Hello, my name's Mrs. Niven, and today we're gonna be talking about how we can separate a soluble solid from a liquid as part of our unit on separation techniques.

Now, you may have inadvertently already experienced some of this, if you've ever noticed some solids forming in the bottom of a honey container or something forming on the neck of a bottle of say, maple syrup, something like that, going forward, this is really important in our journey through science because it help us to consider different ways in which we can obtain some materials for future use and it will also help us answer that big question of how we can explain how substances behave.

So by the end of today's lesson, you should be able to explain how dissolving can be reversed in order to separate a soluble solid from a liquid.

Throughout today's lesson, we'll be using a variety of keywords, but the most important ones we want to be looking at are solute, solvent, evaporation, crystallisation, and crystal.

Now the definitions for these keywords are on the next slide, and you may like to take a moment here to pause the video so you can read through them or make a note of them so you can refer back to them later on.

Today's lesson is composed of two parts.

First, we'll be looking at separating a solution and then we'll move on to this idea of crystallisation.

So let's get started by looking at separating a solution.

Now you may recall that a solution is simply an example of a mixture and that that mixture is made up of a solute and a solvent that have been combined to create that solution.

And that a mixture is in fact a material simply that's been made of two or more different substances that can be physically separated.

And the key point here is that it can be physically separated.

Now, one technique I might be able to use to physically separate this solution would be to use filtration.

So I would start with my solution that is formed for my soluble solute dissolved in a solvent, and I'd pour it into the filter paper which is supported by the funnel and itself supported by the conical flask.

When I pour that solution into the filter paper, any of the insoluble particles would form the residue which is collected in the filter paper, and then any soluble particles would remain in the filtrate and be collected in the conical flask.

Now because my solution has started with a soluble solute, filtering is not going to work to reverse that dissolving because those soluble solu particles would remain in the solvent and form part of my filtrate.

I'm going to need to find a different technique in order to separate that soluble solute from the solvent.

Time for a quick check.

True or false, filtration can separate a soluble substance from a solution? Well done if you said false, but which of these reasons justifies your answer? Is it A or B? You may wish to pause the video here so that you can read through and discuss your choices and come back when you're ready to check.

Well done if you said B, the soluble substance is small enough to pass through the holes in the filter paper so it becomes part of the filtrate, and that's why filtration would not work to separate a soluble substance.

A would not be correct because any insoluble substances form the residue, not the soluble substance, but well done for having a go at that first question.

So if I can't use filtration to separate my solute from the solution in order to separate my soluble sokute, then I'll probably need to remove the solvent.

And this is gonna take place in one of two ways.

The first way would be to boil that solvent away, and the second method would be by evaporation.

Now, both of these methods require energy to be applied to the solution.

In the first one of boiling, you're directly heating that solution.

You are choosing the amount of energy that you are applying to that particular solution by how strongly that Bunsen burner has been applied.

The evaporation then is going to be relying on the energy of the surroundings.

So that could be the energy within the room, simply the temperature of the room or the energy that is available in the surroundings outside.

So both of these methods of boiling and evaporation both rely on energy being supplied to this solution, and there are pros and cons to both of these methods.

So let's look at what those are.

If someone is going to use boiling as a method to remove the solvent from their solution, it is a really fast process.

You can get that Bunsen burner roaring and it would be able to remove that solvent very, very quickly because the boiling is going to take place throughout that entire solvent.

So the energy is being applied to every single particle in that solvent.

The problem with this though is that because it's happening so quickly, boiling, if you recall, will form bubbles and those bubbles then push through the rest of the solution really quickly.

And what that ends up forming then is this spitting action.

So if you are looking at the container, then you'd have these spitting bubbles of the solvent and the solute that's within it would be pushed out of your solution container and being spitting at you.

Now, you might have noticed something like that if you ever cook something on the hob and perhaps the boiling gets a little out of hand and the water gets pushed out of that container, it can be a little dangerous.

If we look at evaporation, the pro here is that you don't have any spitting because you're heating it very gently, very slowly and simply relying on the temperature difference in the environment to help evaporate off that solvent.

The downside of this is that it takes a really long time.

It can take several days in fact to completely separate that solvent because the evaporation or the removal of that solvent is only ever taking place at the very surface of your solution rather than what's going on throughout the entire solution.

So it just takes a little bit longer.

Now, regardless of the method that's used to remove the solvent either boiling or evaporation, what's happening is that the solvent particles in the solution are gaining energy either directly through an apparatus like a Bunsen burner or partly from the surroundings and the temperature differences between them and those solvent particles then gain enough energy to change from the liquid state into the gas state.

Time for another quick check.

What I'd like you to do is to look at these three pictures using the key on the side as well to help you and decide in which order you would put these pictures to arrange and explain what happens when a salt solution is evaporated.

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

Let's see how you got on.

Now the key to answering this particular question is using the information on the side that tells you what each of these different coloured spots represent.

And looking at that, I would've said that the order was C, A and then B.

Now C comes first because we can see that there are only air particles above the beaker and the solution still contains both the salt particles and the water particles.

A comes next because we can start to see that some of the water particles have turned into the gas state and have now mixed with the air particles above it, but some of them are still in that solution, still in the beaker in A.

B comes last because we can see that all of the solvent particles have been removed from the beaker and they are now mixing with the air particles above and we are left with simply the solute or the salt particles in B left behind in the beaker.

So tricky one, well done for having a go.

Now, once all of the solvent has been removed from a solution, what you're left with is sometimes referred to as a crystal.

Now let's imagine that Jacob wants to investigate how the temperature might affect the size of those crystals that are formed from a sugar solution.

Well, Jacob for instance, suggests that maybe in a warmer environment those crystals will be bigger than they might be in a cooler environment.

What do you reckon the temperature of the surroundings might have on crystal size? Why don't you pause the video and have a think.

Now, whenever we start to investigate something like will the temperature affect the crystal size of what is formed when you remove the solvent from a solution, you need to identify three main variables.

So the first one you might want to think about is what is your independent variable? This is what is changed or what you're selecting values for in that investigation.

The next thing you need to think about is what are you going to measure or observe and eventually record in your results table? That's gonna be called your dependent variable.

Now anything else that could possibly change during your investigation needs to be kept the same, and that's so that you have a fair test so that the results that you gather are gonna make some good evidence to make your conclusion that much stronger and those variables are known as control variables.

Let's have a quick check and go back to this idea of Jacob wanting to investigate how the temperature of the surroundings infects the size of the crystals that might form when the solvent is removed from a solution.

What do you think the dependent variable would be for his experiment? Well done if you said the size of the crystals that are forming, very well done, what do you think would be then the independent variable for this investigation? Well done if you said the temperature of the surroundings, so it's the temperature of the surroundings that would be changed or chosen.

And then the thing that is being measured or observed would be the size of those crystals.

For our first task, what I'd like you to do is to create a storyboard method for investigating how temperature might affect the size of those crystals that are forming using those diagrams below.

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

Okay, let's see how you got on.

So the first thing you need to do is you need to make your solution and for this to work the best it can, you want to make that a saturated solution.

So we're trying to dissolve as much solute as possible in the volume of solvent that has been chosen.

The next thing you're gonna do is you're gonna separate that solution that you've created into two different containers and an evaporation dish would probably work the best here.

Then you're going to take those dishes and you're going to place them in two different locations.

One place should probably be on a window sill or somewhere out of the way, and the other one you'd wanna put near a heat source, so probably closer to a radiator or something like that.

And then finally, we're going to observe them as those crystals start to form from the solution, as that solvent is evaporated by the window sill or boiled away near that heat source.

Well done if you manage to get that correct.

Now that we're feeling a bit more comfortable talking about how we could separate a solution, let's take a closer look at what's happening as those crystals form in the process of crystallisation.

So if we cast our mind back, what we're really doing when we're separating this soluble solute from the solution is we're removing that solvent and when it's being lost to the environment, either by boiling directly, by adding the heat with say a Bunsen burner or indirectly, by absorbing the energy from the temperature difference in the environment, at some point as that solvent is leaving that container with our solution, we are creating a saturated solution.

And as a reminder, a saturated solution then is one in which you can dissolve no more solute in the volume of solvent that you have at any given temperature.

So as that solvent is leaving our solution, we have fewer solvent particles present in the solution.

And what that does then is create a solution that looks a little bit like this, and you might be able to see that there are fewer of those solvent particles on the right than there are on the left, but we have the same number of solute particles in each of these diagrams. As those solvent particles are leaving the container, the solute particles represented here by those smaller pink circles, they're moving closer together.

And as those particles move closer together, forces of attraction are able to reform between them.

And we can see that represented here by the arrows between those smaller pink circles.

Time for a quick check.

Which of these statements occur during this process of crystallisation? Pause the video here to read through them.

Maybe have a chat with the people nearest to you and then come back when you're ready to check your answers.

Well done if you chose B and C.

So both the particles are lost to the air, and that's the solvent particles that are lost, not the solute particles and the forces of attraction are reforming between those solute particles, not the solvent, because the solvent particles are leaving the container entirely.

So as more and more of those solvent particles leave our container, the solute particles get closer and closer together, reforming those forces of attraction.

And eventually they form what's is known as a crystal, which is essentially the solute particles that are left and that's it.

So they've returned to their solid state and they've been completely separated from the solution.

So crystallisation in general is a process that forms your solid crystals from a saturated solution by continually removing that solvent.

Let's have another quick check.

True or false, crystallisation is a process that forms solid crystals from a solution as the solvent is dissolved.

Well done if you said false, but which of these statements justifies that choice? Pause the video here if you want to read through them and have a chat with the people nearest to you and come back when you're ready to check your answer.

Well done if you said A, crystallisation forms solid crystals from the solution, when the solvent is evaporated, filtration will not form crystals.

What it does is separate an insoluble solid from the solution.

So we know that crystallisation will form crystals during the process, but it actually depends on the speed that that evaporation of the solvent actually takes place.

For instance, if you are heating your solution really, really quickly, the crystals don't have a lot of time to form, and so you end up having really small crystals.

And in fact, if you were to put your evaporation dish with your solution on top of a Bunsen burner and watch them, you would be able to see tiny little crystals forming just on the edge where that solution is and where the solvent is being boiled off.

Now, if you were to leave that evaporation dish somewhere so that it can lose that solvent slowly, or if you're heating it very, very gently, more likely though through evaporation rather than boiling, you would form very large crystals.

And that's because those crystals have time to actually develop.

Let's have another quick check.

Which of these diagrams shows crystals? Well done if you said A and C.

Now the reason those two show crystals and not a solution is because those blue solvent particles are missing, we're only left with the solute particles, and that's why we have crystals, well done if you got that correct.

Time for our last task, you are now gonna have a chance to create your own crystals from a saturated solution.

So on the next slide, I'm going to show you the method to follow.

And what I'd like you to do then is to record your observations from that method in a table as shown below.

This is the method that I'd like you to follow to create your saturated solution.

If you don't have a beaker to hand, know that you can use any other two containers as long as they are the same exact containers.

So if you have two of the exact same glasses or two of the same bowls, that's absolutely fine, as long as they are exactly the same, that will work.

Let's see how you got on.

Now, if you followed the method as it was stated, you will have put your two dishes in two different locations, one near a window sill and one closer to a heat source.

Now the solution that's put near the window sill will have been in an environment where the temperature compared to near the heat source is actually quite low.

And what that means is with a low temperature, you have a low amount of energy that's available for us to evaporate the solvent from that solution.

So the speed at which evaporation takes place is gonna be really slow.

And as a result of that, the crystals that form as the solute then is all that remains in our solution will end up being very large.

You end up with those large crystals, what you'll find then for the dish that's near the heat source is because it's near a heat source, the temperature there is a lot higher than it would've been near the window sill.

And because of that, it has a lot more energy that's available to help that solvent turn into the gas state and be removed from the solution.

So the speed of the evaporation near the heat source compared to near the window sill will be a lot faster.

Now because that evaporation has happened a lot faster, the crystals don't have a lot of time to form, and therefore you end up with a lot smaller crystal sizes.

I hope you had a good time following that method and watching your crystals grow.

If you wanted to try this again, you could have a go with sugar as well, but maybe put a little bit of food colouring in and see if you can get different coloured crystals forming that way.

Lots of fun.

So let's summarise what we've learned in today's lesson.

We've learned that heating will remove a solvent from a solution and that could lead to the production of a saturated solution.

So one in which no more solute can dissolve in that solvent.

And crystallisation then is a process that could reform a solute as crystals from that saturated solution.

Now a solvent can evaporate into the air around you during that process of crystallisation.

And if you heat the solution, you can make that solvent evaporate even faster.

And the smaller the crystals will form if that solvent is evaporated really quickly and larger crystals will form when the solvent evaporates more slowly.

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