AQA GCSE Chemistry Chemistry Paper 2 (Higher), 2022: Question 6
10 marks · Standard Demand difficulty · Short Answer
Analyze changes in the Earth's early atmosphere using a graph of gas percentages over time, and describe how natural gas deposits were formed.
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Question text
06 This question is about the chemistry of the Earth’s atmosphere.
Figure 3 shows how the percentages of gases in the Earth’s atmosphere may have
changed since the atmosphere was formed.
Figure 3
06.1 Explain the change in the percentage of gas in the region labelled A on Figure 3.
[2 marks]
06.2 Explain the change in the percentage of gas in the region labelled B on Figure 3.
[2 marks]
06.3 Compare the changes in the percentages of gases in the region labelled C on
Figure 3.
[2 marks]
06.4 What process caused the changes in the percentages of gases in the region
labelled C on Figure 3?
[1 mark]
06.5 Natural gas is a fossil fuel.
Describe how deposits of natural gas were formed.
[3 marks]
Mark scheme
Show the mark scheme
Question 6
AO /
Question Answers Extra information Mark
Spec. Ref.
06.1 (the percentage of) nitrogen 1 AO3
increased
AO1
(because of intense) volcanic 1
activity
4.9.1.2
AO /
Spec. Ref.
06.2 (the percentage of) carbon 1 AO3
dioxide decreased
AO1
(because) carbon dioxide do not accept references to 1
dissolved in oceans photosynthesis
or 4.9.1.2
(because of) formation of
carbonate (precipitates /
sediments)
AO /
Spec. Ref.
06.3 (the percentage of) carbon 1 AO3
dioxide decreased 4.9.1.3
and 4.9.1.4
(the percentage of) oxygen
increased
the increase and decrease (in allow the changes (in 1
percentage) occur at the same / percentage) are the same /
similar rate similar in the same period of
time
AO /
Answers Extra information
Question Spec. Ref.
photosynthesis allow a description of 1 AO1 19
06.4
photosynthesis 4.9.1.4
AO /
Spec. Ref.
06.5 plankton (died) allow tiny marine organisms 1 AO1
(died) 4.9.1.4
and (the organisms) were allow and (the organisms) were 1
covered by sediments buried
allow and (the organisms) were
in anaerobic conditions
and subjected to 1
high temperature
and
high pressure
(over millions of years)
Total Question 6 10
How to answer it
The History of Earth's Atmosphere & Fossil Fuels
This question assesses your ability to interpret graphical data showing changes in atmospheric gases over 4.5 billion years. It tests your knowledge of the early atmosphere's composition, the role of volcanic activity, how oceans reduced carbon dioxide, the impact of photosynthesis, and the geological process of fossil fuel formation.
Question 06.1
Explaining early changes in Region A
Correct Answer
- The percentage of nitrogen increased [1 mark]
- Because of intense volcanic activity [1 mark]
Exam Technique
Always use the Key on the graph first! Locate region A (around 4000 million years ago) and trace the dashed line to the key to identify it as Nitrogen. Describe the trend (it goes up) and then state the scientific reason.
Key Knowledge: The Early Atmosphere
During the first billion years of the Earth's existence, there was intense volcanic activity. These volcanoes released gases that formed the early atmosphere, consisting mainly of carbon dioxide (CO₂), water vapour (H₂O), and nitrogen (N₂), which gradually built up in the atmosphere.
Question 06.2
Explaining early changes in Region B
Correct Answer
- The percentage of carbon dioxide decreased [1 mark]
- Because carbon dioxide dissolved in the newly formed oceans [1 mark]
OR due to the formation of carbonate precipitates/sediments.
Common Error Trap
❌ Do NOT mention photosynthesis here!
Region B is at 4000 million (4 billion) years ago. Photosynthetic life had not evolved yet. The examiner's mark scheme explicitly states: "do not accept references to photosynthesis" for this part.
Key Knowledge: How CO₂ first decreased
As the Earth cooled, water vapour in the atmosphere condensed to form the oceans. Carbon dioxide dissolved in this water. Carbonates were precipitated, producing sediments and reducing the amount of CO₂ in the atmosphere.
Question 06.3
Comparing gas changes in Region C
Correct Answer
- The percentage of carbon dioxide decreased AND the percentage of oxygen increased [1 mark]
- The increase and decrease occurred at the same / similar rate (or over the same period of time) [1 mark]
Exam Technique: "Compare" Questions
When asked to "compare" changes on a graph, look for relationships between the curves. Notice how the decrease in the dotted line (CO₂) perfectly mirrors the increase in the solid line (O₂). Mentioning this rate relationship is what secures the second mark!
Question 06.4
Identifying the process in Region C
Correct Answer
Photosynthesis
(An accurate description of photosynthesis is also accepted).
Key Knowledge: Photosynthesis Equation
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
Algae and plants produced the oxygen now in the atmosphere by photosynthesis. As they absorbed carbon dioxide, they released an equal molecular ratio of oxygen, explaining the identical rates of change in Region C.
Question 06.5
Formation of natural gas deposits
Step-by-Step Process of Natural Gas Formation
- Step 1 (Organism): Plankton (or tiny marine organisms) died and sank to the seabed [1 mark] .
- Step 2 (Burial): They were buried and covered by layers of sediment, creating anaerobic (oxygen-free) conditions [1 mark] .
- Step 3 (Conditions): Over millions of years, they were subjected to high temperature and high pressure [1 mark] .
Common Error: Coal vs. Gas/Oil
Do not confuse fossil fuels!
• Coal is formed from giant plants/trees in swamps.
• Crude oil and Natural Gas are formed from ancient plankton in oceans.
Examiner's Tip
To get all 3 marks, you must mention:
1. The correct starting material (plankton/marine organisms).
2. Sediment cover/burial.
3. Both high temperature and high pressure over millions of years.
Topics
Chemistry · C9: Chemistry of the Atmosphere
Question and mark scheme from the AQA GCSE Chemistry examination, Chemistry Paper 2 (Higher), 2022. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.