing everywhere, it's in the solid state.

We've got some orange juice.

You're right, that's not frozen.

We can pour it, so that is juice in a liquid state.

So well done if you wrote that.

Number three, this ice lolly is in its solid state, so it is frozen.

Number four, we've got the fudge.

That's in a solid state.

It's not able to flow, so it is frozen.

The plastic blocks, they are frozen 'cause that plastic is in a solid state.

If we have a look at the treacle, we can see that that is pouring.

So that makes that one not frozen.

The treacle is in a liquid state.

So well done if you've got all six of those correct.

We've learnt all about freezing.

Moving on to our next learning cycle of freezing point.

And finally we're going to have a little look at comparing melting and freezing.

The melting point and the freezing point are actually the same temperature for a substance.

So if we have a think, some examples.

So here's ice, it melts at zero degrees C, but it also freezes at zero degrees C.

We've got chocolate, probably all familiar with this one.

It melts at around 30 degrees, but it also freezes at 30 degrees.

So this shows that the melting point and the freezing point are the same temperature.

Our final example of silver, so silver's got a much higher melting point at 962 degrees C.

So at 962 degrees C, it will melt, but it will also freeze at that temperature.

So the melting point and the freezing point are the same temperature for a substance.

Let's check, see how much you know.

So true or false, the freezing point and melting point are the same temperature for a substance.

Is that true or is that false? I also want you to find one of these reasons.

So are you gonna tick A, are you going to tick B? Which one is a reason for that? So is it water becomes ice at zero degrees C and melts at zero degrees C? Or is it silver melts at a different temperature than it freezes at? Pause the video now, then press play when you've got your answers.

So well done if you said it is true, it is true.

The melting point and the freezing point are the same temperature for every substance, and that means that A, water becomes ice at zero degrees C when it freezes and it also melts and becomes water at zero degrees C.

Water's got a freezing point of zero degrees C.

Below this it's frozen, so it's ice in the solid state, and above it it's melted so it's in the liquid state.

Room temperature's about 20 degrees C, so water would be in the liquid state.

So if you put on here, this is the freezing point or the melting point of water, put that on the scale.

Beneath that, the solid state is water and above that it is a liquid state.

So if we put 20 degrees C on here, you can see that that fits into the liquid state.

So 20 degrees C, that's definitely a liquid state.

What I want you to do is use a scale to add on to tell me what state of matter would paraffin be on at the room temperature? So you need to put the melting point on.

Then you need to figure out where it's in a solid state, where it's in a liquid state.

And then what you need to do is tell me, what state of matter is paraffin at room temperature? Pause now and press play when you're ready for the answer.

So, let's check the answers.

If we were to continue putting an approximate scale, we could put 40 here, 60 here, 80 here.

And then what we would do is we know that paraffin's melting point is somewhere between 60 and 80.

We'd put that there.

We could see that below 67 degrees, it's in the solid state and above, it's in the liquid state.

So if we then have a little look, see where 20 degrees is, we can see that, indeed, it's going to be in the solid state.

Well done if you got that right.

Another check for understanding.

The freezing point of substance A.

So it is just any substance, we've just called it "A", is 15 degrees C.

The room temperature is 20 degrees C.

What state of matter is substance A at room temperature? Is it solid state, liquid state, or neither liquid state or solid state? Pause the video now and press play when you want the answer.

Well done if you said liquid state.

Because the freezing point of the substance is less than 20 degrees C, it's going to be in the liquid state.

Well done.

So we've got substance B now, it's at 30 degrees C.

The room temperature is 20 degrees C.

What state of matter is substance B at room temperature? Is it a solid state, a liquid state, or neither liquid state or solid state? Pause the video now if you need some extra thinking time, then press play when you're ready for the answer.

So here's the answer.

Well done if you said solid state.

Indeed, the freezing point of substance B is 30 degrees C, and the room temperature is 20 degrees C.

So that's gonna be in the solid state.

So here's your task.

I'm going to give you some substances and you are going to tell me if the room temperature is 20 degrees C, is it going to be in a solid state or a liquid state? So we've got gallium, it's got a freezing point of 30 degrees C.

Substance C has got a melting point of 45 degrees C.

Substance D has a melting point of 19 degrees C.

Substance E has got a freezing point of 21 degrees C.

Iron has a melting point of 1003.

Ice cream's got a freezing point of minus three degrees C.

What I want you to do now is pause the video, complete the task, then press play and we'll go through the answers.

Welcome back.

What state of matter would the substances be in at room temperature of 20 degrees C? So remember freezing point and melting point are the same temperature for a substance.

So gallium's got a freezing point of 30 degrees C, that's above 20 degrees C.

So it's going to be a solid, it's not yet melted.

Substance C has a melting point of 45 degrees C.

Again, that's going to be a solid because it's above 20 degrees C.

Substance D has got a melting point of 19 degrees C.

If room temperature's 20, that's just a little bit below, so it would've melted at 19, so it's going to be a liquid substance.

E has got a freezing point of 21 degrees C, so that's just above the room temperature.

So that should be a solid.

Iron has got a melting point of 1003 degrees C.

That is definitely above 20 degrees C.

So that's definitely gonna be a solid.

And ice cream's got a freezing point of minus three.

So that is indeed gonna be a liquid.

We need to cool it down to minus three degrees C before it would become a solid.

Well done if you've got all six of those correct.

Okay, so we're doing really well.

We've done the freezing and the freezing point learning cycles.

Now we're going to go on to comparing melting and freezing.

What's the same? What's different about them? Here's our particle diagrams of solid state and liquid state.

Remember when a substance melts, the substance changes from being a solid state in a regular arrangement, to a liquid state which is a random arrangement.

Remember that's melting.

So then we're gonna have a little look.

What happens when something goes from the liquid state being all randomly arranged into a solid state that's freezing.

So melting and freezing are the reverse of each other.

When a substance melts, the particles go from being in a fixed position in a regular arrangement to being in a random arrangement with the particles sliding past each other.

If we have a look at this diagram, we should see they start off fixed.

Then when they are warm enough, they will melt and you can see that then they are free to move past each other.

Let's watch that again.

You can see that they're free to move past each other in the end.

Then we have a look at freezing.

Remember, freezing is the opposite of melting.

So the particles go from being able to slide past each other in all directions to being in a fixed position in a regular arrangement.

So let's have a look at this.

They start off random and then they eventually, once they're cool enough, it'll be in a fixed position in a regular arrangement.

So that's the difference between melting and freezing.

Melting goes from being fixed to regular and freezing goes from being regular to fixed positions.

So here's an arrow representing temperature increases.

A substance is in a solid state below it's freezing and melting point.

So here's a melting point, here's a freezing point.

Below it, it's going to be a solid, and above it, it's going to be a liquid.

So above that temperature, it's a liquid.

Below that temperature, it's a solid.

Right, quick check for understanding.

When melting occurs, the change of state is.

Does it go from being a solid state to a liquid state? Does it go from a liquid state to a solid state? This is for melting, remember.

Or is there a non-existent change of state? Does it just not happen at all? Pause the video now.

If you need some thinking time, then press play once you want the answer.

Welcome back.

So for melting, it is absolutely going from a solid state to a liquid state.

Quick check for understanding.

After a substance has melted, the particles are randomly arranged.

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

Now I want a reason for this, so I want you to justify your answer.

So is it that particles in a solid state can only vibrate but are fixed in position? Or is it that they are in a liquid state that they can slide past their neighbouring particles? So I want you to write A or B at this point.

Pause the video and if you need a little bit more thinking time, then press play when you're ready for the answer.

Well done if you said that particles in a liquid state can slide past their neighbouring particles.

Here's your final task for today.

Help Alex sort the statements into a table with the headings of melting and freezing.

So you're going to do a table, you're going to write melting, and then you're going to write freezing.

Then I want you to sort the following statements.

Are they statements to do with melting or are they statements to do with freezing? So solid state to liquid state.

Particles become fixed into position.

Liquid state to solid state.

Particles become free to slide over and around each other.

Particles end up in a regular arrangement.

Particles end up in a random arrangement.

I want you to pause the video now, complete the task and then press play when you're ready for the answers.

Welcome back.

How did you get on helping Alex? So a solid state to a liquid state is melting.

A liquid state to a solid state is freezing.

Thinking about the particles.

When something melts, the particles become free to move over and around each other, compared to being fixed into a position, which is what happens when they freeze.

So melting particles end up in a random arrangement 'cause they can move over and around each other, compared to a regular arrangement where they can't move around each other, they are fixed in a position and they can only vibrate.

So well done if you've got all of those six statements in the correct boxes.

Here's a summary.

When it melts, a substance changes from the solid state to a liquid state.

When it freezes, a substance changes from the liquid state to a solid state.

At temperatures below its melting point, a substance is in its solid state.

At temperature's above its freezing point, a substance is in the liquid state.

Here's a little diagram at the bottom just to sum it up.

And then a diagram in the top right-hand corner showing the particles and their different arrangements when they are melted or frozen.

So, thanks for working really hard during this lesson.

I hope you've learned a lot about melting and freezing points.

I look forward to seeing you in the next lesson.