Composition of the atmosphereIB MYP Chemistry: Revision notes
Section 1
What is in the air today?
The atmosphere is a mixture of gases. The approximate percentages by volume of dry air are:
- nitrogen (N₂): about 78%
- oxygen (O₂): about 21%
- argon (a noble gas): about 0.9%
- carbon dioxide (CO₂): about 0.04%
- tiny amounts of other noble gases such as neon and helium
Air also contains water vapour. Its amount changes from place to place and from day to day (it is higher in humid places), so it is left out of the figures for dry air.
Nitrogen, oxygen and the noble gases are elements. Carbon dioxide and water vapour are compounds.
Remember the order: nitrogen (78%), oxygen (21%), argon (0.9%), carbon dioxide (0.04%).
Section 2
Using the percentages
To find the volume of one gas in a sample of air, use:
volume of gas = percentage ÷ 100 × volume of air
Worked example: the volume of oxygen in 5.0 dm³ of dry air is 21 ÷ 100 × 5.0 = 1.05 dm³.
To find the percentage of oxygen from an experiment, pass air over heated copper. The copper reacts with the oxygen and turns from orange-brown to black copper oxide. If 100 cm³ of air shrinks to 79 cm³, then 21 cm³ of oxygen was used, so the air is 21% oxygen. Let the apparatus cool before reading, because a hot gas takes up more volume.
Section 3
The early atmosphere
About 4.5 billion years ago the Earth's surface was hot, with many active volcanoes. Volcanoes released gases that formed the early atmosphere:
- mostly carbon dioxide
- water vapour
- nitrogen
- almost no oxygen
This is similar to the atmospheres of Mars and Venus today.
Section 4
How the oceans formed and carbon dioxide decreased
As the Earth cooled, the water vapour condensed and fell as rain to form the oceans.
Carbon dioxide then dissolved in the oceans. Some became locked up in sedimentary rocks such as limestone (calcium carbonate) when sea creatures made shells that later settled on the sea floor. Carbon was also locked up in fossil fuels such as coal and oil.
These processes removed most of the carbon dioxide from the air.
Section 5
How oxygen built up
About 2.7 billion years ago, algae and later plants evolved. They carry out photosynthesis:
carbon dioxide + water → glucose + oxygen
Photosynthesis removes carbon dioxide from the air and releases oxygen. Over millions of years the oxygen level increased to about 21%. This allowed animals that need oxygen to evolve. Later, some oxygen formed the ozone layer, which absorbs harmful ultraviolet radiation.
The percentage of nitrogen increased because it is unreactive and was added by volcanoes, so it stayed in the air.
Do not say oxygen came from volcanoes. The oxygen in the air came mainly from photosynthesis.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Composition of the atmosphere
- A diving equipment supplier in Cape Town fills cylinders with compressed dry air. Dry air is a mixture of gases. It is mainly nitrogen and oxygen, with small amounts of argon and carbon dioxide.The gases other than nitrogen and oxygen make up about 1% of dry air. Name the gas that is the largest part of this 1%, and state the name of the group of elements to which it belongs.2 marks
- A student in Mumbai measures the percentage of oxygen in air. She puts 100 cm³ of air in a gas syringe that is connected through a tube of heated copper turnings to a second, empty syringe. She pushes the air slowly back and forth over the hot copper until the volume stops changing. When the apparatus has cooled to room temperature, the volume of gas left is 79 cm³.Use the results to calculate the percentage of oxygen in the air sample.2 marks
- Scientists believe that 4 billion years ago the atmosphere of the Earth contained mostly carbon dioxide, with water vapour and nitrogen released by volcanoes, and almost no oxygen. Today, carbon dioxide makes up about 0.04% of the air and oxygen makes up about 21%.Describe how the percentage of carbon dioxide in the atmosphere decreased as the Earth cooled.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).