Lesson video

Hi, I'm Mrs. Nevin, and today we're gonna start our journey through the topic of solutions in which we address that big question of how can we explain how different substances behave? By the end of this lesson, you should be able to describe the difference between a pure substance and a mixture.

You may have come across these ideas before when you were looking at whether or not something might dissolve, but it's also really important to understand the difference between a pure substance and a mixture when we consider how different substances are made in the lab or how we deal with substances out in the environment.

The keywords that we'll be using in today's lesson include pure substance, a mixture, and impurity.

Now the next slide will show you the definitions for these keywords used in a sentence, and you may find it useful to pause the video here to read through them or to maybe jot them down for reference throughout the rest of the lesson.

So in today's lesson, we're going to look at three different things.

We're going to look at what a pure substance is.

We'll look at how we define what a mixture is.

And in the final part of today's lesson, we will look at how scientists are able to distinguish between a pure substance and a mixture.

So let's get started and look at what we mean by a pure substance.

Now the word pure is probably very familiar to you.

You will have come across it in lots of different ways in everyday life.

For instance, you might have seen it in things like a poem with the line, "Blessed are the pure of heart," but you're more likely to have come across it in things like food packaging, describing juices, for instance, as pure orange juice or pure apple juice.

Or perhaps you've actually hear the word pure and you simply think of something that is clean, like pure water.

The way that scientists use the word pure is slightly different to what we use in everyday life.

When a scientist uses the word pure, what they're doing is talking about a substance that is composed of only one type of chemical.

So you may be familiar with particle diagrams from a previous lesson.

And what we have here is a particle diagram showing how particles are arranged in a pure solid carbon sample.

Here we have the particles arranged in a pure liquid bromine sample.

And finally we have particles arranged in a pure oxygen gas sample.

But if you look at each one of these samples, what you'll notice is that all the particles for each of those samples look exactly the same.

This is showing that they are composed or made up of just one type of chemical.

Now crucially, the chemical in a pure substance can actually be made of different types of particles.

Let's look at an example.

Here we have a chemical made of two different types of particles, one black and one red.

And if we look at another example of this chemical, we can see it looks exactly the same.

And another example, again, it looks exactly the same, one black particle and two red particles.

Collectively, this is actually an example of pure carbon dioxide gas.

This is the type of gas you'd find in most of the fire extinguishers you might find at school or around everyday buildings.

There are other types of fire extinguishers though, so be carefully only looking at the one that is labelled as carbon dioxide.

But let's look at another example.

Here we have again, a substance that is made up of two different particles, a red one and a white one.

And if we look at another example of that chemical, it looks exactly the same.

Any ideas what this might be? It is pure water.

So this is the type of water that you're more likely to find in the lab.

Scientists usually refer to it as DI water, we mean as deionized water.

This is not water that has come from the tap.

This is water that has been prepared specially and is therefore composed of only one type of chemical.

And that chemical looks like what is shown here.

So let's see how you're getting on with this idea of what we mean as a scientist by the term pure.

So true or false? A material that contains one type of substance is pure.

What do you think? Well done if you said that it is true, a material that contains one type of substance is pure.

But which of these two statements do you think best supports your choice? Is it that, "Pure materials contain different chemicals," or is it that, "Pure materials are composed of only one type of chemical?" What do you think? Great job if you chose B, "Pure materials are composed of only one type of chemical." Let's try another question.

So in this example, we have three different samples of substances and I want you to think about how confident you are about the statements describing them.

Are you absolutely sure that it's right, could be right, might be wrong, or are you pretty confident that this is a wrong statement? So let's have a look.

Statement A.

"Fresh orange juice is pure." What do you reckon? If you said it is definitely wrong, you are correct.

Fresh orange juice tends to be composed of many different substances.

Vitamin C, water, it might have pieces of pulp in it, lots of different things.

Let's look at B then.

Orange squash, is that pure? Definitely sure, maybe, definitely wrong.

Well done again if you said that that's definitely wrong.

In fact, if you think about how you make a glass of squash, you'll usually pour a little bit from the bottle of whatever flavour squash, orange in this instance, into that glass, and then fill it up with water to however you like that taste.

So already you've putting two different chemicals into a container.

So that's definitely not pure.

What about water is pure? How confident are you that pure is being used correctly here? Well done if you said yes, in fact, we did use this as an example earlier in the lesson that water is pure.

You may have said this might be right, it might be wrong, because we talked about tap water earlier, but I did use earlier that very crucial adjective of being tap.

Because it's just said water here, we can say that it is pure.

Right, let's have a go at a task then.

I'd like you to read through this.

Have a go and pause the video and come back when you are ready to go through the answers.

Right, let's see how you've got on.

So we were being asked to see if the word pure is being used scientifically in this instance.

And well done if you said, "No, pure is not being used scientifically correctly on this label." And there were lots of different reasons why you could say that.

And part two asked you to justify your choice.

So if we look at this, if you looked a little bit more carefully at the ingredients label, you'll see that it contains actually several chemicals and some examples of what it contains are listed, carbohydrates, proteins, vitamin C, potassium, lots of other things.

Now, if you didn't notice that in the ingredients label and you read just a little bit further, you might have seen that it was described as needing to be shaken before serving, a pure substance needing to be shaken.

That makes us think that perhaps there's more than one chemical in that box and they might need to be mixed together before you can actually use it.

So well done if you said any of those things to support your answer that pure was not being used correctly in this instance.

Now on this question, we have a discussion happening between some students and I want you to very carefully read through their statements and decide who do you think is describing a pure substance? Now, crucially in this question, what I'd like you to do is to support your choice using a because clause.

When you use a because clause, you are automatically linking up your thinking with your choice.

So using the word because is really important here.

Let's look at what is being said.

Aisha has suggested that a sample of gas that only contains oxygen might be a pure substance.

Andeep is suggesting that aspirin contains a drug to stop pain and sugar to help the tablet taste better and therefore maybe it is pure.

Whereas Sam is suggesting that his gold earring is made up of lots of gold particles joined together and therefore that is pure.

So have a discussion.

Who do you think might be describing a pure substance? Justify your choice and explain it using a because clause.

And when you're ready to go through the answers, come back.

Well done if you chose Aisha and Sam as being students that were correctly describing a pure substance.

This is because both of their statements were referring to only one chemical.

Aisha was suggesting only oxygen as the chemical, and Sam was suggesting gold as the only chemical.

So very well done.

Okay, now that we're feeling pretty confident about what a pure substance is, let's move on to look at what a mixture is.

Now, scientists have been interested in the world around them from the start and they learned very early on that a lot of the materials that we might use in our investigations are actually found in nature, but that those materials are found as mixtures.

And some of the examples of those starting materials may have come from rocks, they might have come from the sea water or they might have come from air.

Three things that we have quite a lot of on the planet, but they're all mixtures.

So what is a mixture? Well, a mixture is any material that contains two or more different substances that can be physically separated, and that's a crucial point that we'll come to later.

But let's take a closer look at some of these naturally occurring mixtures that scientists will have looked at early on.

Let's take a closer look at our example of rocks.

Now rocks are a mixture of minerals and possibly a useful metal.

Now those useful metals could be something like iron that we use in building works, aluminium that's turned into foil to maybe wrap your sandwich in, and sometimes the metals that you might use in electronic devices like your mobile phones, a tablet, or computer.

Now, if we were able to look at the particle diagrams for a rock sample, we can see a clear difference between this particle diagram of a solid and the particle diagram we looked at earlier for pure carbon.

We can see here that we have different substances in this particular sample, and in fact if we look at it, one of those particles might be the metal that's really useful and the rest will be different materials.

But crucially, what we can see are different substances in this sample.

Another example of a mixture found in nature is sea water.

Now many people might describe it as actually salt water, but it does in fact contain more than just salt and water.

Sea water tends to also contain gases, gases like oxygen that help to support the sea life within the oceans.

Now if we take a closer look at the particle diagram for seawater, again, we can see that it has water and salt particles in there, but they're obviously very different from each other.

So we have two different substances that are in this particular sample of seawater.

Let's look at our last example of a mixture in the environment.

Now here is a mixture of lots of different gases, but primarily it contains things like nitrogen, oxygen, carbon dioxide, water vapour, and argon.

And in fact, if you go outside, you're probably able to see at least one of those gases fairly easily on most days.

Any ideas which one? You'd be correct if you said water vapour.

Now, if we actually took a sample of the air and looked at it in terms of its particle diagrams, we could see very clearly that it is made of several different substances.

And it's the fact that this sample contains many substances that make it a mixture, right? Let's see how you're getting on with our understanding of what a mixture is.

Which of the diagrams below or diagram below do you think shows a mixture? Very well done if you said the answers were B, C, and D.

All three of those are examples of a mixture, because all three of them show more than one type of chemical or substance in that sample.

A is not an example of a mixture, because all the particles are exactly the same, therefore it's a pure substance.

Now importantly, it doesn't really matter how much of one substance is mixed with another.

The result is still known as a mixture.

So if we look at an example using particle diagrams, here we have a pure sample, pure because all the particles are all the same.

All of the substances, all the chemicals in this sample is exactly the same.

It's pure.

If we look at this other example, we can see that there are two different coloured substances and therefore we have a mixture, two different substances mixed together.

Now, if a mixture contains a really small amount of one of those substances, that small amount of substance is known as an impurity.

Why do we care? Because an impurity has the ability to alter or change the properties of a material and that's really important.

So let's look closer at what an impurity can or might be able to do.

Now, sometimes impurities can actually be really useful.

An excellent example of this is looking at bottled water.

It's actually mineral water and it's those mineral impurities in that water that gives it its flavour.

In fact, different bottled waters have different compositions or different amounts of different minerals in those waters, giving it its different flavour.

It's why Volvic might taste different to Highland Spring, to Evian water, to Harrogate water, to the water you get out of your tap.

It's all gonna be different because of the different amounts of minerals that you find within that water.

Another really good example is actually the jewellery or things that would normally be incredibly heavy might contain an impurity in there to lessen the weight of it.

Now, pure gold is actually incredibly heavy.

Nobody wants to be walking around with these masses of rings hanging off their fingers or around their neck or off their ears.

So what might have happened is some impurity would lessen the weight and how heavy that material might feel if you're wearing it.

So a crown, for instance, might contain a few impurities.

Now in other instances, impurities can actually be not very helpful at all.

For instance, we talked about earlier metals being used in electronics, but if there are some impurities found in that metal that's used for electronics, it can prevent the performance or impact the performance of that electrical conductor.

So, it's not something that we wanna see in our electronics.

Another example might be medicines.

If an impurity is found in a medicine, then it could possibly be harmful to the person who's taking that medicine.

So it's absolutely essential that we keep track of whether or not an impurity is found in the medicine during its development.

So let's have a look and see how you're getting on understanding what a mixture is.

True or false? "Mixtures do not contain impurities." Well done if you said false, but which of these statements do you think best justifies your choice? Is it, "A mixture contains two or more substances joined together, or is it, "An impurity is a chemical found in a mixture in very small amounts." You're exactly right if you chose option B, "An impurity is a chemical found in a mixture in very small amounts." A is not an option, because it describes a mixture as having two or more substances joined together.

But if you remember earlier, we said that a mixture is something that can be physically separated.

If it's joined together, that's gonna be quite tricky.

Now, when I first mentioned the idea of a mixture, you may have had an image pop into your mind of a mixture that you come across in your everyday life, because actually a lot of substances can be mixed together, but they might be really difficult to separate once they've been mixed.

An example might be cake batter or pancake batter where you mix things like flour and milk and eggs together.

But once they've been mixed together, they're really difficult to separate.

Same thing with cement.

You might have water, special powder, you might have gravel in there.

When they're all mixed together, it's actually quite difficult to separate them once they've been mixed.

So in science, one of the key features of a mixture is that its different substances can be physically separated from each other and actually quite easily.

So if we use the scientific definition of a mixture, cake batter's not a mixture and neither is the cement.

Okay? So, some models are actually really good to consider in the back of your head to decide whether or not something is a mixture.

Can it be physically separated? It's just useful to have when you are thinking about whether or not what you're looking at is a mixture or not.

So one example might be some sweets.

If you have a pile of sweets, they could be grouped by colour, they could be grouped by flavour.

If you're anything like me, maybe you do that with your M&Ms or Skittles or something, putting them by colour and then eating them in patterns, something like that.

Maybe wooden blocks.

If you can be grouped by the size or the colour, I know whenever I get a new box of LEGO, or if I'm organising those, I will always organise them by colour and then by size so that I can find the pieces a little bit more easily.

Money can definitely be grouped together by denomination.

So it's, put all your five pound notes together, your 10 pound notes together, something like that.

Or you could even do it by country of origin.

If you are travelling by country, you might group all of your money together if you have all your American money in one part or your British money in another or any of your European money in another.

Time for another quick check.

"Which of these pictures do you think shows a model of a mixture?" And crucially, why did you choose that picture? Well done if you chose picture A.

Now I would've chosen picture A as well.

Let's see if you thought the same reason I did.

I chose A, because the sweets could be physically separated.

If I wanted to, I could group them according to colour or flavour.

In B, it looks like all of those, all of those little sweets have been melted together and they're far more difficult to separate physically.

Time for another task.

What I'd like you to do is take a very closer look at these pictures and I'd like you for each one of them, to write a sentence describing each of the models shown.

But crucially, I'd like you to try and use those keywords we've been talking about so far of pure, mixture, or impurity within your sentences.

So pause the video, have a go, and come back when you are ready to go through the answers.

Okay, let's look and see how you got on.

This was a little tricky, so I'm gonna go through each of these individually.

So if we look at jar A, jar A is a model of a pure material, because it contains only one grain, those popcorn kernels.

Now this is one way you could have described A, there might be others, but I would definitely have said that it is pure.

So well done if you said that it was pure.

B, jar B then, I would've said is a mixture model, because it contains two grains.

Those kind of orangy red ones are known as lentils and the others were rice.

So as long as you said that you could see two different grains in there, they could be physically separated or grouped and therefore it's a mixture.

Jar C is also a model of a mixture.

We can clearly see three different substances, the brown rice, those black beans, and then those white beans.

And like jar A, they could be separated into the different piles based on their colours even.

Now jar D, I thought was a little bit of a tricky one, so let's see how you got on.

Jar D is also a model of a mixture.

It does also contain two substances that could be separated.

It appears to have an impurity with those darker grains.

Now there were only a few of them, so don't worry if you didn't quite get that one correct.

It was a very tricky one to see, but well done if you managed to notice it was a mixture and then identified that there was such a small amount of that second substance that meant it was an impurity.

Tricky task, but very well done.

Good job, guys.

Okay, let's move on to the last part of today's lesson where we look at how scientists can distinguish between a pure substance and a mixture.

So one of the easiest ways for scientists to tell the difference between a pure substance and a mixture is to look at its properties.

For instance, the pure substance of iron is actually really soft, and because of that, we can't use pure iron for building.

Instead, we use a mixture of iron with carbon, and that mixture is actually known as steel.

Now, steel of all the different varieties, actually tends to be hard.

It's far stronger than pure iron.

And because of that, it's steel that's used in building works, not pure iron.

Now we don't wanna have to build materials in order to tell the difference between something that's pure or something that's a mixture.

So there's a far easier way to distinguish between those two materials.

The properties that scientists tend to use more frequently than others to tell the difference between a pure substance and a mixture is the substances' melting points and boiling points.

So let's take a closer look at those.

So the reason that scientists use melting points and boiling points as a way to distinguish between pure substances and a mixture is because a pure substance tends to change state at a very definite temperature.

For instance, pure water will boil at exactly 100 degrees centigrade.

If we were to look at a sample of sea water, which if we remember is a mixture of salt and water, depending on the amount of salt in that sample, it could boil anywhere between 102 and 123 degrees centigrade.

That's very different than our pure water sample.

So, let's look at some distinguishing features of the melting point of a mixture.

So if you look at an example here, 24 karat gold is known as pure gold.

And if we were to melt it down, it would need to get to a temperature of 1,064 degrees centigrade for it to melt.

If we look at another type of gold jewellery, this time, we're gonna look at rose gold.

It turns a mixture and it's a mixture of gold and copper particles.

Now, if we were to melt that particular sample down, it would need to get to a temperature of 1,000 to 1,100 degrees.

And what you'll notice here then in our two examples is that the pure substance, it was able to change state at a very specific temperature, whereas our mixtures are changing state over a range of temperatures.

And that's a really important, easy to identify distinguishing feature between something that is pure and something that's a mixture.

So, let's check to see how you're getting on with distinguishing between a pure substance and a mixture.

We have here two different sized cubes of the same pure wax that are being heated.

Now, I've got three students who are discussing what they think might happen.

Who do you think is correct about what will happen as these two different size but same type of wax are being heated? So, we've got Laura who says, "The smaller cube will melt at a much lower temperature." We've got Lucas who thinks that, "Both wax cubes will melt at the same temperature." And then we have Jacob who suggests that, "The larger cube will need to reach a higher temperature before it melts." Who do you think is correct about what will happen as these two pieces of wax are melted? If you said Lucas, you were exactly right, well done.

Because these two pieces of wax are the same pure wax, it doesn't matter the size that is being heated, they will both melt at the same temperature.

Well done if you said Lucas.

Let's move on and have another go.

What does a scientist need to know in order to decide if a sample is pure or if it's a mixture? Is it if things were mixed to make it? Do they need to know if it is natural or has been manmade? Do they need to know what happens when it's added to water or do they need to know the temperature at which it melts? If you said D, you were exactly right, well done.

A scientist would need to know the temperature at which a sample melts to decide whether or not it's a pure substance or a mixture.

Well done.

Okay, now we're onto our last task of the lesson.

We have here that a lab technician has two jars of stearic acid.

Now stearic acid is a waxy solid at room temperature and she suspects that the stearic acid in one of those jars has been contaminated with an impurity.

Oh dear.

Now what you are gonna need to do is to describe an experiment that you could perform to decide which of those jars has been contaminated.

Now, the technician has said that you can use two grammes from each jar in order to conduct any of your tests.

What we need you to do besides describing that experiment, is also say what result would you expect to see if the sample you're testing is pure and what result would you expect to see for your sample that might be contaminated? So have a think, pause the video, jot your ideas down, and come back when you're ready to check.

Okay, so the easiest way to do this would to see what temperature that stearic acid sample melts at.

Because it's a waxy solid, we're gonna look at melting rather than boiling.

So in order to do that, you'd need to heat that sample until it melts and measure the temperature at which the melting actually occurs.

Now, if you had a pure sample, it should melt at a very specific temperature, and the contaminated sample should melt over a range of temperatures, because it would be a mixture.

It contains that impurity, and that changes the properties of the material and causes it to melt over a range of temperatures.

So well done if you managed to get any of the bit correct, it was a tricky task and you've been doing brilliantly.

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

We've learned that a pure sample of a substance is made up of only one type of chemical and that it will change its state, that's melt or boil, at a single distinct, very specific temperature.

A mixture, on the other hand, is made up of different substances that can be physically separated.

And it changes state, so that's melt or boil, over a range of temperatures.

And we've also learned that an impurity is a very small amount of a substance found in a mixture, and what it can do is change the properties of that material, including its melting point and boiling point.

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