Hello, my name's Mrs. Niven and today we're going to be talking about Dissolving as part of our topic on Solutions.

You may be familiar with the idea of something being able to dissolve, based on your previous learning, but today we're gonna be looking at this idea in a little bit more detail, in the hopes that we're able to better answer that big question of how can we explain how substances behave.

So by the end of today's lesson, you should be able to use the particle model, to explain not only how a solution forms, but how it differs from a suspension.

Throughout the lesson, we'll be using a variety of keywords and those include, dissolve, force of attraction, melting, suspension, and insoluble.

Now the next slide shows the definitions of these keywords being used in a sentence.

You may wish to pause the video here to either read through each of these, or perhaps make a quick note of them so you can refer to them later on in today's lesson.

Now today's lesson we'll form three different learning cycles.

We'll first be looking at describing the process of dissolving, again, using that particle model.

We'll then move on to thinking about some key features of dissolving and how it might differ some other processes that you've learned of before.

And then finally, we'll finish off today's lesson by looking at what a suspension is.

So let's get started by looking at the process of dissolving.

So a substance is said to have dissolved once its particles have separated and spread throughout the particles of a solvent.

And when that happens, you can no longer see the substance or the solute.

And we have an example here where we have a white solid solute being put into a colourless solvent.

And when that solute enters the solvent, the particles start to spread out and perhaps once they've been mixed together a little bit, eventually you end up with a clear colourless solution.

What we wanna be able to explain by the end of this learning cycle, is why solute particles separate and why do they then spread out.

Now in order to answer that question of why a solute might dissolve, we really need to consider how those solute and solvent particles interact with each other in order to do that explaining.

We also can use that interaction to help us explain why a substance maybe dissolves in one solvent and not another.

Before we look at the interactions between a solute and solvent particle, it's important to remind ourselves about the fact that there are forces of attractions acting as a pulling force between the particles in both of those substances.

So here we have how the particles might be arranged in a solid solute, and the strength of those forces of attraction are shown by these thick arrows.

In the solvent then, which here is a liquid, we have a particle diagram again showing of a liquid, and we can see that those forces of attraction are slightly weaker than those in our solute, again showing by a thinner arrow.

Now when a solute is put into a solvent, its particles become surrounded by the solvent particles, and when that happens, the solute and solvent particles are now able to interact with each other.

So let's take a closer look at what that interaction involves.

Firstly, we need to remember that brownian motion is a random movement of particles in a fluid, so that's a liquid or a gas.

And that movement is due to collisions between those particles and the particles of the medium.

So in this instance, it would be the particles of the solvent.

So when a solute and solvent are put together and mix, those particles will collide with each other due to brownian emotion.

Here we have a simplified diagram showing the particles of a solute that's just been put into a solvent, and it also shows those forces of attraction between the solid solute particles shown here by the pink circles and the weaker force of attraction between the blue solvent particles.

Now when the solvent particles collide with the solute particles via brownian motion, there's gonna be a force of attraction that interacts between them.

And the key to dissolving, is considering the strength of that attraction between the solute and solvent particles.

Now crucially, for something to dissolve the force of attraction between a solute and solvent particles must be stronger than the force of attraction between the particles of solute themselves.

So if we look at these yellow ovals, showing where the solute and solvent particles have interacted and collided with each other, we can see that those thicker arrows are representing a stronger force of attraction between the solute and solvent particles, and these then will be able to dissolve.

Once that force of attraction, that stronger force of attraction has developed between the solute and solvent particles, the force of attraction allows the solvent particles to pull the solute particles away from the rest of the solute.

Now that sounds a little bit complicated, but if we look at our diagram again, those yellow ovals are showing the movement of our pink solute particles away from their original positions because that stronger force of attraction exists between the solute and solvent particles.

Now what happens next is that because of brownian motion, those particles are constantly moving and colliding with each other.

And other solvent particles then move into the gaps that were left behind by the solute particles having been pulled away, and we can see those here represented by those yellow circles.

What happens with that then, is now a new force of attraction can possibly form between these solvent particles and the solute particles that were left behind.

And we can see here when that happens that these forces attraction are strong because of that thicker arrow.

Those are still strong forces of attraction between the solute and new solvent particles that moved into the gaps left from before.

The thing about brownian motion is that there is no end.

They are constantly moving, and because of this continued brownian motion then, those solute particles are being pulled apart and spread throughout that solvent.

What that means then now is that those solute particles are no longer grouped together.

They're individually spread throughout that solvent, throughout the medium.

That means they can no longer be seen.

That means our solute has now dissolved.

Time for a quick check to see how you're getting on.

Some salt is dissolved in water, but which particle diagram do you think is the best representation of the solution that forms? Have a think, pause the video and come back when you're ready to check your answer.

Well done if you chose diagram C.

Diagram C is the best representation because it shows the individual salt particles distributed rather randomly, but fairly equally across the whole of that diagram.

They're not grouped together.

If they're grouped together, they'll be able to be seen and we don't want to be able to see them for something to be dissolved.

So well done if you chose C.

Let's try another one.

At room temperature, sugar is in the solid state and carbon dioxide is in the gas state.

Now both of those substances, sugar and carbon dioxide can dissolve in water.

Which solution or solutions could this particle diagram represent? These are your options.

Have a think, pause the video maybe if you'd like to discuss your ideas with the people nearest you and come back when you're ready to check your answers.

Well done if you chose D.

This diagram could represent both a solution of sugar and water, or it could represent a solution of carbon dioxide and water.

And that's because those black circles could represent our solute, which are randomly distributed throughout our substance.

And because we don't have a key telling us exactly what that black circle represents, it could be either sugar or the carbon dioxide.

So well done on a trickier question.

Time for our first task of the lesson.

Now I'm gonna show you in a moment some students and each of them have a statement about dissolving.

What I'd like you to do is to put them into the correct order that will explain how dissolving actually occurs.

So here are the statements and our students, what I'd like you to do is put them in order, but you might wanna pause the video here, have a think, come back when you're ready to check your work.

Let's see how you got on.

Personally, I would've started with Andeep whose statement was, a solute is put into a solvent and their particles collide with each other.

Next comes Lucas.

His is an important one because he's talking about the force of attraction between a solute and solvent particles is stronger than the forces between the solute particles.

Next up is Sophia with solute particles are pulled away and solvent particles fill the gaps.

Then Izzy with new forces of attraction between other solute and solvent particles form.

So that was the big key there, that there are new forces between other solute particles, meaning that she should be after Sophia.

And finally then we have Alex who says solute particles are spread throughout the solvent and can no longer be seen.

That was a tricky one, you had to really carefully read those sentences to get them in the correct order.

So don't worry if you didn't get them all correct, but very well done for having a go.

Now that you're filling a little more comfortable using the particle model to describe the process of dissolving, let's move on to look at the key features of dissolving.

Now, a key thing to remember is that both dissolving and diffusion depend on this idea of brownian motion in order to occur.

And remember, brownian motion is when particles collide and then move in random directions.

Now dissolving occurs because the forces of attraction between the solute and solvent particles are much greater than the forces between the solute particles themselves.

Diffusion on the other hand, occurs after this dissolving process takes place.

So after the forces of attraction between the solute particles have been overcome, what you have at that point then is solute particles and quite a lot of them in one particular place.

And then via diffusion, those particles then are able to move and randomly and spread throughout the rest of that medium.

Similarly, dissolving and melting are both physical processes that require a force of attraction to be overcome.

So when a solute dissolves, it is the force of attraction between its solute particles that are being overcome.

When a substance melts, its particles are gaining energy to overcome the forces of attraction between its own particles.

Time for a quick check.

Which two processes happen because of brownian motion? Your options are dissolving, diffusion and melting.

Pause here and come back when you're ready to check your work.

Well done if you chose dissolving and diffusion.

Both of these processes rely on brownian motion to occur.

Let's try another one.

Which process results in a change of state? Is it dissolving, diffusion or melting? Well done if you chose melting, that is when something in the solid state changes into a liquid state.

Well done.

Let's look at another feature of dissolving.

Each of the samples shown in the picture below show the same number of particles, but we can clearly see that their volumes are very different.

The one here on the left is obviously a substance with a large volume taking up a large amount of space.

And the one on the right is a substance with a small volume taking up a smaller amount of space.

Now the difference in volumes is due to the size of their particles.

If we were able to zoom in, we could see that the substance on the right has very large particles, and takes up a larger space.

And then the substance on the right has the same number of particles, but because the particles are smaller, it takes up a smaller amount of space.

So larger particles, larger volume, smaller particles, smaller volume, fairly simple.

What we need to consider is that if these substances dissolve in a solvent, their particle size could impact on the total volume of the solution that's formed.

What we need to remember is that, every particle in a liquid or our solvent, has enough energy to overcome the forces of attractions with its neighbouring particles and that allows the particles to move over and around each other.

It's one of the reasons why liquids are able to flow.

Now, although they're still touching, as we can see here in this particle diagram of a liquid, this movement is gonna create some really small gaps between the particles.

And you can see those here.

These particles are still moving and because of that, those gaps are gonna be different sizes, but the gaps there are significant.

So when a solute dissolves in a solvent, its particles could actually be small enough that they fit in the gaps between those solvent particles.

So we have here the gaps still shown.

The blue circles represent our solvent particles, and if we had a specific type of solute, those particles could be small enough as shown here by the pink circles to fit within the gaps between those solvent particles.

So why do we care? Well, that could have an impact, these gaps and what fits in them could impact on the total volume of the solution that's formed as a result of dissolving.

If we were to put these two substances together, our seven centimetres cubed of solute and are 90 centimetres cubed of solvent, you might expect the volume to be round about 97, so adding them together, centimetres cubed of a solution.

But what really happens is when a small amount of solid solute dissolves in a solvent, the total solution volume actually stays pretty close to the solvent volume.

So we have here a 90 centimetres cubed solution, and that's because the solute particles can fill in the gaps between those solvent particles.

If we look at another example, when a small amount of liquid solute, so we're using a liquid dissolving into another liquid, sometimes the solution's volume then is actually less than the combined volume.

So if I look at this, I would've expected bringing 40 centimetres cubed of volume of ethanol and adding that to 40 centimetres cubed of water, I would expect the volume to be around about 80 centimetres cubed.

It actually ends up being slightly less about 78 centimetres cubed.

And that's because there's actually a stronger force of attraction between those solute and solvent particles.

And what that does is it makes the gaps between those particles even smaller, taking down the total volume to smaller than what you might've expected.

So the volume of a solution can change depending on the size of the gaps between the particles, and the forces of attraction between those particles.

Time for a quick check, true or false? Volume is always conserved when a solution is formed using a liquid solute? While done if you said false, but why is that? Is it that the total volume of a solution is always equal to the volume of its solute and solvent combined? Or is it that sometimes there are stronger forces of attraction between the solute and solvent particles making the gaps between them even smaller? Have a think and come back when you're ready to check your answer.

Well done if you said B.

The volume is not conserved when using a liquid solute, and that could be because there is a stronger force of attraction between the solute and solvent particles.

Tricky questions guys, we're getting in some nitty gritty ideas here and you're doing supremely well, really proud of you, well done.

Time for another task.

What I'd like you to do first of all, is to match the name of the process to the correct description.

Pause the video here and come back when you're ready to check your answers.

Okay, let's see how you got on.

So diffusion should be the middle description that it occurs because of brownian motion and because there are different amounts of particles in an area.

Dissolving then is the bottom description that a solute and solvent particles have a stronger force of attraction between each other than those between the solute particles.

And then melting, the key here that meant it was the top answer is that it's talking about a change of state from the solid state to a liquid state.

Well done if you matched those up correctly.

For the next part of this task, I'd like you to consider this.

We have two students notice that when salt dissolved in water, the total volume of the solution was less than the volume of the water used.

I want you to discuss their comments and then correct any mistakes that you might observe.

Firstly, we have Aisha.

She says the volume of salt used will have no impact on the volume of the solution, but Jacob says the salt particles fill up the gaps between the water particles.

Have a think, correct any mistakes and come back when you're ready to check your answers.

Okay, let's see how you got on.

We'll start by looking at Aisha's comment.

She said that the volume of salt used will have no impact on the volume of the solution.

Well, maybe, maybe not.

I would've said that the volume of salt used will have an impact on the volume of the solution.

We said earlier that only a small amount of solute might not impact the total volume of a solution, but if you use more solutes or more salt, that could actually eventually increase the solution's volume.

So that was a really tricky one.

Let's move on to Jacob's comment.

He said that the salt particles fill up the gaps between the water particles.

Now he could be correct because the solution's volume is less than the volume of the water, and that suggests that perhaps the salt particles have moved into the gaps between the water particles.

That was an incredibly difficult task.

So I hope you had some really good discussions and that you were able to gather some ideas about how we can talk about dissolving in different ways.

Okay, let's move on to the last part of this task.

So we have some salt that's been added to a beaker of water and after stirring the salt could no longer be seen.

And the students were asked, where is the salt? So some students have been discussing their ideas and these are some of the comments that they've made.

What I'd like you to do is to read to these comments and decide which student you agree with and why.

You might want to pause the video here and come back when you're ready to check your answers.

Okay, let's see how you got on.

If you recall, Aisha suggested that when the salt goes into the water, it turns into a liquid and mixes into the water.

And she's slightly misunderstood what mixing actually is and has tried to explain it in terms of the salt changing state by suggesting that it's turned into a liquid.

But when we have a change of state, the particle arrangements have simply changed and you have just that same material, and she's not really recognised that we're mixing it into a second substance into the water.

If we look at Jacob's suggestion that the salt has joined with the water and made a new type of liquid called salty water, what's happened here is that he hasn't recognised that the salt is still present in its normal form, it's just been broken up slightly.

So we've not made anything new When things are being mixed together.

Sam suggested that the salt has melted, and what he's done here is he's misused that keyword of melting.

'Cause melting is a change of state, again, changing from the solid state of how the particles are arranged to a liquid state in terms of how the particles are arranged, and that's going to have a different explanation.

Laura, on the other hand, has said that the salt breaks apart and mixes into the water.

And because of that, that's probably the closest explanation of what's happened to the salt as it's put into the water and dissolved.

So very well done, if you managed to agree with Laura.

I hope that these comments that some of our students suggested has stimulated some great conversations and you enjoyed discussing their suggestions and who you decided was the most correct.

Tricky, tricky task, very well done with having a go.

Okay, let's move on to the last part of today's lesson and we are going to look at suspensions.

If you remember at the start of today's lesson, one of the outcomes was to be able to distinguish between a solution and a suspension.

So we're gonna start with a very similar particle diagram that we use to looking at a solution by the process of dissolving.

So we still have our pink solute particles and we can see the force of attraction is quite strong between them.

It's in the solid state as the particle diagram shows.

And then we have our solvent particles represented by the blue circles here.

Now those forces of attraction between the solute particles are really important because remember, for something to dissolve, the force of attraction between the solute and solvent particles needs to be quite strong in order to overcome those forces of attraction between the solute particles.

What we can see in this particular situation is when we look at that interaction between the solute and solvent particles, the arrows are smaller, they're thinner, and that represents a weaker force of attraction between the solute and solvent particles.

When that happens, the solute is unable to dissolve and it's gonna be described as insoluble in that solvent.

There is not a strong enough force of attraction between the solute and solvent particles in this instance to overcome the forces of attraction between the solute particles themselves.

Now if a substance doesn't dissolve and is insoluble, bits of it can still be seen in that possible solvent.

That medium and the mixture that was formed isn't clear you can still see bits.

If the insoluble particles become dispersed or spread out throughout that medium.

The result is known as a suspension.

And an example is shown here where we have flour particles that's baking flour particles suspended in the water as our medium.

You're probably actually really familiar with suspensions in everyday life.

A good example here is mud that's suspended in water, or you might have seen dust suspended in air.

This is particularly visible on bright sunny days when people are probably inclined to clean a little bit 'cause that's when you can see all the dust floating around.

And you might have also seen sometimes medicines that are suspended in water, and that's one of the reasons why you need to shake up a medicine bottle before it's being used because that suspension needs to be fully dispersed within the medium before it's dispensed.

Now the thing is, insoluble particles do not remain suspended in a solution or in the medium indefinitely.

Eventually, the heavier insoluble particles will settle out of that medium.

And a really good example is something we see every day, everywhere, dust particles suspended in air particles.

Over time that dust settles and will collect on nearby objects.

A really good example is dust settling on bottles, but you've probably noticed it on bookcases, on windowsills on many different things, and it's one of the reasons why dusting, along with sweeping, is a daily, if not weekly chore, usually in most homes.

Now, how quickly those insoluble particles will settle out of that suspension depends on their size and mass.

The heavier insoluble particles tend to settle really quickly.

Things like sand, you can see that really easily if you go to the beach and the wave washes some sand up and the sand settles down quite quickly.

More lightweight or insoluble particles like flour or cornstarch can take a really long time to settle out of suspension.

So we have an example here where water is our medium and flour is suspended in it.

After 20 minutes, you can see that there is still some flour suspended throughout the medium, but a lot of it is also collected on the bottom.

Having said that, it's still not clear.

After three hours of leaving this particular mixture settled there, we can see far more of the flour has settled to the bottom of our mixture, but there still is a little bit of flour particles suspended throughout it, so it's taking an incredibly long time for those lighter weight insoluble particles to settle out of our solution.

Let's do a quick check to see how you're getting on.

True or false, suspensions contain soluble particles dispersed in a medium.

Well done if you said false, but what's the reason why? Is it that suspensions contain insoluble particles dispersed in a medium, or is it that suspensions contain soluble particles dispersed throughout a medium? Well done if you said A, suspensions contain insoluble particles dispersed.

Soluble particles means you wouldn't be able to see them.

And the key about a suspension is that you can see the particles suspended throughout that medium.

Well done if you got that correct.

Let's try another one.

Which of these diagrams do you think best represents a suspension? Pause the video here and come back when you're ready to check your answer.

Well done if you said B is a good representation of a suspension.

It is because we have clumps of the solute particles represented by the black circles that are suspended or dispersed throughout our water particles, which is our solvent or medium.

It can't be A because those solute particles, whilst they're all broken up, have all collected at the bottom of the container, bottom of that particular diagram, so they're not suspended or spread throughout the medium.

C is more likely to be a diagram showing a solution because the solute particles are all broken apart to single particles rather than the solid that it might have started as and they are randomly distributed throughout the entire medium.

D definitely can't be it because there are so few water particles.

The medium is actually the black particles there and doesn't show to be a suspension.

Okay, last task of the lesson.

June is wondering why some drinks like orange juice with pulp have pieces that don't sink to the bottom right away, while others like apple juice can stay clear and don't have things floating in them? What I'd like you to do is write a few sentences to answer June's question.

Include key terms like forces of attraction, dissolve, insoluble and suspension in your answer.

This might take a little bit of time and you might wanna discuss some of your ideas with the people nearest you.

Plan your answer out, pause the video and come back when you're ready to check your work.

Okay, let's see how you got on.

Now there were a lot of different ways that you could have tackled this question.

Everybody's gonna write things in a different way.

What I've done here is shown a few sentences and highlighted those key words to show how they've been used within those sentences.

So your answer may have included some of the ideas that's listed here.

For instance, that orange juice with pulp in it is an example of a suspension.

This is because the bits of pulp are insoluble in the juice, and they would settle to the bottom if they're left for a while.

In the apple juice, any bits that might be in it are probably soluble, and that means that they've dissolved.

That's why they can't be seen.

This happens because the particles that have dissolved have a stronger force of attraction to the solvent that they're in, than they do to each other.

As I said, this was not an easy task, so well done even if you got only one or two sentences done and well done, if you managed to include those key words.

They are very easy to use incorrectly, so the main thing here is have you used your key words, in an appropriate way in the sentences in which you've put them? Not an easy task, so very well done for having a go.

Really proud of you.

Let's review what we've learned in today's lesson.

We've learned that dissolving and melting are both physical processes that require a force of attraction between the particles to be overcome.

We've also learned that in order for a soluble substance to dissolve, the forces of attraction between the particles of the solute and those of the suitable solvent, have to be stronger than the forces between the solute particles themselves.

We also found that in a solution, the solute particles may fit into the spaces between the solvent particles, and that could possibly make the total volume less than what we might expect.

We've also learned that an insoluble substance can't dissolve and it may instead form a suspension in which the particles of our solute are dispersed throughout a medium.

I hope you've had a good time learning with me today, and I look forward to seeing you again soon.