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 "Mixing substances of different temperatures." So let's get on with today's learning.

During today's lesson, you will predict the resulting temperature when two substances of different temperatures are put together.

Here are the keywords for today's lesson.

Volume, particle model, temperature, and collide.

Pause the video here, read through those definitions, and make any notes you feel you need to.

Today's lesson is divided into three parts.

"Mixing hot and cold water," "Particles in a mixture of hot and cold water," and "Adding ice to a drink." So let's start with "Mixing hot and cold water." Now you'll know from your maths lessons how to calculate volume of a cube and volume of a cylinder, but remember that volume is the amount of space an object takes up and it's often measured in centimetres cubed in science.

There we can see we've got a cube of solid steel and we can measure the volume of that cube by multiplying together the length, the breadth, and the height of that cube.

And then we have 100 centimetres cubed of liquid water this time inside a beaker and it's a cylinder, but we're using that graduated measuring scale at the side to show us that we've got 100 centimetres cubed of liquid water.

You're going to be mixing together volumes of water at different temperatures today to find the resulting temperature, and you can measure the volume fairly accurately using the scale on the beaker for this experiment.

So if we took these two beakers here, we've got the first beaker and the second beaker and you can see they've got different volumes of water.

So what I'd like you to do is read the scale, that's the graduated scale, work out the volume of water in each of those beakers.

What would the total volume of water be at the end? So in the first beaker, we've got 50 centimetres cubed of water.

In the second beaker, we've got 150 centimetres cubed of water.

So if we were to mix those together, we'd end up with 200 centimetres cubed of water.

So for accurate temperature results, take measurements quickly because the water will reach room temperature if left for long enough.

Here's a question based on that learning.

Which of the following beakers of water will reduce in temperature if left in a room at a temperature of 20 degrees Celsius? So you need to look at the room temperature, 20 degrees Celsius, and then look at the temperature of each of those beakers of water, and which one will reduce in temperature if left at room temperature? So pause the video here, answer the question, and I'll see you when you're finished.

Welcome back, so the answer is A, because that's the only beaker that is above room temperature.

Beaker B will increase in temperature until it reaches 20, and beaker C is already at 20 degrees Celsius.

We are now going to do Task A, and this is a practical activity.

So you're going to measure the temperature of different mixtures of water.

You're gonna use cold water at room temperature and hot water from a kettle.

So here you've got different volumes of cold water and different volumes of hot water that you're going to mix together and then you're gonna measure the resultant's temperature, so that's the temperature when the two liquids are mixed.

So pause the video here, carry out the experiment, and I'll compare your results to my results when you're finished.

Welcome back, so let's have a look at my results compared to your results.

This is a sample set of results that may well be similar to the ones that you gained.

Obviously all results are going to be slightly different, but these are the results that I obtained.

Now we're going to move on to part two of the lesson.

This is "Particles in a mixture of hot and cold water." Now you will remember the particle model, you've learned about this already, and this is the idea that all objects are made of tiny particles and these particles are constantly moving.

And we've got some great images here, so we've got water at room temperature and you can see the particles there moving, passing by each other and moving quite vigorously.

And then you've got a room temperature steel and you can see here that in the solid the particles are vibrating, they're in a fixed position, but they are still moving.

So in a solid, remember, the particles have a regular arrangement, and in a liquid the arrangement is random.

And we've used some marbles here to show that arrangement.

And as it says, just bear in mind that this is a model and the spaces between the particles has been made bigger than it really is to make it easier to see their arrangement.

So just bear that in mind when you're looking at these diagrams. So we've got a regular arrangement there in a solid, so the particles are all in nice neat rows, and then a random arrangement in the liquid.

And remember in a liquid, when you're drawing diagrams in a liquid, then normally about 90% of them should be touching each other, and we've obviously got larger gaps than that in the liquid and we've got larger gaps in the solid.

So here we've got representation of those marbles now at a higher temperature.

So the higher the temperature, the faster the particles move and the more often they will collide with each other.

So we've got that model at a lower temperature and then at a higher temperature.

And you can see at a higher temperature the particles are moving much more quickly and they're colliding much more often with each other.

And again, remember, this is only a model, so the space between those particles is bigger than it would be in reality.

And again, another model.

So when particles collide with each other, they push on each other.

So we've got before the collision and after the collision.

So look at the slow-moving marble there, the yellow one, and then the bluer one is hitting it, it's travelling at a much faster speed.

So look at the speed of those marbles before and after the collision and think about what's happening as a result.

So collisions result in the faster particles slowing down and the slower particles speeding up.

So we've got a diagram here of those marbles from the previous example.

So before the collision, we've got that fast-moving marble there hitting the slower-moving marble, and during that collision they push against each other.

So look at the direction of the arrows there, so they're both pushing against each other.

And then after the collision, they're both travelling at a medium speed.

So the slower marble has increased in speed and the faster marble has decreased in speed.

So here's a question based on that learning.

So a fast particle, X, and a slow particle, Y, are travelling in the same direction.

What happens when they collide? Now you can see X is a faster particle, faster-moving particle, 'cause the lines there, the grey lines before it are slightly longer.

So what happens when these two particles collide? 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 C.

So particle X will slow down and particle Y will speed up, so well done if you've got that answer correct.

So two times the volume of liquid contains two times the number of particles.

In the first one we've got 100 centimetres cubed of water, and in the second one we've got 200 centimetres cubed of water.

Now there were a lot of zeros there, so don't worry too much about the number of zeros, but in the first one you can see we've got 3 followed by all those zeros, and then in the second one we've got 6 followed by the same number of zeros.

So there are twice as many particles in that 200 centimetres cubed of water than in 100 centimetres cubed of water, and that's what you need to remember.

So here we've got a graph that's gonna represent what's happening to the speed of the particles when we mix liquids together.

So equal volumes of hot and cold water contain almost equal number of particles.

So the particles in hot water, however, are moving faster.

So we've got the same number of particles but the particles are moving much more quickly.

And in these diagrams, that block, so we've got a single block, that represents a volume of 50 centimetres cubed.

So in this example, we've got 100 centimetres cubed of particles in cold water and 100 centimetres cubed of particles in hot water.

When we mix together these two liquids, the faster particles slow down and the slower particles speed up, remember.

So this means the resulting temperature will be in between the two temperatures, the hot and the cold water.

So remember when we were talking about particles earlier and we said faster particles slow down, slower particles speed up? And remember, in these liquids the particles are all moving around and they're colliding with each other.

So what happens is when we mix together these two liquids, we end up with a temperature in between the two, you can see in the column there, so we've got 200 centimetres cubed of water at a medium particle speed, so the temperature and the speed of the particles is in the middle.

So these are the particles in the warm water.

If the volume of hot water is greater than the volume of cold water, there are more fast-moving particles than slow-moving particles.

So remember when we talked about doubling the volume doubles the number of particles? Here we've got three times as many particles moving quickly as those particles moving slowly.

So if we're to mix these two together, obviously the resulting temperature will be closer to that of the hot water because we have more fast-moving particles present before we mix the two liquids together.

So here the particles are in quite hot water.

Reversely, if the volume of cold water is greater than the volume of hot water, there are more slow-moving particles than fast-moving particles.

So this time we end up with particles that are quite slow and they're in quite cold water.

So here we have a true or false question based on that learning.

If you mix 190 centimetres cubed of water at room temperature with 10 centimetres cubed of hot water, the resulting temperature will be close to room temperature.

Is that true or false? And explain your answer.

So pause the video here, think about your answer before you write it, and I'll see you when you're finished.

Welcome back, so the answer is true, and that is because there is much more room temperature water than hot water, and this means there are many more slower-moving particles than faster-moving ones at the start.

The increase of temperature of room temperature water will be very small when we mix it with the hot water.

So well done if you've got an answer that's similar to that.

Now we're going to do Task B and this time it's a fill in the gaps, so fill in the gaps using the words hot or cold to explain what happens when hot and cold water are mixed.

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

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

So when mixed, faster particles in the hot water collide with the slower particles in the cold water.

Particles in the cold water speed up and those in the hot water slow down.

The resulting temperature is between the temperature of the hot and the cold water.

If we start with more hot water than cold, the resulting temperature will be closer to the original temperature of the hot water.

So well done if you got that correct.

Let's move on to part three of the lesson, "Adding ice to a drink." So, as you know, we can cool a drink at room temperature, which is normally 20 degrees Celsius, using a solid cube at freezer temperature, which we're going to say in this instance is -19 degrees Celsius.

And we've got different types of ice cube.

You might be used to a cold water cube, which is where we freeze water, but you can also put things like steel into a freezer and cool that down to the same temperature.

So to start with, the particles in the drink are moving much more quickly than those in the cube, and we've got those great diagrams again here showing that happening.

So we've got faster-moving particles in that liquid than the slower-moving particles in the solid.

So we're making a comparison between the two, so we're saying faster and slower, so we're using comparative language.

Now, when they're put together, collisions between the particles increase the temperature of the ice cube and decrease that of the drink.

So remember, these particles are colliding with each other and the faster-moving particles will slow down and the slower-moving particles will speed up.

We did that with marbles earlier, if you remember.

Here's a question based on that learning.

Again, true or false.

So when a steel cube from the freezer is added to a drink at room temperature, the steel cube's temperature decreases.

Is that true or false? And you need to explain your answer.

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 false.

And this is because the particles in the drink are moving more quickly than the particles in the steel because the temperature of the drink is higher.

The particles in the drink collide with the particles in the steel, causing them to speed up, and this increases the temperature of the steel.

So well done if you've got an answer similar to that.

Now when a cube of frozen water or ice is added to a drink at room temperature, the increase in the ice cube temperature can cause it to melt.

So it doesn't get hot enough for the steel block to melt, but it can for the ice cube to melt.

So if we look at the scale there, we've got a number line.

We're going from approximately -19 all the way up to room temperature, which is 20.

And remember, the melting point of ice is the same as the freezing point of water, so that's 0 degrees Celsius.

And we can see the particles moving here.

So in the solid ice, before it melts, the particles are in very much fixed positions and they're vibrating around those fixed positions and they're in nice and neat rows.

And then we get to the liquid there, we can see the particles are moving much more freely, they're passing each other and they've got more energy, so they're moving much more quickly.

The outside of the ice cube is the first bit to melt, so particles at the edge of the frozen water are the first ones to move into the drink.

And you can see here the water particle in the ice cube, and you've got a water particle at the edge of the ice cube moving into the drink, and there you've got the particle in the drink.

So the darker blue is an ice cube and the lighter blue is the particle in the drink, and you can see those edge particles are moving away.

Eventually, all the ice melts and both the water and the drink will be in the liquid state.

The mixture will be at a lower temperature than the initial temperature of the drink.

So if a cube was added that was at a higher temperature than the drink, it would warm the drink up.

So this time we've taken the steel cube and we've heated it and then we've added it to the drink at room temperature, and what happens is the drink warms up and the cube cools down.

So which of the following situations will cause a drink to cool down? So read through those statements carefully, answer the question, pause the video here, I'll see you when you're finished.

Welcome back, so we're looking for adding a cube that is at a lower temperature than the drink.

So the steel at freezer temperature will cool down the water at room temperature 'cause it's at a lower temperature.

The water at freezer temperature will also decrease the temperature of the milk because it's at lower temperature.

And the steel at room temperature, which is 20 degrees Celsius, will reduce the temperature of the milk because the milk is at 60 degrees Celsius.

So well done if you've got that correct.

We're now going to do Task C, and for this task, again, you're going to fill in the gaps.

So fill in the gaps using just the words steel or water to explain why water at room temperature will warm up if we add a hot steel cube to it.

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

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

So at the beginning, particles in the steel are moving faster than those in the water.

When we add the steel cube to the cold water, collisions between the particles increase the speed of particles in the water.

The same collisions reduce the speed of particles in the steel, and the temperature of the water increases and the temperature of the steel decreases.

So well done if you got that correct.

Here is a summary of today's lesson.

If we add a cold solid or liquid to hot water, the faster-moving particles in the water collide with the slower-moving particles in the added solid or liquid.

Collisions between particles speed up slower-moving particles and slow down faster-moving particles.

The temperature of a cold solid or liquid increases when added to hot water and that of the hot water decreases.

A cube of frozen water or an ice cube can be used to cool a drink.

As the drink cools, the temperature of the ice cube will increase.

The ice may melt and mix into the drink.

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