AQA GCSE Chemistry Chemistry Paper 2 (Higher), June 2025: Question 4

✅ Correct Answer

The percentage of nitrogen increased [1 mark]

because of (intense) volcanic activity [1 mark]

💡 Key Knowledge

• Volcanoes released large amounts of nitrogen (N₂), along with water vapour, carbon dioxide, and small amounts of methane and ammonia.
• Nitrogen is very unreactive, so once released, it gradually built up in the atmosphere over billions of years.

🧠 Exam Technique

The command word is "Explain": you must give the trend (increased) and the reason/cause (volcanoes produced it). Giving only one part loses a mark.

❌ Common Errors

• Stating that nitrogen decreased (confusing nitrogen with CO₂).
• Claiming living organisms or plants produced nitrogen (plants need fixed nitrogen from soil; volcanoes were the original atmospheric source).

✅ Correct Answer (Any two points)

• (Carbon dioxide) dissolved in water / oceans [1 mark]
• Carbonates formed sediments / precipitates [1 mark]

💡 Timeline Knowledge

• Between 4.5 and 3.0 billion years ago, the Earth cooled, water vapour condensed, and oceans formed.
• CO₂ gas dissolved in these oceans and reacted to form insoluble carbonate precipitates, which settled as sediment on the sea bed.
• Algae did not evolve until approximately 2.7 billion years ago, so photosynthesis had not begun yet!

❌ Common Errors (Zero Marks!)

• Writing photosynthesis — organisms had not yet evolved at this early stage!
• Writing formation of shells of marine animals — complex shell-building sea life evolved much later.
• Writing formation of fossil fuels (coal/crude oil formed much later from dead plants/plankton).

🧠 Exam Technique: Check the Dates!

Always check the dates given in the question stem. Questions referring to 4.5 to 3.0 billion years ago exclusively test physical/chemical processes (oceans condensing, CO₂ dissolving, carbonate sediments), not biological processes.

📐 Data Points to Plot

• Point 1: x = 4.5 , y = 98
• Point 2: x = 4.0 , y = 54
• Point 3: x = 3.5 , y = 30
• Point 4: x = 3.0 , y = 21

✅ What the Graph Should Look Like

Plots: Four neat crosses plotted accurately within ±½ a small square tolerance.

Line of Best Fit: A single, smooth curved line sloping downwards from left to right, passing close to or through all four plotted points. It must not be drawn with a ruler point-to-point.

🧠 Graph Drawing Rules

• Use a sharp pencil and plot using small neat 'x' marks or dots with circles.
• Notice the x-axis scale goes backwards in chronological time (4.5 → 4.0 → 3.5 → 3.0), as it represents "billions of years ago".
• Work out grid values: On the vertical axis, 1 large square = 20%, meaning 1 small square = 2%. Therefore, 98% is 1 small square below 100; 54% is 2 small squares above 50.

❌ Common Errors

• "Dot-to-dot" straight lines connecting each pair of points with a ruler (loses the line mark).
• Feathery, thick, or multiple overlapping pencil lines.
• Misreading small square values (e.g. plotting 54 as 52 or 58).

✅ Correct 3-Mark Answer

1. Carbon dioxide decreased and oxygen increased [1 mark]

2. (Because of) photosynthesis [1 mark]

3. By algae / plants (or cyanobacteria) [1 mark]

💡 Key Chemistry & Biology

• Word equation:
  carbon dioxide + water → glucose + oxygen
• Symbol equation:
  6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
• Algae first produced oxygen about 2.7 billion years ago. Over the next billion years, plants evolved and oxygen levels gradually increased, allowing animals to evolve.

🧠 Exam Technique: Securing All 3 Marks

Notice the 3 distinct parts needed for full credit:

1. Describe the trends for BOTH gases (CO₂ down, O₂ up).
2. Name the process (photosynthesis).
3. Name the organism (algae / green plants).

❌ Common Errors

• Saying respiration instead of photosynthesis (respiration uses O₂ and makes CO₂).
• Only describing one gas (e.g. "oxygen increased") and forgetting to state what happened to carbon dioxide.
• Referring back to ocean dissolving — the mark scheme explicitly states: "ignore dissolving in oceans / ignore sedimentation" because by this time period, biological activity was the dominant driver.