AQA GCSE Combined Science: Trilogy Chemistry Paper 2 (Foundation), 2024: Question 1

✅ Correct answers

• Nitrogen → 80%
• Oxygen → 20%

Marks are awarded separately for each correct line drawn. One mark per correct match.

💡 Key knowledge

• Today’s atmosphere is mainly nitrogen and oxygen.
• Approximate composition:
  • Nitrogen = 78% (often rounded to 80%)
  • Oxygen = 21% (often rounded to 20%)

🧠 Exam technique

• Use the approximate values the exam expects, not exact science textbook values.
• Draw one line from each gas to one percentage box only.

❌ Common errors

• Swapping nitrogen and oxygen.
• Choosing 60% or 40% because they “look reasonable”.
• Drawing more than one line from the same gas label.

✅ Correct answers

• Dissolving in oceans
• Photosynthesis

Each correct tick = 1 mark. You need two correct boxes for full marks.

💡 Key knowledge

• Carbon dioxide was removed from the atmosphere by dissolving in the oceans.
• Plants, algae and cyanobacteria used photosynthesis to take in CO₂.
• Photosynthesis can be remembered as: carbon dioxide + water → glucose + oxygen

🧠 Exam technique

• Only choose options that clearly remove CO₂.
• The question asks for reasons why CO₂ decreased, not reasons for oxygen increasing.

❌ Common errors

• Combustion of fuels adds CO₂, so it is wrong here.
• Deforestation reduces photosynthesis, so it increases CO₂, not decreases it.
• Respiration releases CO₂.

✅ Correct answer structure

Top-mark description:

• The percentage of carbon dioxide is constant at about 0.03% from 1800 until about 1850.
• It then increases steadily to 2020.
• You can support this with data, e.g. it rises to about 0.04% by 2020.

This is worth 3 marks: one for stating the flat trend, one for using a time point, and one for describing the increase.

💡 Key knowledge

• “Describe the trend” means say what happens over time.
• Use words like constant, increases, rises, and include approximate values from the graph.
• For this graph:
  • Starts at about 0.03%
  • Stays level until around 1850
  • Then increases to about 0.04% by 2020

🧠 Exam technique

• Always read values directly from the graph and include a time period.
• For 3 marks, give three clear points: start, middle change, end.

❌ Common errors

• Saying only “it goes up” without any data.
• Using the wrong years, such as saying it starts increasing in 1900.
• Calling a small rise “constant” all the way through.

✅ Correct answer

• Methane

Only methane is a greenhouse gas from the options given.

💡 Key knowledge

• Greenhouse gases absorb and re-emit infrared radiation.
• Common greenhouse gases include carbon dioxide, methane and water vapour.

❌ Common errors

• Argon is a noble gas, not a greenhouse gas.
• Nitrogen and oxygen are the main atmospheric gases, but not greenhouse gases.

✅ Correct answer

• Climate change

This is the direct environmental problem linked to greenhouse gases.

💡 Key knowledge

• More greenhouse gases means more heat is trapped in the atmosphere.
• This leads to global warming and wider climate change.

🧠 Exam technique

• If the question asks for an environmental problem caused by greenhouse gases, the safest answer is usually climate change.

📐 Calculation — step by step

1. Write the formula: CO₂
2. Count the atoms: 1 carbon and 2 oxygen atoms
3. Substitute the values:
   M_r = 12 + (2 × 16)
4. Calculate:
   M_r = 12 + 32 = 44

Final answer: 44

✅ Correct answer

Relative formula mass of carbon dioxide = 44

You can score full marks by showing the substitution and the final answer.

🧠 Exam technique

• Always use the formula exactly as written.
• Multiply the oxygen Ar by 2 because there are two oxygen atoms in CO₂.
• Check that your final answer is a whole number.

💡 Must-know facts

• Nitrogen ≈ 80%, oxygen ≈ 20%
• CO₂ decreased because it dissolved in oceans and was used in photosynthesis
• CO₂ rose from about 0.03% to about 0.04% between 1800 and 2020
• Methane is a greenhouse gas
• Climate change is a problem caused by greenhouse gases
• M_r of CO₂ = 44

🧠 What top answers do

• Use precise science words: constant, increases, dissolving, photosynthesis.
• Read data from the graph instead of guessing.
• Show calculations clearly and include the correct final value.