Lesson focus: What is in the air we depend on?
Describe air as a mixtureTwo or more substances mixed together but not chemically joined, like sand and salt. and distinguish major and trace gasA gas found in only a tiny amount in the air..
Build on earlier learning
MixturesIdentify a mixture from the relationship between its components.
You will be successful when you can…
- Recall the main proportions of dry air.
- Explain why air is a mixture.
- Distinguish dry air from variable water vapour.
- Explain why concentration is not the same as importance.
Words that carry the science
- atmosphere
- Layer of gases surrounding Earth.
- mixture
- Substances together without all being chemically joined.
- nitrogen
- Most abundant gas in dry air, about 78%.
- oxygen
- Second most abundant gas in dry air, about 21%.
- trace gas
- Gas present at a very small concentration.
Represent a mixture without hiding its small but important components
01A rounded 100-unit model
Dry air is approximately 78% nitrogenThe most common gas in the air, making up about 78% of it. and 21% oxygenA gas that makes up about 21% of the air; we need it to breathe and things need it to burn.. Most of the remainder is argon, with small amounts of carbon dioxide and other gases.
02Dry air is a defined comparison
Water vapour varies with place, weather and altitude, so standard dry-air proportions exclude it. This does not mean the real atmosphereThe layer of gases that surrounds the Earth. contains no water.
03Small proportion can have a large role
Carbon dioxide supports photosynthesis and absorbs/emits infrared radiation. Effect depends on properties and change, not only percentage.
How the atmosphere changed
The early atmosphere was mostly carbon dioxide from volcanoes, with almost no oxygen. Oceans absorbed carbon dioxide, and photosynthesis by algae and plants added oxygen. Today it is about 78% nitrogen and 21% oxygen.
Controls
Early percentages are illustrative estimates; scientists still debate them. Today’s values are measured.
Convert a proportion to volume
A sealed sample contains 200 dm³ of dry air. Estimate its oxygen volume using 21%.
- Write 21% as 0.21.
- Multiply 200 × 0.21.
- Obtain 42.
- Attach the volume unit: about 42 dm³ oxygen.
How to make the reasoning visible: The estimate assumes the rounded dry-air proportion applies to the sample.
Atmosphere model audit
Interpret the rounded model and state its limits.
| Component | Approx. units/100 | Role/example | Caution |
|---|---|---|---|
| Nitrogen | 78 | Major atmospheric gas | Rounded |
| Oxygen | 21 | Respiration/combustion | Rounded |
| Argon | Most of remainder | Unreactive noble gas | Hidden in 1 unit |
| Carbon dioxide | Trace | Photosynthesis/infrared | Too small for main scale |
| Water vapour | Variable | Weather/greenhouse gas | Excluded from dry air |
Why is air a mixture?
Why use a rescaled trace-gas inset?
Use the table. Carbon dioxide is only a trace gas. Give one reason why it is still important.
Check your understanding
Answer from memory, then check against the suggested answer.
What is atmosphere?
What is mixture?
What is nitrogen?
Which gas makes up about 78% of dry air?
About what percentage of dry air is oxygen?
Why would removing all carbon dioxide matter despite its small proportion?
Why can local humidity change while the dry-air percentages remain a useful reference?
Why is air a mixture and not a compound?
Exam Question Practice
Exam Question Practice
Mark points shown after marking
State the two most abundant gases in dry air.
(2)Most abundant: . Second most abundant: .
Explain why air is a mixture.
(2)Total for this question: 4 marks
Command word: Do not call air a compound; link mixture to component independence.