AQA GCSE Chemistry Chemistry Paper 2 (Foundation), June 2022: Question 3

📐 Step-by-Step Calculation

✅ Correct Answer

X = 78.09%

🧠 Exam Technique

Always double-check that you used the correct column! Table 5 has two columns: "early atmosphere" and "atmosphere today". Make sure you do not add the 1.8% from the early atmosphere column.

❌ Common Errors

Writing 78% or 80% from standard recall memory rather than calculating the precise value from the given data table.

✅ Correct Answers

• ✔ Ammonia (NH₃)
• ✔ Methane (CH₄)

💡 Key Knowledge

Intense volcanic activity released mainly carbon dioxide and water vapour, with small amounts of methane, ammonia, and nitrogen.

❌ Common Errors

Selecting Coal or Limestone. These are solids/rocks, not gases! Poly(ethene) is a modern synthetic plastic polymer.

🧠 Exam Technique

Look at state of matter hints in the question text: "Which two other gases...". Filtering out solids instantly leaves only Ammonia and Methane.

✅ Correct Answer

✔ Photosynthesis

💡 Key Knowledge

Algae first produced oxygen around 2.7 billion years ago. Photosynthesis word equation:

carbon dioxide + water → glucose + oxygen

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

✅ Correct Answers

• ✔ Dissolving carbon dioxide in oceans
• ✔ Formation of sedimentary rocks

💡 Key Knowledge

• Oceans formed: Water vapour condensed; CO₂ dissolved to form soluble carbonate compounds and marine shells.
• Sedimentary rocks: Locked up carbon in rocks like limestone (calcium carbonate, CaCO₃).

❌ Common Errors

Selecting "Burning fossil fuels" or "Eruption of volcanoes". Both of these processes increase atmospheric CO₂, rather than decrease it.

✅ 03.5: Oxygen at 8%

1500 millions of years ago

✅ 03.6: Nitrogen Constant

2500 millions of years ago

🧠 Exam Technique: Reading Figure 3

• For 03.5: Find 8% on the vertical axis (each small square = 2%). Follow across to the solid line (Oxygen), then trace directly down to the horizontal axis to find 1500 .
• For 03.6: Find the point where the dashed line (Nitrogen) stops rising and levels out completely flat at 80%. This occurs around 2500 million years ago.
• Note the scale on the x-axis: it counts backwards from 4500 to 0 (millions of years ago).

❌ Common Errors

Mixing up the line styles from the key (dotted line = CO₂, dashed = N₂, solid = O₂). Always trace the line with a ruler to avoid reading the wrong trace.

✅ Correct Answer

✔ Plankton

💡 Key Knowledge: Fossil Fuel Origins

• Crude oil & natural gas: Formed from deposits of plankton buried under high heat and pressure in seabed mud over millions of years.
• Coal: A sedimentary rock formed from thick plant deposits (trees and ferns in ancient swamps).

✅ Correct Answer (Any one of the following)

• There is limited evidence / lack of proof.
• It occurred a very long time ago (billions / millions of years ago).
• No humans were alive to take measurements or keep records.

🧠 Examiner Insight

Examiners look for the word "evidence". A common weak response is "because they are guessing"—this does not get the mark. State that evidence is limited because it happened 4.6 billion years ago.