Hi, I'm Mrs. Hudson, and today, we're going to be looking at a lesson called "Pure Substances." This is a chemistry lesson and it comes under the unit titled "States of Matter." So let's get going.

The outcome of today's lesson is, I can describe what a pure substance is and identify a pure substance from melting and boiling point data.

During today's lesson, there will be some key words that are frequently used throughout the lesson, and today those words are pure substance and property.

So let's have a look at the definitions of those keywords.

A pure substance in chemistry is a single element or compound that is not mixed with any other substance.

And property is a feature or characteristic of a substance that can be used to classify it or describe how it behaves.

If you want to write down those definitions, then please do and then press play when you're ready to carry on.

Today's lesson is going to be split up into two different parts.

In the first part of the lesson, we're going to be looking at pure substances.

And then in the second part of the lesson, we're going to be looking at melting point analysis, but let's get going, first of all, with pure substances.

In everyday language, pure means something that has had nothing added to it.

For example, pure orange juice.

And you might have been in the supermarket and seen orange juice, which is labelled as being pure orange juice.

So for example here, we've got a sign which is saying "Pure Florida Ice Cold Orange Juice." And it's saying that the orange juice has got nothing added to it.

A pure substance in chemistry is a single element or compound, not mixed with any other substance, for example, pure gold.

So here, we can see some bars of pure gold and they would only contain gold.

That would be the only element and the only atoms that would be within pure gold.

Whereas pure orange juice, it's a mixture of different elements and different compounds.

So in a chemical sense it's not pure.

Whereas pure gold, which only contains atoms of gold, is a pure substance.

Pure elements and compounds have specific melting and boiling points.

So pure gallium consists of only one type of atom, which would be just gallium atoms. And you can see a picture there of pure gallium on somebody's hand.

And the melting point of gallium is 29.

76 degrees Celsius.

And if it was pure gallium, it would always have that very specific melting point of 29.

76 degrees Celsius.

Pure water consists of only one type of compound.

So the chemical formula of water is H2O.

So you've got two hydrogen atoms and one oxygen atom chemically bonded together in compound.

And the melting point of water is zero degrees Celsius and the boiling point of water is 100 degrees Celsius.

And again, if you have pure water, then the melting of boiling points will be very specific at those temperatures.

Beeswax can be melted to form candles and we can see here an image of some bees on a honeycomb and then making beeswax.

In everyday language, beeswax is considered to be a pure substance because nothing is added to it.

The melting point of beeswax is 62 to 65 degrees Celsius.

This is a melting range and not a specific temperature.

So if you compare that to the previous slide where water's melting point was zero degrees exactly, and the boiling point was 100 degrees, that is a specific melting point.

Whereas beeswax here, has a melting range between 62 to 65 degrees.

Beeswax is chemically not a pure substance.

It is made up of more than one type of compound and we can tell that because it has a melting point range rather than a specific melting point.

Let's check our understanding so far.

Select all statements about chemically pure substances that are correct.

A, they are made up of a single element or compound.

B, they are everyday substances that have nothing added to them.

C, examples include orange juice and beeswax or D, examples include platinum and carbon dioxide.

So this is A and D.

So chemically pure substances are made up of a single element or compound and the example would be platinum and carbon dioxide.

B is wrong because the everyday use of the word pure is that something has had nothing added to them, but that's not true for the chemical meaning.

And C is incorrect because those were both examples of impure substances.

So great job if you've got those right.

Next question, urea is a pure substance.

A pure substance will have.

A, a specific melting point, B, a melting point temperature range, or C, a melting point of exactly zero degrees.

This is A, urea is a pure substance, which means it will have a specific melting point.

Remember, impure substances have a melting point temperature range, and the reason that C is wrong is because urea doesn't have a melting point of exactly zero degrees.

We said that water had a melting point of zero degrees, so not every single substance has the same melting and boiling point as water.

We're gonna look now at a typical heating curve.

So a typical heating curve for a chemically pure substance is shown below.

A temperature of the substance is measured at regular time intervals for a set time.

So what we've got is a graph there which is showing time along the x-axis in seconds, and then the temperature in degrees celsius on the y-axis.

And if you are performing a heating curve, then you have to measure the temperature of the substance at regular time intervals over a period of time.

And a typical heating curve for any substance will look like this.

So when the temperature is much lower, the substance will be in the solid state and as the temperature increases, the temperature of the substance will increase, but then it will get to a point where the line becomes horizontal.

And then when you heat the substance even more, again, you will get an increase and that will be the substance going into the liquid state.

And then we'll get another period of it being horizontal where there's no temperature increase of the substance.

But then once again, if you continue to heat that substance, the temperature will increase and the substance will enter the gas state.

Now the horizontal sections are showing the changes of state where there's no increase in the temperature of the substance, but the actual state changes taking place.

So the ringed area here, this is where the substance melts at a specific temperature and you can see that there is no change in the temperature at that point, but that is the melting point of that particular substance.

And then when you're going from a liquid state to a gas state, the next horizontal point between those that is going to be the substance boils at a specific temperature.

So the horizontal sections of the graph of the heating curve are showing you the change of state.

And if you extrapolate the horizontal sections across onto the temperature, then you can see the specific melting point of that substance.

And the same if we extrapolated the boiling point across to the temperature, we could work out the specific boiling point of that substance.

So the key things from this slide are that the horizontal sections of a heating curve are showing you the change of state for that substance.

Now if we compare this to a typical heating curve for an impure substance, you should be able to notice the different shape of the graph.

So this is what a typical heating curve will look like for an impure substance.

And notice that there are no horizontal sections on this graph, but we do still have the idea that as the temperature is increasing, we're going from the solid states to the liquid state to the gas state.

However, those horizontal sections between the states do not exist.

So at this point here, we can see that the substance is melting over a temperature range and therefore it's not totally horizontal, it's not a specific temperature if the melting point is occurring over a range of temperatures.

And the same can be said for the boiling point.

So the substance boils over a temperature range and we don't have that horizontal line representing the change of state.

And so therefore, if we extrapolate across, you'd have to go from the bottom to the top of that temperature change and that would give you the melting point range of the impure substance.

And the same for the boiling point range, we go from the bottom to the top, but the key thing here is that impure substances have melting and boiling point ranges.

There is no horizontal change of state line on that graph.

Let's check our understanding of that.

Which part of the graph shows melting of an impure substance, A, B, or C? This is B.

Well done if you got that right.

It's not A because A is showing a fixed melting point, so that must be the melting point of a pure substance.

And the reason it's not C is because that is showing the boiling point range of an impure substance.

So both B and C are impure substances, but B is showing the melting point and C is showing the boiling point range.

We're ready now to move on to task A of the lesson.

And for the first part of the task, you need to use the data to complete the table by identifying if the substances are chemically pure or impure.

So you've been given a list of substances there, the melting point range and the boiling point range, so you need to say if they are chemically, pure or impure.

So pause the video, give it your best go, and then press play when you're ready for me to go through the answers.

Let's see how we did.

So mineral oil is showing a melting point range and a boiling point range, so therefore, it must be chemically impure.

Nitrogen has got a very specific melting point and boiling point and therefore, is chemically pure.

Honey has got a melting point range, so therefore, is chemically impure.

Coconut oil also has a melting point range, so it is chemically impure.

Ethanoic acid has got a very specific melting and boiling point, so it is chemically pure.

And ethanol also has specific melting and boiling point, so it is chemically pure.

Fantastic job if you've got those right, well done.

In part two of task A, you need to complete the graphs to show the differences between a heating curve for a chemically pure and impure substance.

And you've been given a graph here which has shown you the chemically pure substance.

And you've got a graph here showing you the chemically impure substance.

So you need to draw on the heating curves for both the chemically pure and chemically impure substance.

And then for the third part, you're going to add labels to show the boiling point, the boiling range, the melting point, and the melting range.

And for part four, you're going to explain the difference between melting and boiling for a chemically pure and impure substance.

So press pause and then play when you're ready for me to go through the answers.

Let's see how we did.

So first of all, looking at a chemically pure graph, you should have a line that looks like this where we go up and then across which is horizontal, then up again another horizontal line and it goes back up.

The key thing there is that we've got a really flat horizontal line, then we should have labelled the first horizontal point as the melting point and then the second horizontal line as the boiling point.

Now we're looking at a chemically impure substance.

So the line again is going up with the temperature, but it should go up quite steeply and then have a more shallow diagonal line and then go up more steeply again, and then another shallow diagonal line.

But there should be no flat horizontal sections of this graph.

And then from the bottom to the top of the more shallow diagonal line, you're going to label that as the melting range.

And then again, we're going to have the boiling range also labelled.

So fantastic job if you manage to get that right.

Well done.

Looking now at part four, explain the difference between melting and boiling for a chemically pure and impure substance, your answer should be similar to this, but it might not be exactly the same.

A chemically pure substance will have a specific melting point and boiling point seen as a horizontal line on a heating or cooling curve.

Will not have a melting range or boiling range.

Or a chemically impure substance will have a melting range or boiling range seen as a sloped line on a heating or cooling curve and it will not have a specific melting point and boiling point or a horizontal section on the graph.

So if you want to pause the video to check your answers and add any detail in, then please do and then we will carry on with the rest of the lesson.

Great job so far.

So we know about pure substances, but now let's look in more detail at melting point analysis.

Melting point equipment can be used to determine the melting point of a substance and melting point equipment looks like this.

So what we can see is we have got a piece of apparatus where you place a substance in that you want to find the melting point of.

And in there, there is a capillary tube, which is a really tiny tube which has a sample inside of it.

And the sample is a substance that you're measuring the melting point of.

And then, we've got a thermometer which is also measuring the temperature.

Now if we looked in that in more detail, you can see that small circle is a viewing window for you to look at the sample.

And this diagram here is showing you in kind of closer detail what that looks like.

So that would be the capillary tube.

And then inside the capillary tube you've got the sample and then the thermometer sits inside of the capillary tube.

And then the substance is also inside of that capillary tube in the solid state, because remember, we're trying to measure the melting point.

So we're measuring the temperature which the substance goes from being a solid and turns into a liquid.

So you need to observe the state of substance as the temperature increases.

And then the idea is that when the substance turns into a liquid, you measure the temperature at which that happens.

So how to use the melting point equipment? Well, first of all, you've got the substance in the solid state and you're going to heat that substance up.

And the whole time you're going to be looking at this sample and looking at when that substance changes state into a liquid and reading off the temperature at which that occurs at.

So when the substance melts and becomes in the liquid state, read the temperature on the thermometer.

And we can see there that we're showing the eyes looking at the thermometer to show that and record the temperature at which that substance has melted.

Let's check our understanding.

True or false.

When measuring the melting temperature of a substance using melting point equipment, you need to record the starting temperature only.

True or false? That is false.

And now justify that answer.

A, the temperature needs to be recorded every 30 seconds, or B, the temperature needs to be recorded when the substance changes from a solid to a liquid.

This is B.

So when measuring the melting temperature of a substance, you just need to record the temperature of which the substance changes from a solid to a liquid.

So fantastic job if you've got that right.

The melting points can be used to assess the purity of a substance.

Pure substances have a specific melting point at a single temperature rather than over a temperature range.

So here, we've got pure aspirin crystals.

Pure aspirin, acetyl salicylic acid has a melting point of 135 degrees Celsius.

And this is a very specific melting point at that particular temperature.

When aspirin is manufactured, the melting point is tested using melting point equipment and this will tell the manufacturers whether the aspirin is pure or impure.

And if it melts very specifically at 135 degrees, they know that the aspirin is pure.

However, if the aspirin melted over a range, then they would know that there was something else inside of the aspirin making it impure.

We can use the principles of melting point analysis with either heating or cooling curves to find out whether substances are pure or impure.

So we've looked specifically at heating curves so far where we apply heat to the substance and at regular intervals, check the temperature, but you can also do cooling curves would involve heating up a substance to its melting point so that the substance melts.

And then you remove the heat and measure the temperature at set intervals.

And what you can see on the graph on the left, so this cooling curve, you can see that there are, again, horizontal sections of that graph which are showing the fixed points at which the substance is freezing and also condensing.

So the circled section there is showing you the fixed freezing points of that substance.

Whereas if we look at the cooling curve on the right, we can see this is an impure substance because when that substance is freezing, it's freezing over a range of temperatures, which is showing that there must be an impure substance there.

Let's check our understanding.

Select two options.

When carrying out a cooling curve experiment.

A, the substance starts in the solid state and is heated continuously throughout the experiment.

B, the substance is heated until it is in the liquid state, the heat source is then removed.

C, and the temperature is measured after a set time and D, and the temperature is measured at regular intervals for a set time.

So this is B, the substance is heated until it's in the liquid state and then the heat source is removed to allow the substance to cool.

And then D, and the temperature is measured at regular intervals for a set time.

So you're having to monitor the temperature at regular intervals, not just measuring the temperature after a certain amount of time.

Great job if you got that right, well done.

We're ready now to move on to task B of the lesson.

And you're going to carry out two cooling curve experiments to compare paraffin wax to a substance selected by your teacher, for example, stearic acid or salol.

And then you're gonna plot a graph of temperature versus time.

Example cooling curve data is provided in the additional materials.

From the graph, identify the melting point range for each substance.

And then in part three, write a conclusion to say if the substance is a pure or impure substance.

Use the sentence starters below if needed.

So the sentence starter, name of substance, so that's the substance you're doing the cooling curve for is an impure substance because.

And their name of substance is a pure substance because.

Sure, you're gonna have a really good time doing this, give it your best go and then press play ready for me to go through the answers.

So let's look at the cooling curve for paraffin wax and salol.

So here, your graph should look something like this.

it might not be exactly the same, but you should have the x-axis labelled as time with the unit seconds and the y-axis labelled as temperature and degrees celsius.

And the key here is that the green is the paraffin wax and the blue is the salol.

And what we should see is that salol has a very specific melting point at 41 degrees Celsius and that should be very specific.

Whereas paraffin wax melts over a temperature range between 54 to 68 degrees Celsius.

So that should be what your cooling curve looks like.

And then writing a conclusion to say the substance is pure or impure, the paraffin wax is an impure substance because it has a melting range from 54 to 68 degrees Celsius, whereas salol is a pure substance because it has a specific melting point at 41 degrees.

So fantastic job if you manage to get those right.

Really well done.

Let's look now at summarising everything we've learned this lesson.

So today we've been looking at pure substances and then melting points and using heating and cooling curves.

So we started off by saying a pure substance in chemistry is a single element or compound, not mixed with any other substance.

And the definition of pure means something different in its everyday usage.

So remember that pure orange juice is not actually chemically pure because pure orange juice contains lots of other substances as well.

Pure elements and compounds have a specific melting and boiling point, which we can see on a heating and cooling curve because there's a horizontal section at a very specific temperature.

And melting point data can be used to recognise substances with specific melting points as pure substances.

So if you wanted to investigate whether a substance was pure, you could measure the melting point and if it was over a range, it would be an impure substance if it has a specific melting point then it is a pure substance.

I really enjoyed today's lesson.

I hope you have too, and I look forward to seeing you next time.