KS3 · Chemistry

Composition of the atmosphere

Take a deep breath. What did you just breathe in? Most people say oxygen. But most of that breath wasn't oxygen at all. Let's find out what it really was.

Chemistry · What's in the air?

Guess first: how much of the air is oxygen?

No peeking. Drag each gas to the percentage of the air you think it makes up, then check your guesses.

Chemistry · Where the air is

Climb up through the atmosphere

A slice through the atmosphere, from the ground upwards. Every zone is drawn the same height, so this is not to scale.

↑ Higher up

↓ Earth's surface

Zone 1 of 3

Above about 15 km

Keep climbing past about 15 km and the air becomes very thin. There are very few particles up here.

  • Very few particles

Going up, the air thins out: about 80% of it hugs the Earth in the troposphere, and above about 15 km there are very few particles left.

Watch out: The atmosphere does not stay equally thick all the way up. Most of the air sits low down, close to the ground.

Chemistry · Zoom in on the air

A sample of air, zoomed in
A sample of air (particle numbers are illustrative)N₂O₂ArCO₂H₂O

View: With labels. Showing 3 layers: Sample, Particles, Formula labels

View

Layers

Explore

Tap a particle to explore it. Switch views to test yourself.

Every particle is named by its formula.

Tap each particle to read its formula. Then switch to 'Test yourself' to hide the labels and name them from memory. The numbers of each particle drawn are only for illustration, not the real amounts.

Chemistry · What each gas does

Which gas does this job?

Sort each use or role into the gas it belongs to.

Still to sort

Nitrogen (N₂) (0)

Very unreactive; the most common gas in the air.

Oxygen (O₂) (0)

Needed for respiration and burning.

Carbon dioxide (CO₂) (0)

Used by plants, and in drinks and some fire extinguishers.

9 of 9 still to sort.

Three gases from the air, nine jobs. Pick a job, choose the gas, then read why.

Watch out: Don't match a job to the gas you hear about most. Oxygen gets all the attention, but nitrogen and carbon dioxide have important uses too.

Chemistry · Why the air keeps us alive

How cold would Earth be without greenhouse gases?

Today Earth's average temperature is about 14 °C. Imagine the greenhouse gases were taken out of the air. What would the average temperature be? Slide to your guess, then lock it in.

Your estimate

0°C

-40°C40°C

Watch out: Some gases in the atmosphere block much of the Sun's harmful ultraviolet radiation, but not all of it. 'Much' is not 'all'.

Chemistry · Identifying gases

Mystery gas: follow the tests

Start with the mystery gas. At each test, pick what you see and follow that route.

Glowing splint test → Limewater test

  • First test: light a splint, blow out the flame so it is still glowing, and put it into the gas.

3 possible results.

On A test tube of colourless, odourless gas. 2 branches to choose from.

Nitrogen, oxygen and carbon dioxide are all colourless and odourless. To find out which gas you have collected, you have to test it.

Watch out: Don't mix the tests up. The glowing splint is the test for oxygen; limewater is the test for carbon dioxide.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

What is actually in the air you breathe, what each gas does, and how to tell look-alike gases apart.

What you need to know

  • The atmosphere is about 78% nitrogen (N₂) and about 21% oxygen (O₂), with small amounts of argon, carbon dioxide, water vapour and other gases.
  • Air is a mixture: its gases have not reacted together and could be separated.
  • The atmosphere is essential for life: greenhouse gases keep Earth warm, some gases block much of the Sun's ultraviolet radiation, and it provides oxygen for respiration.
  • Oxygen relights a glowing splint; carbon dioxide turns limewater milky.

The big picture

Earth's atmosphere is a layer of gases, with about 80% of it within 7 to 15 km of the surface. It is a mixture: about 78% nitrogen, about 21% oxygen, and small amounts of other gases such as argon, carbon dioxide and water vapour. It keeps us warm, blocks much of the Sun's harmful ultraviolet radiation and gives us oxygen to breathe. Each gas has its own uses, and oxygen and carbon dioxide can be identified with simple tests.

Key points

1The atmosphere is a layer of gases around the Earth. About 80% of it is within 7 to 15 km of the surface, in the troposphere; above about 15 km it is very thin.
2Air is mostly nitrogen (about 78%), not oxygen (about 21%). Nitrogen is very unreactive, which is part of why there is so much of it.
3Carbon dioxide makes up much less than 1% of the air. Argon and water vapour are also present, along with tiny particles of liquids and solids.
4Air is a mixture: the substances in it have not reacted together and could be separated.
5N₂ and O₂ are diatomic (two atoms bonded as a pair). CO₂ is one carbon atom bonded to two oxygen atoms.
6Greenhouse gases (carbon dioxide, methane, water vapour) keep Earth at about 14 °C instead of about −18 °C.
7Nitrogen: fertilisers and food packaging. Oxygen: respiration, burning fuels, steel, chemicals. Carbon dioxide: photosynthesis, fizzy drinks, fire extinguishers.
8Test for oxygen: a glowing splint relights. Test for carbon dioxide: limewater turns milky.

Worked example

Problem

Nitrogen is about 78% of the air and oxygen is about 21%. Roughly how many times more nitrogen than oxygen is there in the air?

⚠ Watch out

Mixing up the two gas tests. The glowing splint is for oxygen: it relights. Limewater is for carbon dioxide: it turns milky. A glowing splint that does not relight only tells you the gas is not oxygen.

🧠

Memory hook

Picture the air as 100 particles: about 78 are nitrogen, about 21 are oxygen, and every other gas squeezes into what's left. Nitrogen is the Number one gas; Oxygen is Only about a fifth.

✓

Check yourself

Cover the page. Can you give the rough shares of nitrogen and oxygen in the air, say why air is a mixture, and describe the tests for oxygen and carbon dioxide?

Flashcards

(15)
Where is most of the atmosphere found?
About 80% of it lies within 7 to 15 km of the Earth's surface, in the troposphere.
What is the atmosphere like above about 15 km?
Very thin, with very few particles.
Roughly what percentages of the air are nitrogen and oxygen?
Nitrogen about 78%, oxygen about 21%.
Why is there so much nitrogen in the atmosphere?
Nitrogen is very unreactive, which is part of the reason there is so much of it.
Apart from nitrogen and oxygen, what else is in the air?
Small amounts of other gases, including argon, carbon dioxide (much less than 1%) and water vapour, plus tiny particles of liquids and solids.
Why is air a mixture?
The substances in it have not reacted together, so they could be separated.
What does diatomic mean, and which gases in the air are diatomic?
Two atoms bonded as a pair. Nitrogen (N₂) and oxygen (O₂).
What atoms make up one molecule of CO₂?
One carbon atom bonded to two oxygen atoms.
What do greenhouse gases do for life on Earth?
They keep Earth's average temperature at about 14 °C rather than about −18 °C. Examples: carbon dioxide, methane, water vapour.
How does the atmosphere protect us from the Sun?
Some of its gases block much, but not all, of the Sun's harmful ultraviolet radiation.
Give two uses of nitrogen.
Making fertilisers (plants need nitrogen to make proteins) and food packaging (keeps crisps fresher for longer).
Give four uses or roles of oxygen.
Respiration in cells, burning (combustion) of fuels, making steel, and making many different chemicals.
Give three uses or roles of carbon dioxide.
A reactant for photosynthesis, the bubbles in fizzy drinks, and fire extinguishers that smother electrical fires.
What is the test for oxygen?
Put a glowing splint into the gas. Oxygen relights it.
What is the test for carbon dioxide?
Bubble the gas through limewater. Carbon dioxide turns it milky (a white precipitate forms).

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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