2.3.1Inorganic Chemistry
Composition of Air — Edexcel International GCSE Science Double Award 4SD0 - Chemistry Revision Notes & Topical Questions
5 min read 10 parts Lesson 6 of 52
Part 1 of 4What the atmosphere is made of
| Gas | Approximate proportion | Notes |
|---|---|---|
| Nitrogen, N2 | About 78% (roughly four-fifths) | Very unreactive, so it mostly just dilutes the other gases. |
| Oxygen, O2 | About 21% (roughly one-fifth) | Needed for respiration and for combustion. |
| Argon and other noble gases | About 0.9% | Completely unreactive; argon is the most abundant of them. |
| Carbon dioxide, CO2 | About 0.04% | A tiny amount, but an important greenhouse gas. |
| Water vapour | Variable, up to a few per cent | Amount depends on the weather and location, so it is usually left out of 'dry air' figures. |
Part 2 of 4Key facts to remember
Two gases dominate
- Two gases dominate Nitrogen and oxygen together make up about 99% of dry air.
- In exam answers it is acceptable to give the values as approximately 80% nitrogen and 20% oxygen.
A stable composition
- A stable composition The proportions of the main gases have changed very little over about 200 million years.
- Plants and algae release oxygen by photosynthesis, while respiration and burning use it up, so these processes roughly balance.
Small amounts, big effects
- Small amounts, big effects Carbon dioxide makes up only a very small fraction of the air, yet it traps heat and so contributes strongly to global warming.
- Rising levels from burning fossil fuels and making cement are linked to the enhanced greenhouse effect.
Part 3 of 4Finding the percentage of oxygen using phosphorus
1
Float a small dish holding a little phosphorus on water inside a trough, and place an upturned bell jar over it so that the water level is the same inside and outside the jar.
2
Ignite the phosphorus; it burns and combines with the oxygen in the trapped air to form a solid oxide.
3
As the oxygen is used up, the pressure inside the jar falls and water is pushed in from the trough, so the water level inside the jar rises.
4
Wait until the apparatus cools and the burning has stopped, then measure the volume of air that was lost.
5
Divide the volume lost by the original volume of air and multiply by 100 to get the percentage of oxygen. The answer should be close to 20%.
Part 4 of 4Evaluating the phosphorus method
Burning phosphorus in a bell jar
Advantage
- Advantage Phosphorus burns readily and keeps burning until essentially all the available oxygen has been used, so the result is reliable.
Disadvantage
- Disadvantage Phosphorus is toxic and ignites easily, so it must be handled with great care and only in small quantities.
Why nitrogen is left
- Why nitrogen is left Only oxygen reacts, so the remaining gas, mostly nitrogen, stays in the jar and the water cannot rise any further.
Key terms
| Term | Meaning |
|---|---|
| Atmosphere | The layer of gases surrounding the Earth. |
| Noble gases | The very unreactive Group 0 gases such as argon, neon and helium. |
| Greenhouse gas | A gas such as carbon dioxide or methane that absorbs heat radiation and helps warm the Earth. |
| Water vapour | Water in the gaseous state; its proportion in air varies from place to place. |
| Dry air | Air with the variable water vapour left out, used when quoting standard percentages. |
Avoid these mistakes
1
Saying air is mostly oxygen. Air is about four-fifths nitrogen; oxygen is only about one-fifth.
2
Claiming carbon dioxide is unimportant because there is so little of it. Even at about 0.04% it has a large effect as a greenhouse gas.
3
Stating that the water rises in the bell jar because oxygen is 'sucked' out. The phosphorus reacts with oxygen, lowering the pressure inside, and the outside pressure pushes the water in.
4
Forgetting that the percentages are approximate and that water vapour is excluded from the standard figures.
Last updated:
\n