AQA GCSE Chemistry Chemistry Paper 2 (Foundation), June 2025: Question 6

✅ Correct Action

Draw a smooth curved line continuing from the point at 3.50 billion years ago, passing smoothly through or very close to the points at 3.25, 3.00, and ending at 2.75 billion years ago.

🧠 Exam Technique

• Turn the paper so your wrist acts as a natural pivot to draw a single smooth curve.
• Look ahead at the target points rather than looking right under your pencil tip.
• Do not use a ruler for curved trends.

❌ Common Errors

• Dot-to-dot lines: Joining points with straight ruler lines loses the mark.
• Feathered/hairy lines: Multiple overlapping sketchy pencil strokes.
• Stopping before 2.75 billion years ago.

✅ Correct Answer

40 %

🧠 Step-by-Step Method

1. Locate 3.75 on the horizontal axis (halfway between 3.50 and 4.00).
2. Follow the grid line vertically up to meet the plotted point on the curve.
3. Track horizontally left to the vertical axis to read the value: exactly 40.

❌ Common Errors

Misreading the horizontal axis scale (each large grid square = 0.50 billion years, so each small square = 0.05 billion years).

✅ Correct Box to Tick

☑ Carbon dioxide was dissolved in oceans.

💡 Key Knowledge

As the Earth cooled, water vapour produced by volcanoes condensed to form the oceans. Huge amounts of CO₂ dissolved in this water, forming soluble carbonate compounds and precipitates (sediments).

❌ Why Other Options Are Wrong

• Combustion of fossil fuels: Fossil fuels had not formed yet, and combustion releases CO₂, it does not remove it.
• Released during volcanic activity: Volcanoes increased CO₂, rather than decreasing it.

✅ Correct Box to Tick

☑ Nitrogen was released during volcanic activity.

💡 Key Knowledge

Intense volcanic activity in the first billion years produced large volumes of nitrogen gas (N₂). Because nitrogen is an unreactive gas, it gradually built up in the atmosphere over geological time.

🧠 Exam Tip

Remember the volcanic emissions: mostly carbon dioxide and water vapour, with small but increasing amounts of nitrogen, plus traces of methane (CH₄) and ammonia (NH₃).

✅ Correct Box to Tick

☑ 2.5 billion years ago

💡 Key Knowledge

Today's atmosphere is composed of approximately 78% to 80% nitrogen. Looking at Table 2:

• 3.0 billion years ago = 75%
• 2.5 billion years ago = 78% (matches modern levels!)

❌ Common Trap

Selecting 4.5 billion years ago by confusing the age of the Earth with when the atmosphere reached modern composition.

✅ Correct Boxes to Tick (Select Two)

• ☑ Ammonia (NH₃)
• ☑ Methane (CH₄)

💡 Key Knowledge

The early atmosphere resembled the atmospheres of Mars and Venus today. Along with CO₂, H₂O, and N₂, volcanoes released trace amounts of:

• Methane (CH₄)
• Ammonia (NH₃)

❌ Distractors to Avoid

• Chlorine (Cl₂): Far too reactive to exist free in early atmospheric air.
• Helium / Hydrogen: Extremely light gases that easily escaped Earth's gravitational pull into space.

✅ Correct Answer

Photosynthesis

💡 Key Knowledge

Around 2.7 billion years ago, algae first evolved and began producing oxygen gas via photosynthesis:

carbon dioxide + water → glucose + oxygen
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Over the next billion years, green plants also evolved, boosting O₂ levels to allow animals to evolve.

❌ Common Errors

Writing "respiration" instead of photosynthesis. Aerobic respiration consumes oxygen, whereas photosynthesis produces it!