Lesson video

Hello, my name's Mrs. Navin.

And today we're gonna be talking about diffusion as part of our solutions topic.

Now some of you may have heard of that word diffusion before, and some of you may be never at all, and that's okay.

But by the end of today's lesson, all of you will be better prepared to help us answer that big question of how can we explain how substances behave? So what exactly are we doing today? Well, by the end of today's lesson, all of you should be able to explain how particles spread throughout a fluid.

Now throughout the lesson, we'll be using a variety of key words.

These include random, Brownian motion, fluid, medium, and diffusion.

Now the next slide will show the definitions of these words being used in a sentence.

You may wish to pause the video here to either read through them or record them to refer to later on in the lesson.

There's going to be three main learning cycles as part of today's lesson.

We'll be looking at Brownian motion, then diffusion and finishing up today's lesson by looking at factors affecting the speed of diffusion.

So let's get started by looking at what we mean by the term Brownian motion.

So we know that science has changed over many hundreds of years, but the ideas that we use in science have their origins and the observations people have made in the world around them.

And those observations have been improved because of the apparatus that they were using in making those observations.

And one such apparatus is the microscope.

Now during the 18th century, so that's the 1700s, technology improved such that microscopes became a really popular way of making observations, specifically observing the very small in our environment.

Now one such scientist who used the microscope quite a lot was a man named Robert Brown.

He was a Scottish botanist at the turn of the 19th century, so that's the late 1700s into the early 1800s, and he used the microscope to study plants.

As part of his observations, in 1827, Robert Brown looked at pollen grains under a microscope and he observed that these grains were randomly moving in water.

And that got him thinking, why? Why were these pollen grains just randomly moving around in water? So his initial thought was that maybe these movements were because those pollen grains were alive.

So like any good scientist, Brown decided to test his observations, but this time using small particles that he knew were not alive.

Things like really fine dust or soot particles.

And he looked at those under a microscope, and what he found was quite beguiling.

He noticed that these other particles, these non-living particles that find us the soot particles, things like that also moved about in a really similar random way, but he couldn't quite explain why.

But he noted these observations down.

And in the early 20th century, so that's nearly 100 years later in the early 1900s, work by other scientists were able to confirm and also explain the observations that Robert Brown initially made in the early 1800s under his microscope.

Now because the work of those early 20th century scientists relied upon the observation that Robert Brown first made back in the early 1800s, these random movements are now known as Brownian motion in his honour.

But what is Brownian motion? Well, Brownian motion is the random movement of particles in a fluid.

Now that fluid tends to be a liquid or a gas, and that's because the particles in a liquid or a gas can flow over each other, okay? So this random movement of particles in a fluid happens because these particles are colliding with particles in the surrounding medium.

It's a bit convoluted.

So let's take a look at this picture in a little bit more detail.

This yellow spot is tracking that random movement of just one particle as it collides with the other black particles.

So we can see that random Brownian motion of that one particle as it collides with other particles.

So for Brownian motion to occur, particles need to be found in a medium.

And this medium is also a fluid, so that's a gas or liquid material.

And it needs to be a fluid because other substances then can mix into or move through those particles.

In this picture here, the water represents the medium and the blue dye shows the random movement of another fluid through that particular medium of water.

So the key to Brownian motion is that the particles of one fluid randomly collide with the particles of another fluid here called the medium.

In this diagram, we can see that the larger blue dots represent the medium and the smaller red dots represent another fluid, and you can see them randomly colliding with each other.

So it's these collisions that result in the unpredictable movement of both particles in those fluids.

It's that unpredictable movement, that random that causes these two particles to mix together.

Time for a quick check.

True or false? Brownian motion is the orderly movement of fluid particles.

Well done if you said false.

But why? Is it because the particles movement is not impacted by the movement of other particles? Or is it because particles can collide, pushing other particles onto a new random path? Have a think and come back when you're ready to check your answer.

Well done if you said B, particles can collide, pushing other particles onto a new random path.

Let's have a go at our first task of today's lesson.

So Laura attended the theatre at the weekend and she's wondering why the smoke seemed to dance around the stage when there was no wind present.

What I'd like you to do is to use the words in the box to complete the explanation below.

May want to pause the video here and come back when you're ready to check your work.

Let's see how you got on.

So if you put the words in the correct spaces, your explanation should read like this.

The smoke particles are colliding with air particles above the stage.

When the smoke and the air particles collide, both are pushed into random unpredictable directions.

This action is known as Brownian motion.

Well done if you've got those correct.

It's a tricky thing to try and explain Brownian motion.

There's some really key ideas in there, but well done if you got that correct.

Let's move on to the next part of our lesson, looking at diffusion.

Now we said that Brownian motion is the random movement of particles when they collide with each other, but they also help us to understand some really common experiences.

For instance, being able to smell freshly baked bread or cakes from another part of the house, maybe being able to smell flowers from another part of the room or even across the field.

It can also be used to help us understand how we can see water changing colour, perhaps as you make a cup of tea.

But all of these experiences are actually as a result of diffusion.

So let's look a little bit more closely at what diffusion actually is.

So diffusion is the process of particles spreading out in a medium.

But crucially, those particles are moving from an area where there are lots of those particles until they are randomly distributed throughout the medium.

Let's look at an example.

If we think about smells that are noticed in your nose, they've reached you from the object that's making those smells in the first place.

So if we think about it from a particle model, we could look at something like this where the blue dots represent the particles causing that baking bread smell and the red dots are the air particles.

And we can see that those blue particles started really close on the left and then randomly distribute throughout the air medium until they reach the right hand side towards your nose.

So here we have an image of some incense that's burning, and the particles that cause that incense smell are represented in the centre particle diagram by the blue dots.

The red dots then represent the air into which those blue incense smell causing particles are being diffused into.

Okay, they represent the medium, those red dots.

Regardless, both of them, the air and the incense smell causing particles are fluids.

Now these fluid particles are able to diffuse because they are constantly moving and colliding with each other.

So blue particles with blue particles, red particles with red particles, but also the red and the blue hitting each other as well.

And it's these collisions that are so important because every single collision pushes each particle into a random new direction.

And if you recall, the collisions that then result in that particle moving in a new random direction is Brownian motion.

So diffusion, which is the particle spreading out from an area where there's a load of them into a random new distribution within a medium is caused and a possible because of this Brownian motion.

We can also observe diffusion taking place in substances that exist in the liquid state.

For instance, when you're making a cup of tea.

So in our particle model here we have some blue dots, which represent our medium of the water, and the red dots that represent the tea colour causing particles.

And via Brownian motions, so that's the collisions between the particles.

We can see that these tea colour causing particles are randomly being distributed throughout the medium of the water.

And as a result, as that continues all the way through the medium, eventually there'll be a fully randomly distributed and you'd now have a cup of tea that rather than looking like the one on the left now looks like the one on the right.

Let's see how you're getting on.

What name is given to the process of particles spreading out, moving from an area where there are lots of them until they're randomly spread throughout a medium.

Have a think and come back when you're ready to check your answer.

Well done if you said diffusion.

The key here is that the particles are moving from an area where there are lots of them until they're randomly spread throughout a medium.

Brownian motion may have been a choice some of you made, but the key with Brownian motion is that there's just random movement based on collisions, not the movement where there are lots of particles until they're randomly distributed.

So tricky one there.

So well done if you got it correct.

Time for another task.

So on his way to his history lesson, Lucas could smell cupcakes.

The food technology room though is at the other end of the corridor from the history classroom.

What I'd like you to do is to put Lucas's thoughts into a sensible order to explain why he can smell the cupcakes.

Here are his thoughts.

So you may want to pause the video whilst you have a think, put them in order, and come back when you are ready to check your work.

Let's see how you got on then.

So from the very start, we should have, there are many particles causing that cupcake smell in the food technology room.

As the food technology door opens, they collide.

So they, being the cupcake smell particles, collide with the air particles in the corridor that Lucas is travelling through.

The particles then spread throughout the air in the corridor via Brownian motion to where there are fewer particles causing that cupcake smell.

So they're randomly moving from where there was a lot of those particles to where there were fewer of them.

This entire process then is known as diffusion.

Well done if you manage to get those in the correct order.

It can be a quite a tricky thing to do when these processes and the sentences sound very similar.

So you had to take a bit of time with that.

Very, very well done though if you managed to even get one or two of those sentences in the correct place.

Very well done.

Okay, let's move on to our final learning cycle for today's lesson.

Looking at the factors affecting how quickly diffusion takes place, there are a variety of factors that are going to affect how quickly particles undergo diffusion.

The ones we're gonna look at in today's lesson are the number of particles present, the size of the particles undergoing diffusion, but also the amount of energy that those particles have when they collide, and that will be able to look at by checking the temperature of those substances.

Now the main thing to remember when we look at these factors is that this is going to affect the particles of both the medium and the diffusing fluid.

So all of the particles that are involved in diffusion, both the fluid undergoing diffusion and the medium through which that fluid is diffusing.

Now what we need to remember as we go forward here is that diffusion depends on collisions that are occurring randomly via Brownian motion.

So we're talking about collisions taking place between the fluid particles themselves, between the medium particles themselves, and the collisions between fluid and medium particles.

So there's lots of collisions taking place here.

But in general, if you have fewer particles, there'll be fewer collisions; and if there are more particles, there are more collisions.

An easy way of thinking about this is maybe if you're going down a corridor.

If there's fewer people in the corridor, there's less chance of colliding with someone.

And if there's more people in the corridor, there's more chance of colliding with someone.

Now, because diffusion depends on collisions in order for it to take place, it stands to reason that the more particles you have, the faster the diffusion takes place.

The key here though, is that it happens to a point.

Because once you start putting too many particles into that medium, either from the medium or from the diffusing fluid, they can actually get in the way of each other.

They can prevent that movement from one area where there are a lot of particles to be randomly distributed throughout that medium.

Again, if we go back to that corridor idea, if there's too many people in that corridor, it's gonna be really difficult for you to get through it from one end to the other, right? So there is a limit to the number of particles that we can add before diffusion is actually slowed down.

Now in general, the size of the particles will also impact the speed at which diffusion takes place.

So the smaller particles tend to diffuse a little bit quicker because they're more lightweight and they're gonna be able to travel a little bit faster because of that.

The larger particles are gonna require collisions that have a lot more energy in order for them to move at speed that's gonna allow them to travel from one end of our diagram to the other.

So those larger particles tend to have slower diffusion because those collisions need to be more energetic.

There needs to be more force behind them for them to be able to move.

So if my larger particles need more energy or more force, then the next thing I'm gonna think about is how to give it to them.

And that's how our temperature comes into play here.

So an increase in temperature means that particles are gonna have more energy.

And if those particles have more energy, it's likely that they're moving faster.

Those particles are gonna collide more often, and they're also gonna be colliding with a little bit more force.

So that means that an increase in temperature is gonna result in faster diffusion.

We have here a diagram already showing that these particles at air a low temperature, so they're a lot colder, and the diffusion is a lot slower.

Their particles are moving, they're still colliding, but those particles aren't diffusing or moving throughout that medium very quickly.

If I increase the temperature here, so those particles have just a little bit more energy, so they're moving a little bit more quickly, they're colliding a little bit more often and colliding with a little bit more force, we can see that more of those blue particles, the diffusing fluid, are able to move through the medium, represented by the red particles, and reach the right hand side of our diagram a little bit more quickly.

And not only that, but more of those diffusing particles are making it to the other side and throughout that medium.

Let's check and see how you're getting on.

True or false? Having more particles involved in diffusion always speeds up the process of diffusion.

True or false? Well done if you said false.

But why? Is it because more particles mean there are fewer collisions pushing particles into new positions, therefore diffusion is faster? Or is it that if there are too many particles colliding, this can make it difficult for particles to spread out or diffuse very quickly? Have a think and come back when you're ready to check your answer.

Well done if you said B.

If too many particles are colliding, this can actually make it difficult for particles to spread out very quickly.

Well done if you managed to get that correct.

Let's try another one.

In which of these temperatures of water would the tea diffuse the fastest? 60 degrees centigrade, 75 degrees centigrade, or 90 degrees centigrade? Well done.

90 degrees centigrade would make the tea diffuse the fastest because those particles would have the most amount of energy and be able to randomly move throughout that medium a lot faster.

Well done if you got that correct.

Time for our final task of today's lesson.

So Sam's bus passes a sewage treatment works on the way to school.

And the smell from the facility is not very nice, but it's definitely more noticeable in June than it is in February.

And what I'd like you to do is explain why that is.

Why is it more noticeable in June than in February? Try to use some of our ideas that we've talked about in today's lesson in your explanation.

Remember that when we say explain, we'd like you to use that word because as part of your answer.

And what you might like to do is pause the video here while you get to your ideas down on paper and come back when you're ready to check your work.

Let's see how you got on.

So the smell is more noticeable in June because of a variety of reasons.

So your answer may have included a few of these points.

That is identifying that the temperature of the air is actually warmer in June than it is in February.

That means at higher temperatures the particles have more energy and are able to collide more often.

This means that the diffusion of the particles causing that sewage smell happens faster when the temperature is higher in June rather than it is in February.

That was a tricky, tricky question.

Well done if you got any of those ideas into your answer and incredibly well done on today's lesson so far.

Well done.

So what did we learn in today's lesson? Well, we learned that particles released into a fluid, that's a gas or a liquid, can move around due to Brownian motion.

And Brownian motion is caused by particles randomly colliding into each other.

We also learned that diffusion happens because of Brownian motion, and diffusion is specifically the movement of particles from an area where there are a lot of them until those particles are randomly distributed throughout a medium.

We also learned that there are a variety of ways in which to cause diffusion to happen faster or slower.

And that speed can be affected by the number and size of the particles that are colliding, as well as the amount of energy that those particles have when they collide.

I hope you've had a good time learning with me today and to see you again soon.