This lesson is called Gases in the Atmosphere, and is from the unit, Carbon Cycle and Climate Change.

Hi there.

My name's Mrs. McCready and I'm here to guide you through today's lesson, so thank you very much for joining me today.

In our lesson today, we're going to name the main gases in the atmosphere and describe the test and some of the uses for each one.

Now we're gonna come across a number of keywords in our lesson today, and they're listed up here on the screen for you now.

You may wish to pause the video and make a note of them, but I will introduce them to you as we come across them.

So in our lesson today, we're going to first of all consider the earth's atmosphere and what that is and where it's located, then we're going to have a look at some uses for gases before we consider how we might test to see whether those gases are present.

So, are you ready to go? I certainly am.

Let's get started.

So the earth's atmosphere is a layer of gases that surrounds the earth, and you can see in the diagram there that roughly 80% of the atmosphere is located within seven to 15 kilometres of the surface.

That's what we would consider to be air, and that's what we breathe, so the atmosphere is mainly located within 15 kilometres of the surface of the earth.

The atmosphere does extend much further than that, but beyond 15 kilometres, it gets very, very thin and is made up of very, very few particles.

Now, that atmosphere up to 15 kilometres away from the earth is called the troposphere, and that contains about 80% of the atmosphere.

Now, the atmosphere is made mainly of nitrogen and oxygen.

About 78% of it is nitrogen, about 21% is oxygen, and then there are very, very small quantities of other gases and particles of liquids and solids, and these include argon, carbon dioxide, and a whole host of other gases and particles.

And you can see that represented there in the pie chart.

So what percentage of the earth's atmosphere is nitrogen? About 78%, about 21%, about 1% or much less than 1%? I'll give you five seconds to decide.

Okay, so you should have said that of nitrogen, the earth's atmosphere is made up of about 78%.

Well done if you got that correct.

And what about the percentage of carbon dioxide in the atmosphere? The same values.

Which one's correct for this one? Okay, so for carbon dioxide, you should have said much less than 1%.

Well, I dunno if you got that correct.

So the atmosphere is a mixture.

It's made up of lots of different substances, lots of different gases, some liquids and solids, and we know that a mixture is a combination of two or more substances that are mixed up, physically mixed up, but they can be separated.

So they haven't reacted together with each other, they are just sitting alongside each other, and so, if we were clever and careful enough with it, we would be able to separate those substances out into their various different parts.

So the atmosphere is made up of nitrogen, N2, oxygen, O2, argon, Ar, carbon dioxide, CO2, and water vapour H2O.

So which of the following is the most accurate description of the earth's atmosphere? Is it A, a mixture of a few types of gas? B, a mixture of many types of gas, C, a mixture of a few types of gas and particles of liquids and solids, or D, a mixture of many types of gas and particles of liquids and solids.

I'll give you five seconds to consider.

Okay, so the most accurate description is D, a mixture of many types of gas and particles of liquids and solids.

Well done if you've got that correct.

So we've seen that the earth's atmosphere extends well beyond the surface of the earth, but 80% of it is contained within the lowest 15 kilometres in what is called the troposphere.

Now, greenhouse gases, which are present within the troposphere, keep the average temperature of the earth at about 14 degrees.

So overall across the earth, the average temperature is about 14 degrees centigrade, but if those greenhouse gases weren't there, then the earth's atmosphere wouldn't be warmed by them, and therefore the earth's surface temperature would instead be an average of about -18.

Very, very cold indeed.

In fact, -18's about the temperature of your freezer.

So if you imagine living inside your freezer, that would be very, very cold indeed, and therefore wouldn't be very suitable for life.

Life in fact wouldn't exist on Earth if it weren't for the greenhouse gases present within the atmosphere, because of the way they radiate heat around the earth and keep the earth warmer than it would otherwise be if those gases weren't present.

So the earth's atmosphere is essential for human life and it's also essential for all of the other life that is present on Earth.

Now, I've said that this is maintained by the greenhouse gases present within the atmosphere, and these include carbon dioxide, methane, nitrogen dioxide, and water vapour.

So all of these gases are greenhouse gases and all contribute to keeping the earth warmer than it would be if they weren't present within the atmosphere.

There are other gases present within the atmosphere as well, and some of them block the really harmful rays of ultraviolet radiation that are coming from the sun.

Now, they don't stop all of that radiation coming through the atmosphere and down to the surface, but they do block out the vast majority of them, and that means also, that life on earth can exist, because if those gases weren't present within the atmosphere, then those harmful UV radiations would get through the atmosphere and essentially destroy DNA and stop life from existing on Earth.

So, the atmosphere has multiple purposes and all about keeping conditions suitable for life on Earth.

Now, present also within the atmosphere is oxygen, and we've seen that there's about 21% oxygen within the atmosphere, and oxygen is essential for life on Earth because the vast majority of life on Earth requires oxygen in order to be able to survive, and that's because oxygen is necessary for a process called respiration, and respiration is an essential life process.

It is a key feature of all cells, and this is a way of transferring energy from one place to another, and so in most cells across life on Earth, oxygen in the atmosphere is combined with glucose within the organisms' body, within the organisms' cells, and that makes water and carbon dioxide, and also releases energy as part of that, and then the energy can be used to power all the other life processes that that organism needs in order to be able to live and survive.

So, we've got a number of different features of the atmosphere that enable life on Earth to exist.

The presence of oxygen, the presence of UV radiation reflecting molecules, and the presence of greenhouse gases which help to keep the earth warmer than it otherwise would be.

So, why is the earth's atmosphere essential for human life? Is it because it maintains a warm temperature, or because it blocks all harmful ultraviolet radiation, or because it provides oxygen for cellular respiration? I'll give you five seconds to decide.

Okay, so you should have decided that the earth's atmosphere is essential because it maintains a warm temperature and it provides oxygen for cellular respiration, but it doesn't block all harmful radiation.

It only blocks some harmful ultraviolet radiation, so well done if you spotted that amendment.

So, what I'd like you to do is to summarise this part of the lesson by firstly describing the composition and structure of the earth's atmosphere, and then using the data in the table to draw a bar chart to show the different percentages of the different gases in the earth's atmosphere.

So use the data in that table and present that as a neat graph.

Remember to use a pencil and a ruler whilst drawing it.

So, pause the video and come back to me when you're ready.

Okay, let's see what you wrote.

So firstly, I asked you to describe the composition and the structure of the earth's atmosphere, and you should have said that it is firstly a mixture of many different gases and particles of solids and liquids, and it surrounds the earth, that most of it, about 80% of it is very close to the surface of the earth, and you might have said within the first 15 kilometres, and some of it extends much, much, much further out into space as a very, very, very thin gas.

Well done if you mentioned all of those points, and even better if you added extra bits in that I haven't got there on the screen.

Very good job indeed.

Then, I asked you to draw the data as a bar graph to show the different percentages of gases in the atmosphere, and so you should have put the gases along the X axis and labelled them, and then the percentage that they appear within the atmosphere in on the Y axis.

And you can see my scale is going up in twenties all the way up to 80, and I've listed nitrogen, oxygen and argon as separate substances, and then the last one, all the other gases and particles of solids and liquids as a very, very thin bar across the bottom because of the extremely small quantity that they are present within the atmosphere.

So just check your graph over.

Have you used a ruler? Have you got the scales correct? Have you labelled up all of the different parts of the graph, included the columns, the axes, and included a title for your graph as well? Well done if you've done all of that.

Good job indeed.

Okay, let's move on to have a look at what these gases are used for.

So let's start with nitrogen then.

So nitrogen has a molecular formula of N2, N standing for nitrogen and two meaning that it comes as a pair, one nitrogen atom attached to another nitrogen atom as a molecule.

Now, nitrogen gas is colourless and it is odourless, and it's also very unreactive, which is why there is so much of it within the atmosphere.

Now, it's got many different uses as well as being present in the atmosphere.

It's also used in the production of fertilisers, because plants need nitrogen in order to make proteins, and then we use those proteins to make our own proteins.

It's also used as food packaging to fill up bags of food and it's used because nitrogen doesn't cause crisps and things like that to go stale, whereas the general atmosphere, and, in particular, carbon dioxide does.

So, if you read on the back of a crisp packet for instance, you'll see that it's packaged in a protective atmosphere.

Well that just means that it is packaged within nitrogen because that will keep it fresher for longer.

Oxygen has a molecular formula of O2, so that's also diatomic.

That means it appears as a pair, oxygen bound to oxygen, and it is also, just like nitrogen, colourless and odourless.

Now, we've already seen how oxygen is needed in respiration in cells, and also for combusting fuels.

So burning fuels, that requires oxygen as well, but it's also used in the production of steel and it is also used in the manufacture of many different chemicals.

So, oxygen has plenty of uses beyond the cell and beyond burning things.

Carbon dioxide is also colourless and odourless.

This is why the atmosphere doesn't really smell.

And carbon dioxide is CO2, that's its molecular formula, carbon and oxygen, two oxygens, bound together in a three-atom molecule.

Now, carbon dioxide has many uses beyond being a greenhouse gas, including being a reactant for photosynthesis.

So plants use carbon dioxide and water in order to build their bodies by making glucose.

Also, carbon dioxide is used to make the bubbles in fizzy drinks.

So your Coke and Fanta and things like that are all bubbly because of the carbon dioxide that is fizzing through them.

It's also used in some fire extinguishers, it's because it can be used to put out things like electrical fires, because it won't burn in those sorts of fires, but it can smother them to stop them from burning.

So again, carbon dioxide has plenty of uses.

So which of these gases are colourless and odourless? Nitrogen, oxygen, methane, carbon dioxide.

I'll give you five seconds to decide.

Okay, so you should have chosen nitrogen, oxygen, and carbon dioxide as colourless and odourless.

You might also have chosen methane, which is correct also, but we tend to be able to smell methane because it's often mixed with another substance that makes it smell really quite unpleasant, and that means that we can detect it.

So I dunno if you selected all four of those.

That was a bit of a trick there.

Okay, so what I'd like you to do is just to summarise that part of the lesson by giving for each of these gases, the molecular formula, and stating one use.

So for nitrogen, oxygen, and carbon dioxide, what is the molecular formula for each of these and one use for them? So pause the video and come back to me when you're ready.

Okay, let's check our work.

So for nitrogen, you should have said that its molecular formula is N2 and it is used in the production of fertilisers and in food packaging.

For oxygen, it's molecular formula is O2, and it is used in respiration, in the combustion of fuels, in the production of steel, and in the manufacture of chemicals.

And for carbon dioxide, that molecular formula is CO2, and it is used in photosynthesis, in carbonated drinks, and in fire extinguishers.

Now please note that all of those letters are capital letters, and all of those numbers are subscript.

That means they're tiny and written sort of below the line.

So please just check the way you've written those letters, make sure you've written them correctly, and well done.

Okay, let's move on to the last part of our lesson which is about how we test for these gases, because these gases can be tested for using some very straightforward scientific procedures, and we need to be able to state these and also have a go at doing them.

So, to test for the presence of oxygen, we can take a splint and we can light it and then allow it to burn for a few seconds.

Then if we blow the flame out and leave it as a glowing splint, we can then insert that into oxygen and the splint will relight because oxygen is present.

So the glowing splint, and it has to be glowing, it can't be completely grey and cold, it has to be glowing, that will relight if oxygen is present.

So that's the test for oxygen gas, and you can see that in the video.

The test for carbon dioxide is to bubble carbon dioxide gas through a liquid called limewater, and if carbon dioxide is present, then as it bubbles through limewater, a white precipitate will form.

That means that the liquid will turn cloudy, 'cause it's normally colourless and completely see-through, but when you bubble carbon dioxide through it, it turns cloudy, this precipitate, this solid substance, is formed and the liquid turns a milky white, it goes cloudy, and you can't see through it anymore.

So watch the video to see how this experiment is conducted.

Okay, let's check our understanding.

What is the positive test for oxygen gas? Is it that it will relight a glowing splint, or it will make a squeaky pop sound when lit, or it will turn limewater cloudy? I'll give you five seconds to decide.

So you should have said that the positive test for oxygen gas is relighting a glowing splint.

Well done.

What about the positive test for carbon dioxide gas? Same options.

Which one for CO2? So, for carbon dioxide gas, you should have said that it will turn limewater cloudy.

Well done.

So what I'd like you to do is to firstly follow the method to prepare oxygen gas, and then test for the presence of that oxygen gas.

So you are going to use hydrogen peroxide and manganese dioxide to produce oxygen, and then you're going to light a splint, blow it out, but leave it glowing, and then insert that splint into the test tube to see whether oxygen has been produced.

If oxygen is present, the splint will relight.

Once you've had a go at doing that, I would then like you to prepare and test for carbon dioxide gas.

So to do that, you'll need to put limewater into a boiling tube, then add hydrochloric acid and some marble chips to produce the carbon dioxide, bubble the gas through the limewater and see whether it turns cloudy.

If it does, then carbon dioxide has been produced.

And once you've done both of those practicals, I would like you to then describe how we can test for the presence of oxygen gas and also how we can test for the presence of carbon dioxide gas.

Write those down.

So pause the video and come back to me when you've finished all of that.

Okay, let's just check what you've written.

So, to test for the presence of oxygen gas, you light a splint, blow out the flame and leave it glowing, and then insert that into the gas.

If oxygen is present, then the splint will relight.

As for testing for carbon dioxide gas, we bubble the gas through limewater, and if carbon dioxide is present, a white precipitate will form turning the limewater milky.

Did you use that word precipitate? If you haven't, maybe you can add that into your notes now, and well done indeed for writing those descriptions.

Okay, we've come to the end of our lesson now.

I hope you've enjoyed all those practicals as well.

What we found out today is that the earth's atmosphere is located mostly within seven to 15 kilometres of its surface, and is a mixture of 78% nitrogen, 21% oxygen, and then very small amounts of other gases and liquids and solids.

Now, the earth's atmosphere is essential for life on Earth, partly because it contains greenhouse gases which help to keep the earth's temperature warm.

Also, because some of the gases within the atmosphere block out some of the most dangerous ultraviolet radiation from the sun, and also because of the presence of oxygen, which is required for respiration in living cells.

We've also seen how nitrogen gas is unreactive, which is in part why there's so much of it in the atmosphere, we've seen how oxygen will relight a glowing splint, and we've also seen how carbon dioxide reacts with limewater to form a white precipitate.

So I hope you've enjoyed our lesson today.

Thank you very much for joining me, and I hope to see you again soon.

Bye.