What dry air is made of, percentage and fraction calculations, and how volcanoes, oceans and living things turned an early carbon dioxide atmosphere into the air we breathe.
What you will learn
- Give the composition of dry air as percentages and fractions
- Calculate the percentage of a gas in a sample
- Describe how the early atmosphere changed into today's
What dry air is made of
For the last 200 million years or so, the proportions of gases in dry air have stayed about the same.
Dry air today is nitrogen about 78% (4/5), oxygen about 21% (1/5) and about 1% other gases: mostly argon (about 0.9%), with carbon dioxide about 0.04%.
percentage of a gas = volume of the gas ÷ total volume × 100
You are expected to know the rough figures (four fifths nitrogen, one fifth oxygen) and to work percentages out yourself.
- A 400 cm³ sample of breath contains 66 cm³ of oxygen
- percentage of oxygen = 66 ÷ 400 × 100 = 16.5%
- Estimate for 400 cm³ of dry air: oxygen ≈ 1/5 × 400 = 80 cm³
Divide by the total volume of the sample, not by the volume of the other gases.
The rest of this lesson and its 9 practice questions are locked.
In this lesson
- What dry air is made of
- The early atmosphere
- Oxygen arrives, carbon dioxide is locked away
- Try it: Estimate a volume
9 practice questions, marked as you go; about 20 minutes in all.
Key terms
Volcanic activity, Carbon dioxide, Water vapour condensed, Carbon dioxide dissolved, Algae, Sedimentary rocks
In this unit: The Earth's atmosphere
The air now and how it formed, greenhouse gases and climate change, carbon footprints and the pollutants from burning fuels. Its later lessons on water, life cycles, materials and the Haber process stay here too; Using resources teaches them in full.