Lesson video

Hello and welcome to today's lesson.

We're going to look at what happens to particles when the temperature changes today.

This lesson is part of the heating and cooling unit.

My name's Mrs. Clegg.

So let's begin.

Here are today's learning outcomes.

So by the end of today's lesson, you should feel a lot more comfortable in being able to describe what happens to the particles in a solid or a liquid when the temperature increases or decreases.

Here are today's key words.

Expand means to increase in volume.

Contract, decrease in volume.

A particle is the smallest part of a substance, and vibrate means to repeatedly and regularly move back and forth.

So listen out for those key words during the lesson.

You might like to note down what they mean.

So if so, pause the video and join us when you're ready.

Today's lesson is split into three parts, expanding solids, melting solids, and expanding liquids.

So let's get started.

Substances in a solid state have a fixed shape.

Now, water, when it's frozen solid, it has a fixed shape, and so does steel at room temperature.

in the solid state, particles are in a regular arrangement.

If we zoom in to the spoon here, we can see the particles in the solid.

They're vibrating, but they're not moving past each other.

Let's have a quick check.

Imagine you could see the particles inside a steel spoon, which image would match what you would see? Which of those matches what you would see? Well done if you said C.

Now you might wonder why it's not B, because the particles are vibrating there, but the reason is that they're not in a regular arrangement.

When we heat a solid, it expands length ways, width ways, and in height.

So if we heat this steel cube, it will increase its dimension in all three ways.

It expands.

The explanation of why the steel cube expands, is because its particles vibrate faster and then move further.

So the space between the particles increases.

Compare the two diagrams here.

The first one shows the particles in a solid at a lower temperature and the second diagram shows the same particles in a solid at a higher temperature, and you can see there's more space between the particles there because the particles are vibrating faster and moving further.

Let's have a quick check.

Which of the following statements explains why a steel bar expands when it's heated? Well done if you said C, the particles vibrate faster and they move further.

It's the only correct answer there.

If you cool a solid, then the opposite happens.

Its length, its width, and its height decrease, and we call that contraction.

So our steel cube there is at a higher temperature, we cool it down to a lower temperature, and it contracts in all three dimensions.

The reason for this is if we lower the temperature of the solid, it makes it contract because the particles vibrate slower, and they move closer, causing the space between them to decrease.

So they can see the top diagram is particles in a solid at a higher temperature.

And then we've got particles in a solid at a lower temperature, and you can see there's less space between the particles there, so the substance would contract.

Now we need to understand this because in structures like buildings and bridges, they can collapse if they're built using rigid connectors.

So in the diagram, we've got two metal strips, and they're being connected by a solid rigid metal connector.

So if the metal strips contract, then they will snap or shear the metal connector, and that would not be very useful in a structure like a bridge.

You might have noticed in roads, for example, flexible rubber connectors.

So when the road surface cools down and contracts or it heats up, the flexible rubber connectors can change shape without affecting the road.

Let's have a quick check.

So on a cold day, a steel structure contracts.

Which of the following best describes the reason why? Well done if you said B.

The space between the particles and the steel decreases.

Let's have a look at task A now.

We've got Aisha and Lucas discussing why solids contract when they are cooled.

Use the statements below to explain why Aisha is correct and Lucas is incorrect.

Pause the video and come back when you're ready.

Let's have a look how you did.

So Aisha is correct because slower particles don't move as far away from each other when they vibrate, and so the gaps between the particles are smaller.

Lucas is incorrect because smaller particles of the same substance would have less mass, and cooling a solid doesn't affect mass.

Well done if you got those correct.

Let's move on to the second part of our lesson.

Melting point is the temperature at which a substance changes from a solid to a liquid.

So we've got some ice cubes there, which are solid water melting to form liquid water.

The melting point of water is naught degrees Celsius.

And there we've got the melting point of wax and steel.

And you can see there's quite a difference there.

Different substances have different melting points.

In a solid, the particles are held together by the forces of attraction.

At the melting point, the particles are moving fast enough to partially overcome these forces of attraction.

They start to move away from their fixed positions and past each other.

Melting point is the property of a substance.

It doesn't depend on the size of the object that is melting.

So here we've got two containers with ice cubes of different sizes, and the ice cubes will melt at naught degrees Celsius no matter what size they're.

Let's have a check.

Which of the following statements about the two steel spoons shown is correct? Well done if you said C, 'cause it doesn't matter about the size.

It matters about the substance.

And since they're both made from the same substance, they'll both have the same melting point.

Let's have a look at task B.

So complete the sentences below by choosing the correct word each time.

So look at the words in bold and choose the correct one.

Pause the video and come back when you're ready.

Welcome back.

Let's have a look how you did.

The temperature at which an object melts doesn't depend on its size.

It does depend on the substance it's made from.

Before a solid melts, its particles are in a regular arrangement, and they can't move past each other.

After a solid melts, its particles are in a random arrangement, and they can move past each other.

Amazing if you got those correct.

Well done.

Let's move on to the last part of our lesson, expanding liquids.

Substances at a temperature above their melting point are in the liquid state.

So water at 20 degrees Celsius is a liquid and steel at 1,600 degrees Celsius is a liquid.

In the liquid, the particles have this random arrangement, and they can start to move past each other.

If you warm a liquid up, the particles will start to move faster and they collide with each other much more often.

So in the first beaker, we've got slower-moving particles at 20 degrees Celsius.

And then we've got a beaker of liquid at 50 degrees Celsius, and you can see the particles are moving much faster there.

And the higher the temperature, the larger the space between the particles.

If we just stop the simulation for a moment, you can see the difference in spaces between the particles there at 20 degrees Celsius and 50 degrees Celsius.

The three diagrams show the particles in a liquid at different temperatures.

In which diagram is the liquid at its highest temperature? Well done if you said A, and the reason we know that is because the space between the particles is much more in A in comparison to B and C.

The expansion of a liquid can be much more easily seen when we use a narrow container, like a glass tube here.

Remember, expansion means the volume gets bigger.

So if you look at these two diagrams, we can see that the beaker with a glass tube at 50 degrees Celsius, the water has expanded much more up into the glass tube.

And a glass thermometer works in a similar way by expansion.

So we've got the bulb at the bottom there which contains the coloured liquid.

And when the bulb is warm, the liquid expands and rises up the tube.

When the bowl cools, then the liquid contracts and drops down the tube.

Let's do a quick check.

The liquid in a glass thermometer expands when it is heated.

Which of the following statements explains why? Well done if you said B.

It can't be A because the space between the particles is getting smaller, and so the liquid would take up less space.

And it can't be C because particles don't get bigger.

Let's have a look at task C.

So use a coin, a ruler, a pencil, and paper, and draw the arrangement of particles in a liquid when it is cooler and when it is hotter.

So draw a square box that is four coins high and four coins wide.

Place a coin inside the box and draw around it to represent a particle, and then use the coin to draw more particles inside the box showing the arrangement of particles in a liquid.

And then repeat this process to draw the particle arrangement for the same liquid at a much higher temperature.

Pause the video and come back when you're ready.

Welcome back.

How did you do? Let's have a look.

So your diagram should be something like these.

So the cooler liquid will have particles which are much closer together, less space.

And the hot liquid, there'll be more space between the particles.

Well done if you did that.

Excellent.

So we've come to the end of the lesson now.

Let's have a look at our summary of learning.

Heating a solid makes it expand because its particles vibrate faster and further, and so the space between the particles increases.

At its melting point, a solid turns into a liquid because its particles vibrate quickly enough to partially overcome the forces of attraction between them.

The particles start to move past each other.

Heating a liquid makes it expand because the particles move past each other faster, collide with each other much more often, and the space between them increases.

And then cooling both solids and liquids makes them contract because the space between their particles decreases.

Great job.

We've come to the end now, so I hope you feel a lot more confident about describing what happens to particles that make up a solid or liquid when the temperature increases or the temperature decreases.

Well done.

I look forward to working with you next time.