Hello, my name's Mrs. Nevin, and today we're going to be talking about crystallisation as part of our topic on separating substances.

Now, you may have some experience of crystallisation from some of your previous learning or if you've ever noticed things kind of forming on the lips of containers like honey or syrups and things like that.

But what we learn in today's lesson will help us to better understand the process of crystallisation, but also answer those big questions of what are substances made out of and how can we explain how those substances behave? So by the end of today's lesson, you should be able to explain the process of crystallisation and how it can be used to separate substances.

Now throughout the lesson I'll be referring to some key terms, and these include solvent, soluble, saturated solution, and crystallisation.

Now the definitions for these keywords are given in sentence form on the next slide, and you may wish to pause the video here so that you can read through them or perhaps make a note of them so you can refer to them later in the lesson or beyond.

So today's lesson is going to be formed from two parts.

We'll be looking at how we might separate a solution in the first place and then move on to talk about crystallisation.

So let's get started by looking at how we might separate a solution.

Now, if we're going to talk about how we can separate a solution, we should probably remind ourselves what we mean by what a solution is in the first place.

So a solution is simply a mixture and it's a specific type of mixture that's been made of a solute and a solvent.

So the solute has dissolved in the solvent to create our solution.

And a mixture, if we remember, is a material that is made up of two or more different substances and that they can be physically separated, and that's the key point about any mixture, they can be physically separated.

Now, if a solution is a mixture, that means a solution should be able to be physically separated as well.

Now, you may be familiar with this particular setup, which is used for filtration, and if you take a mixture and you are able to pour it through the filter paper, then usually that can separate some substances.

But if that mixture is made of a soluble solute that's been dissolved in your solvent and that is your mixture, so we're talking about solutions here, and you were to pour that in, we would expect that any insoluble particles would collect in the filter paper, forming our residue, and that any of the soluble substances would remain in the filtrate and in that liquid, the solvent here, and be collected in the container.

Now, this is not possible when you have a solution, because the solution is formed of a soluble solute that's been dissolved in that that liquid, that means that this soluble substance would then flow through the filter paper and collect in that container, and it will not be able to separate out our solute from our solvent.

So in this situation, filtering for this type of mixture would not work.

Let's have a quick check.

True or false, soluble substances will collect in the filter paper during filtration.

Well done if you said false, but which of these statements best supports that answer? Well done if you said B, the soluble substance is small enough to pass through the pores of the filter paper, and because of that, a soluble substance will not collect in filter paper during filtration.

Well done if you got that correct, great job guys, fab start.

So if we're talking about separating a solution, we're talking about separating a soluble substance that's dissolved in a solvent and filtering isn't gonna work because filtering only works for an insoluble substance in a liquid.

So in order to remove our soluble solute from a solution, we're going to have to remove the solvent instead.

And when that happens, the solvent is lost to its surroundings.

That might be okay if all you want out of this mixture is the solute.

Now, there are two ways in which this can occur.

You can either boil your solution and in which case what you're doing is you're actually applying heat to this solution.

So in this instance we're using a bunsen burner to supply that heat, which the solution then is absorbing and using to boil that solvent away.

The other method we could use is evaporation.

And in this case, what's happening is that solution is absorbing some of the energy from its surroundings, and it's that energy that is allowing the solvent to then evaporate out of the solution and eventually leave that soluble solute behind.

Now there are two methods then, we've said boiling and evaporation, and there are pros and cons to using each of these.

So you really need to decide which is most appropriate for what you want out of this separation.

If we boil our solution in order to separate the solvent from that soluble solute, well, it's really quick, okay? Because you can turn that bunsen burner up to its highest heat and that is going to cause a boiling to occur throughout the entire solvent, and it happens very, very quickly.

The problem we have with this is that actually it starts to create bubbles.

So anybody who's ever boiled anything on a hob will notice that these bubbles, sometimes if you're not paying attention, your solution could boil over, it could start to spit, and if it starts to spit this solution, you might actually lose some of that solute that you were trying to separate out and keep.

So there is the possibility that you're losing what you want.

If you're doing evaporation, there's no spitting, okay? Because you've got just that one temperature.

But the problem is evaporation only occurs on the surface of that solution, so it takes a lot, lot longer, sometimes days if not longer, for this to be completely separated out, for that solvent to be completely lost from the solution.

So we know that we can separate a solution into its components by applying heat energy, but which statement describes what happens when energy is actually supplied to a solution? Well done.

If you said C, the solvent particles change from the liquid state to the gas state.

Solute particles are not changing here in the liquid to the gas state, it's the solvent.

So really that narrows it down to B and C.

And when the solvent or liquid particles are being heated, they're gonna change from the liquid to the gas, not gas to liquid, that would be condensation.

So well done if you chose C for the answer to this question.

Fab start, guys, well done.

Let's try another quick check then.

So looking at these pictures and these diagrams and using the key on the side, I'd like you to tell me which order these particle diagrams should be arranged so that it shows what would happen when a salt solution is heated.

You may wish to pause the video here so you can take a good look and double check that the order is the one you like and then come back when you're ready to check your answer.

Well done if you said the order should be C, A, and then B.

What we can see with C is that we have both the salt and water particles, so therefore our full solution with the solute and solvent particles in the beaker and only air particles above.

Moving to diagram A, we can start to see that some of the water particles have moved from the liquid state into the gas state and started to mix with the air particles above it.

And then finally, B shows us that all we have left in our beaker is the solute or salt particles and that all of the solvent has been removed as it's heated.

So really well done if you've got those in the correct order of C, A, B.

Great job guys.

Now we've mentioned that you can use boiling and evaporation in order to separate the solvent, and Laura here wants to investigate how the temperature of the surroundings might affect the time it takes for the solvent to be completely removed from a solution.

And she thinks that the solvent will be lost faster if the solution is left in a warmer environment.

What do you think? Do you think that the temperature of the surroundings will affect the time it takes for the solvent to be completely removed from the solution? Well, before Laura can actually begin her investigation into answering that question, she needs to identify three main variables.

She'll need to identify the independent variable.

Now this is a factor that she's changing or which she's selecting values for.

She'll also need to define what her dependent variable is.

This is gonna be what's measured or observed in order to get the results, the evidence she needs to answer that question.

And finally, she'll need to decide the control variables.

Now these are factors that need to be kept the same to make sure that the investigation she's carrying out is valid, to make sure that it's a fair test.

Now remember, Laura wants to investigate how temperature of the surroundings affects the time it takes for the solvent in a solution to be completely removed.

But in this investigation, what do you think is the independent variable? Well done if you said the temperature of the surroundings, this is something that Laura is going to decide before she starts that investigation.

So in this same investigation, what do you think is the dependent variable? Well done if you said the time taken for the solvent to be removed.

Remember, the dependent variable is something that is measured or observed.

So she'd be measuring the amount of time it takes for the solvent to be completely removed from solution, and that's why the answer is B.

Well done if you also answered B.

Time for the first task of today's lesson, what I'd like you to do is to create a storyboard method using the statements and some of the images below that we could use to investigate how the temperature of the surroundings would affect the time it takes for a solvent to be completely removed from the solution.

Now, this might take a little bit of time, you might want to discuss your options with the people nearest you.

So I'd like you to pause the video here please, and come back when you're ready to check your work.

Okay, let's see how you got on.

Well, the first thing you need to do is actually make the solution that you are going to be investigating.

So that's step one, make your solution.

So you're going to dissolve a certain amount of solute in a certain volume of solvent.

Make that solution to ensure that it is fully dissolved.

Then you're going to need to separate that solution into two evaporating dishes.

And you might have clarified that you need to measure out a specific volume so that you have the same amount of volume in both of those evaporating dishes.

At this point then, you're going to place those dishes in two different temperature locations, ideally, would be a place where the temperature is going to be kept constant.

So you might place one dish in a 20 degree Celsius oven and place another one perhaps in a 60 degree Celsius oven or at least a different temperature.

Finally, then what you're going to need to do is set that timer and keep track of how long it takes for that solvent to be completely removed from the solution.

Very well done if you managed to put those in the correct order and a spectacular job if you managed to clarify a few situations here where we didn't include the control variables just yet, but if you added a few, fantastic work, you're really starting to think around how you would carry out an investigation to make sure that the results you collect are valid.

Well done guys.

Okay, now that we're feeling a little bit more comfortable about how we might be able to separate a solution which is made from a soluble solute and the solvent by applying heat to it, let's move on to crystallisation.

Now, we said that in order for us to be able to separate a solution which is made up of a soluble solute that's dissolved in a solvent, we're going to have to remove the solvent because filtration will only ever remove the insoluble substances.

And here we've got a soluble substance.

Now we're doing that by removing the solvent, by applying heat to it.

And at some point along this process, a saturated solution actually starts to form.

But what is a saturated solution? Well, we actually use the word saturated in lots of different things in everyday life.

For instance, you might describe a sponge is saturated.

You might have heard the ground being described as saturated or the atmosphere being saturated.

And in each one of these examples, saturated is referring to the thing as being full, it's holding as much as it can.

So our saturated sponge actually has absorbed as much liquid as possible.

When you pick it up, it might actually drip out the end.

A saturated ground is holding as much liquid as it possibly can, and this is when flooding occurs because no more of that water can be absorbed into the ground.

And a saturated atmosphere means that it's holding as much liquid into the atmosphere as well.

And when that becomes saturated, we start to have rain or snow, sleet and the like falling out of the atmosphere.

So saturated essentially means full.

So if we take that idea and apply it to a solution, a saturated solution is simply a solution in which no more solute can dissolve in a solvent at a particular temperature, the solvent is full, it can hold no more solute within it.

Now you can tell the solution is saturated because if you were to add more solute into your solution, it wouldn't dissolve, you would still be able to see it visibly in the solution, the little bits in it because it's not dissolving.

And we can see that in our diagram here on the left, those solid pieces that are sticking together and kind of collecting at the bottom of our container.

Now, the key thing about a saturated solution is that it occurs at a particular temperature.

So let's take a closer look at what we mean by saturated solutions at particular temperatures.

Well, if we have a saturated solution that only forms at a specific temperature, meaning it's dissolved as much solute as it can at this temperature.

Well, if the temperature lowers and that solution then cools, the solute that had dissolved can't stay dissolved any longer because it's not at that temperature needed in order for the solute to dissolve.

And a few things can happen as this solution cools.

One thing that might happen is that that so solute then undissolves from the solution and starts to collect at the bottom of the container.

And if you've ever had a hot chocolate drink, you might have noticed this because if you don't drink it fast enough and it's able to cool, when you finally get to the bottom of that mug, you might actually find cooled drink, but with some of this undissolved powder, some of the stuff that has fallen out of our solution as it cooled.

Now, you might also find that solid particles of that solute, of what you tried to dissolve in the first place, start to form.

So if you have, for instance, a sugar solution and it's become saturated, so you've dissolved as much sugar as you possibly can into it, you can't see any more of the sugar and there maybe one or two grains, that's about it.

Once you leave that to cool then, you might actually see sugar crystals start to develop, start to form.

And this is actually one of the ways that they form things like rock candy, starting with a saturated sugar solution, you might hang a piece of string in it and the solid sugar crystals then will grow on the string as that solution cools.

So the formation of a saturated solution and what happens upon continued removal of that solvent really depends on what's going on when the solute and solvent particles interact with each other as more and more of that solvent is removed from the solution.

So one of the first things you might notice is that as these solvent particles start to leave the solution, the pink or smaller solute particles that we have in our model here would actually move closer together.

So as we start with our solution on the left, it's heated up a bit and some of the solvent particles, so those larger blue solvent particles, have left my solution, the solute particles, the small pink ones, move closer together.

Now, what that does then is it allows for the forces of attraction to reform between those solid solute particles that were originally overcome to create the solution in the first place.

So we now have solute particles closer together and held more strongly together by these reformed forces of attraction between those particles that are now closer together.

So we already have some solute particles that are closer together and have reformed those forces of attraction.

And as you continue to heat the solution, you are continually removing more and more of those solvent particles, those larger blue particles in our diagram here, and eventually what you're left with are only those solute particles, those soluble solute particles.

And what they form then, as the solvent particles leave that container, is they start to grow crystals.

Okay, so crystallisation then summarised as simply a process and it's a process that removes a soluble solute from the solution and it forms then solid crystals from this saturated solution.

And this is possible because we're continually removing these solvent particles from that solution.

So we're heating up our solution, removing the solvent, and we're leaving behind that soluble solute particles, which eventually come together creating forces of attraction between those particles, and we're left with these beautiful crystals.

And that entire process is known as crystallisation.

Phew, we've gone through quite a lot of stuff, so let's stop quick for a quick check.

Which of these statements occurs during crystallisation? You may wish to pause the video here and come back when you're ready to check your answer.

Well done if you said B and C.

So during crystallisation, solvent particles are lost to the surroundings, not the solute as A says.

And C then is also correct, forces of attraction reform between the solute particles, which go on to then form our crystals.

The forces of attraction between the solvent particles are actually overcome, and that's why the solvent particles then leave the solution.

So well done if you managed to choose B and C as your answers.

Good job, guys.

Okay, let's try another one.

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 best supports your answer? Well done if you chose A, solid crystals form when the solvent is removed, not dissolved, it's the solute that's been dissolved, but we need to remove that liquid solvent in order for the crystals to form.

Solid crystals won't form a solution in filtration because filtration is removing an insoluble substance, not a soluble one.

So well done if you chose false and A to support your answer, good job.

Okay, so we've learned that we can remove a soluble solute from a solution by continually removing the solvent from that solution by applying some heat energy.

And when we do that, we force those solute particles closer together and they will eventually form crystals, and that entire process is known as crystallisation.

But the size of the crystals that form during crystallisation depend on how quickly that solvent is removed from the solution.

So if we remove that solvent really quickly, you end up with quite small crystals, okay? Because there isn't enough time for those forces of attraction to form between getting all those solute particles as close together as possible.

If you remove the solvent more slowly through some really gentle heating, more likely via evaporation, those solute particles have a little bit more time to get closer to each other, to reform more of those forces of attraction.

And then you end up with quite large crystals as a result.

Let's have another quick check, which two of these diagrams show crystals? Well done if you said A and C, crystals tend to have just the one particle, so we're looking at something that has none of the solvent particles there, so none of the blue ones and the answers then leave us with A and C.

So well done if you've got at least one of those, and great job if you manage to get both of the diagrams correctly.

Good job, guys.

Okay, time for the final task in today's lesson.

What I'm going to do in a moment is I'm going to show you a method that I would like you to follow in order to create crystals from a saturated solution, and then I'd like you to record your observations in this table below.

Now, if you take a closer look at this table, you'll see the word relative comes up quite frequently.

Now, relative in this point is very similar to what we mean when we're talking about a relative as say a cousin or a sister or brother.

In this context of chemistry, the term relative means compared to one another.

So we can see here that we're gonna have two spaces for the location of our solution, so relative, so that's comparing your two locations of solution, is the temperature going to be higher or lower than the other location that you are leaving your solution in? So just to give you a heads up on what we mean in terms of relative in this context.

So I'll pass you over to the method and then you can have a go at creating your own crystals and completing your observations in the table.

You may wish at this point to pause the video so you can follow it a little bit more easily and then come back when you're ready to check your answers.

Okay, let's see how you got on.

Now, depending on where you've actually put your different sample solutions, your relative temperatures in the rest of your table might look slightly different.

So I'm going to relate what I would've put in my table if I had used a window sill versus perhaps leaving the other solution near a heat source, so that's either in an oven or perhaps near a radiator or something like that.

So when I compare that then, the window sill will have a relatively lower temperature than the solution that is left near the heat source, that's gonna have a far higher temperature.

That means then that the heat source location will have more energy available to my solution than the window sill will.

And because of that, the window sill is going to then lose the solvent a lot slower than the solution that is left near the heat source.

So the higher the temperature, the more energy there is available, and that results in a faster loss of solvent, and that then will impact on the size of the crystals that form as that solvent is lost.

So you should have found that wherever you put the lower temperature solution would end up with large crystals and those that had the higher temperature location solution would end up with smaller crystals.

Well done guys, that was a tricky one for you to have a go at.

It's a little tricky sometimes to decide what exactly you should be recording within this table, and I hoped that I gave you a little bit of guidance and you enjoyed creating your crystals.

So we've had a long day, we've learned a lot of new things.

Let's summarise what we've managed to learn today.

Well, we've learned that crystallisation is a method that can be used to separate out a soluble solute from a solution, and it tends to result in the formation of crystals.

We've also learned that crystallisation occurs the best after you've formed a saturated solution.

And then what you might do, so what you're doing is you're applying a little bit of heat energy to create that saturated solution, and then you would possibly leave that solvent then to evaporate slowly into the surroundings so that you can create the biggest crystals if that's what you're aiming for.

We've also found then that small crystals will form if a solvent is removed quite quickly, and that tends to happen at a very high temperature of heat source, and that larger crystals will form when the solvent is removed more slowly, and that's usually when your solution is left in a cooler temperature.

I had a really good time learning with you today.

I hope you did as well with me, and I hope to see you again soon.

Bye for now.