Geologists study very old rocks to find out when oxygen first built up in the Earth's atmosphere.
Grains of the mineral pyrite, FeS2\mathrm{FeS_{2}}FeS2, are worn away and carried by rivers. They survive in river deposits only when there is little or no oxygen in the air, because oxygen reacts with them.
'Red beds' are rocks that contain iron(III) oxide, Fe2O3\mathrm{Fe_{2}O_{3}}Fe2O3. They form on land only when iron is oxidised by oxygen in the air.
The table shows the results of a study of rocks of different ages.
| Age of rock / billion years | River deposits contain pyrite grains | Red beds present |
|---|---|---|
| 3.5 | yes | no |
| 3.0 | yes | no |
| 2.5 | yes | no |
| 2.2 | no | yes |
| 1.8 | no | yes |
| 1.0 | no | yes |
Deduce, using the table, when oxygen began to build up in the atmosphere. Justify your answer.
Write the balanced equation for the reaction of iron with oxygen to form iron(III) oxide.
A student concludes, "These results prove that the first organisms that could photosynthesise appeared about 2.3 billion years ago."
Evaluate the student's conclusion.
The Earth formed about 4.6 billion years ago. Suggest why there is very little evidence about the atmosphere during the first few hundred million years.
97 exam-style questions on Edexcel GCSE Chemistry 9.2 Earth and atmospheric science, covering 9.2.1 The Earth's early atmosphere, 9.2.2 How the atmosphere evolved, 9.2.3 Chemical test for oxygen, and 9.2.4 The greenhouse effect and climate change. Each one has a worked solution and a mark scheme showing where the marks go.