AQA GCSE Combined Science: Trilogy Chemistry Paper 2 (Foundation), November 2020: Question 2

✅ Correct Answer

96 %

Note: Any value in the range of 95–97 % is accepted.

🧠 Exam Technique: Reading Grid Scales

Always check the value of each grid square before writing your answer:

• On the y-axis, there are 10 small squares between 80 and 100.
• Each small square = 2 % (20 ÷ 10).
• The peak line touches 8 small squares above 80:
  80 + (8 × 2) = 96 % .

✅ Correct Answers (Tick Two)

• ☑ Formation of sedimentary rocks
• ☑ Photosynthesis

💡 Key Knowledge

• Photosynthesis: Algae and primitive plants evolved (~2.7 billion years ago) and absorbed CO₂:
  6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ .
• Sedimentary rocks: CO₂ dissolved in oceans forming carbonate precipitates, which became locked up in rocks like limestone (calcium carbonate, CaCO₃).

❌ Common Errors

Students often incorrectly select combustion or volcanic activity. Both of these processes release CO₂ into the atmosphere, rather than reducing it.

✅ Correct Answer (Tick One)

☑ Carbon footprint

💡 Key Term Breakdown

• Carbon footprint: The total amount of carbon dioxide and other greenhouse gases emitted over the full life cycle of a product, service, or event.
• Global dimming: Caused by particulate matter (soot) reflecting sunlight back into space.
• Greenhouse effect: Trapping of thermal energy by gases in the atmosphere.

✅ Table 1 Answers (02.4)

Mass of CO₂ dissolved in 1 dm³ of water:

• At 5 °C: 3.3 g (allow 3.2 – 3.4 g) [1 mark]
• At 15 °C: 2.0 g (allow 1.9 – 2.1 g) [1 mark]

📐 Step-by-Step Calculation (02.5)

Difference in mass:

1. Identify values: Mass at 5 °C = 3.3 g , Mass at 15 °C = 2.0 g
2. Subtract smaller from larger:
   3.3 - 2.0 = 1.3 g

Answer: 1.3 g [1 mark]

🧠 Error-Carried-Forward (ECF)

If you misread a value in Table 1 for part 02.4, you can still gain the full 1 mark in 02.5 if you correctly subtract your two values! Never leave calculation steps blank.

❌ Common Error: Axis Scales

On Figure 3, each small grid square along the y-axis represents 0.1 g, and along the x-axis represents 1 °C. Reading 5 °C requires finding the midpoint between 0 °C and 10 °C.

✅ Correct Order (Tick One)

☑ D, B, C, A

💡 The Greenhouse Mechanism Explained

1. Stage D: Shorter wavelength radiation enters the atmosphere from the Sun (UV and visible light).
2. Stage B: Radiation is absorbed by the Earth, warming the surface.
3. Stage C: Longer wavelength radiation is emitted by the warm Earth (infrared thermal radiation).
4. Stage A: Carbon dioxide stops longer wavelength radiation escaping by absorbing infrared and re-radiating it back towards Earth.

❌ Wavelength Trap

Remember: Sun radiation = short wavelength (passes straight through greenhouse gases). Earth radiation = longer wavelength / infrared (absorbed by greenhouse gas bonds).

✅ Acceptable Answers (Any Two)

• Rising sea levels
• Melting ice caps / glaciers
• Extreme weather events (e.g. severe storms, hurricanes)
• Coastal flooding and erosion
• Droughts and water shortages
• Forest fires / wildfires
• Agricultural problems (crop failure, food shortages)
• Habitat loss / change or extinction of species

🧠 Exam Technique: Be Specific!

Examiners look for clear, descriptive consequences:

• Write "coastal flooding" or "sea levels rising" rather than just "water".
• Avoid vague answers like "harming nature" or "bad weather". State explicit effects like "droughts" or "changes in animal migration/habitats".