Lesson video

Hello, my name is Mrs. Collins, and I'm going to be taking you through the learning today.

This lesson forms part of the topic, Heating and Cooling, and is called Cooling, so let's get started on the learning.

During today's lesson, you will learn how to describe and explain what happens to very hot water as it cools down.

Here are the keywords for today's lesson, surroundings, cooling rate, cooling curve, and dissipate.

Pause the video here, read through those descriptions, and write down any notes you feel you need to.

Today's lesson is split into three separate parts.

We're going to discuss cooling down, cooling curves, and energy.

So let's start with cooling down.

Now remember, a hot drink cools down because its particles collide with slower particles in the surroundings, and we talked about that the last lesson.

So you've got slower particles in the surroundings of the air, which is at room temperature, and faster-moving particles in the drink, and remember, we've got gas particles there in the air, and then we've got water particles there in the coffee.

Collisions with the particles in the drink and the surroundings slow down the particles in the drink.

So remember, when a very fast particle hits a slower-moving particle, the slower-moving particle increases in speed, and the very fast particle decreases in speed.

So the air particle speeds up and the drink particle slows down after the collision, you can see that happening in the diagram.

So the hotter the drink, the more often the particles collide, and so the quicker it will cool down.

So the more collisions there are, the faster the rate of cooling.

So you've got a hot drink there and a very hot drink, and you can see there are more collisions in the very hot drink.

So here's a question based on that learning.

How often do the particles in each beaker collide? So look at the volume, the volume is the same, so we're interested in the temperature.

So we've got beaker 1 and beaker 2.

So pause the video here, answer the question, and I'll see you when you're finished.

Welcome back, so the answer is particles in 1 collide less often than those in 2, and that's because the particles in 2 are at a higher temperature.

Well done if you got that correct.

So to find out how quickly an object cools, we can measure the fall in temperature over time.

So here we've got an example, we've got hot coffee.

We measure the temperature at 0 minutes.

So that was at the start and we found it was 89 degrees Celsius.

And then we measured it one minute later and it had fallen, the temperature had fallen into 85 degrees Celsius, and that's a drop of 4 degrees Celsius.

So the fall in temperature over the first minute is a 89 degrees Celsius minus 85 degrees Celsius, which gives us a value of 4 degrees Celsius.

Now we've got a second drink, this time, we've got some hot tea, and the starting temperature was 76.

The final temperature after a minute was 73.

So we can see that that's a temperature decrease of 3 degrees Celsius.

So this shows us that the coffee has a higher cooling rate, so the rate that it cools down is faster than the rate that the tea has cooled down.

So here's a question based on that learning.

Which of the following has the highest cooling rate? So look at those different statements, decide which one's got the highest cooling rate.

So pause the video here, answer the question, and I'll see you when you're finished.

Welcome back, so the answer to that question is coffee cooling from 77 degrees Celsius to 70 degrees Celsius.

And that's because that one has cooled by 7 degrees Celsius.

The tea, for B, has cooled by 5, and the hot water at the bottom, has cooled by 8, but it's cooled by 8 in two minutes.

So in one minute, it's likely to have cooled by 4, although we'll see later on that might not be the case.

So be careful, you need to look at the length of time as well as the temperature.

So well done if you've got that right.

Now, you are going to be investigating the cooling rate for very hot water from a kettle, and your apparatus is going to be set up something like this.

So you've got a clamp and stand, which is holding a thermometer in place, a beaker of hot water from the kettle, and you're going to monitor the temperature over time.

So remember, glass thermometers are fragile and should be held gently in the soft claws of the clamp.

And setting up the equipment before you pour in the hot water from the kettle reduces the risk of burns.

Here is a question based on that learning.

So when investigating the cooling rate for very hot water, how should you use the thermometer safely? So pause the video here, read through those statements, answer the question, and I'll see you when you're finished.

Welcome back, so hopefully, you recognised that there are a couple of correct statements there, you've got set it up before adding hot water to the beaker, and hold it gently in a clamp with the soft inside edge.

So the soft inside edge is actually holding onto the thermometer.

So well done if you got that correct.

We're now going to do Task A.

So first of all, you're going to predict whether you think the cooling rate of hot water will be higher, lower, or the same after 5 minutes, so that's the rate it's cooling.

Explain the reason for your answer to Question 1, for your prediction.

And then you're going to use the equipment shown to measure the starting temperature of hot water, and it's a temperature every minute for 5 minutes as it cools, and compare your cooling rate from the first minute to that at the last minute.

So remember the rates, we're looking at the difference in temperature over time.

So were your prediction and explanation correct? Try to write a better explanation than your first one.

So pause the video here, make your prediction, carry out the practical, and I'll see you when you're finished.

Welcome back, so let's have a look at my answers then.

So I predict the cooling rate will get lower as the hot water cools, and the particles don't hit each other as often when the water is cooler.

We've got a set of sample data there to show that's the case.

Over the first minute, the temperature drops by 4 degrees Celsius, in the last minute, it drops by 2 degrees Celsius, which is less than at the start.

The particles in cooler water move less quickly and collide with particles of air less often, so they slow down more slowly.

So well done if you've got something similar, obviously your prediction may have been different to mine, but hopefully, your data shows that that cooling rate is slowing over time.

Let's move on to the second part of the lesson then, cooling curves.

So a cooling curve is a line graph that shows how the temperature of an object changes with time.

So you've got time on the X axis and temperature on the Y axis, and you can see here that the temperature is dropping over time, it's falling over time.

So the cooling rate is shown by the slope.

So here we can see, we can work out, we can actually calculate the slope or we can look at it and see how steep it is.

So this is the slope, the fall in temperature over 1 minute.

So how much has the temperature dropped over that minute? So the steeper the slope, the higher the cooling rate.

So if you look at the second graph there, you can see the line is steeper, that green line is much steeper than in the first graph.

So there's a larger fall in temperature over the same length of time.

The cooling rate for a hot drink gets smaller as it cools, which we saw from the practical in the first part of the lesson, so the slope of its cooling curve decreases over time.

So we can see that on this here, we've got a curved line instead of a straight line, so the steepness of the line is changing over time.

And that striped line there, the dotted line there is showing you room temperature.

At the start, when the drink is very hot, its particles collide with particles in the air very often.

The collisions speed up the particles of the air and slow down the particles of the drink.

So those particles are moving very quickly.

The cooling rate is higher for a greater temperature difference.

So there's a much bigger temperature difference there between the drink and room temperature, and you can see that on the graph.

As the temperature of the drink drops, so do its particles' speed, and so its particles collide less often with particles of the air.

And you can see this on the graph because as we move down the graph, the slope is less steep and the gap between the green line and room temperature has decreased, and this means the particles are going quite slowly, there are fewer collisions, and therefore, the rate is much slower.

And then close to room temperature, the particles of the drink are moving just a little bit faster than the particles of the air.

Collisions don't change their speed very much.

So the particle speed is medium at this point, and the slope is even less steep, and the difference in temperature between that line and room temperature is much less.

So the cooling rate is lower, the closer to room temperature that you get.

So here's a question based on that learning.

What happens to the slope of the cooling curve as a hot drink cools down.

So pause the video here, answer the question, and I'll see you when you're finished.

Welcome back, so the answer is the slope gets less steep.

So well done if you got that correct.

How warm the surroundings get depends on the number of particles in the surroundings compared to the hot drink.

So here we've got to remember those particles in the air, hitting the particles in the drink and colliding with the particles in the drink.

And because they're moving more slowly when they hit them, their speed increases, and the speed of the drink particles decreases.

So air particles collide with each other, as well as particles in the drink, and you can see that happening in the diagram there.

For a hot drink in a big room, the temperature rise of the surroundings is usually too small to measure.

There are so many air particles in the air, it's actually, the temperature increase is negligible, it's very, very small, and we can see that in the graph here, so we've got the coffee temperature decreasing over time, and the rate of that decrease, decreasing over time too, and then you've got air temperature with a very, what we'd call negligible or incredibly small increase in temperature, so small that we can hardly measure it.

So the smallest change a thermometer can measure is the smallest division on its scale, and the increase in air temperature is so small that it doesn't even go up one graduation on the thermometer.

So here's a question based on thermometers and reading thermometers, what is the smallest change in temperature that the thermometer below can measure? So we've increased the size of the scale there, the graduation, so you can see them, what is the smallest change in temperature that that thermometer can measure? So pause the video here, answer the question, and I'll see you when you're finished.

Welcome back, so the answer to that question is 1 degrees Celsius.

So you can see, each individual graduation, each individual line is worth 1 degree Celsius, so that's the smallest change in temperature that that thermometer can measure.

We're now going to do Task B.

The cooling curve below is for a hot drink in a room.

What does the dotted line represent? How does the graph show the cooling rate is decreasing? Explain why the cooling rate decreases in terms of particle collisions, and why is it hard to measure the change in temperature of the room? So pause the video here, answer the questions, and I'll see you when you're finished.

Welcome back, so let's go through those answers then.

So the first one, the dotted line shows room temperature, remember.

The decreasing cooling rate is shown by the decreasing slope of the graph.

So when you're answering this question, talk about slope or gradient of the slope, gradient of the line.

As the drink cools, the speed of its particles decreases.

The particles in the drink collide less often with each other and with the particles in the surroundings, so the cooling rate decreases.

So it's all about the speed of the particles for Question 3.

Question 4, there are many more particles in the surroundings than there are in the drink.

The speed of each air particle will only increase a little, and so the temperature of the air will only increase a small amount.

So well done if you got that correct.

Let's move on to part three of the lesson then, energy.

So if an object has energy, it can do a useful job.

So hot water can keep us warm.

It has more energy than the same amount or the same volume of cold water.

So if we look here, we've got two hot water bottles, and each hot water bottle contains the same volume of water.

But we can see that one has a temperature of 20 degrees Celsius, and the other has a temperature of 60 degrees Celsius.

So because they've got the same volume, the hot water bottle that's at a higher temperature has more energy.

So warming up water gives the water more energy, and when it cools, the energy decreases.

So we've got that shown in the diagram here with the kettle.

So the kettle's warming up the water, it's heating the water, so the energy in the water is increasing.

And then in the second example, we're allowing the energy to dissipate or to transfer to the surroundings, and then dissipate into the environment.

So the energy isn't lost, it's transferred to the air around the water, and then dissipated to the rest of the surroundings, as I mentioned.

So here, we've got the water particles, look, with the energy, so the air particles are colliding with those water particles.

The speed of the air particles is increasing, and the speed of the water particles is decreasing because the water particles start off with more energy, and then those air particles go on to collide with other air particles, and this dissipates the energy around the environment.

So three pupils are discussing where the energy that a cup of tea goes when it cools down, which pupils are correct? So read carefully through their statements, decide which students are correct.

So pause the video here, answer the question, and I'll see you when you're finished.

Welcome back, so let's have a look at the answer to that question then.

So Jacob is correct.

The energy is spread out, and Izzy is also correct, the energy dissipates.

Okay, so the energy is dissipated.

So well done if you've got that correct.

Here is Task C.

So complete the following sentences using the words "increases" or "decreases" to describe what happens to the energy.

So pause the video here, answer the task, and I'll see you when you're finished.

Welcome back, so the answers to that question, raising the temperature of the water, increases the amount of energy it has.

As the water cools, its energy decreases.

The energy isn't lost.

The amount by which the water's energy decreases is the same as the amount by which the energy of the surroundings increases.

So well done if you got that correct.

Here's a summary of today's lesson.

When fast-moving particles in a hot object collide with slower particles in the surroundings, the object cools.

As they collide, the speeds of the particles in the hot object decrease and the particles in the surroundings speed up.

As the object's temperature drops, the difference in temperature between the object and the surroundings decreases, and so the cooling rate decreases.

Smaller differences in the speeds of particles causes smaller changes in the speeds when they collide.

As hot objects cool, energy is dissipated to the surroundings.

They warm up, but the temperature rise is often too small to measure.

So thank you very much for joining me for today's lesson.