AQA GCSE Combined Science: Trilogy Chemistry Paper 2 (Higher), 2022: Question 5

✅ Correct answers

• Nitrogen increased.
• This was due to emission from volcanoes.
• Oxygen increased.
• This was due to photosynthesis.

💡 Key knowledge

• Early Earth had little oxygen.
• Volcanoes released gases, including nitrogen, into the atmosphere.
• Photosynthetic organisms, such as algae and plants, released oxygen.
• Over time, oxygen built up in the atmosphere.

🧠 Exam technique

• Make two separate points: one for nitrogen, one for oxygen.
• Use the exact cause from the mark scheme: volcanoes and photosynthesis .
• Write in cause-and-effect form: “Nitrogen increased because...”

❌ Common errors

• Saying nitrogen “decreased” or “stayed the same”.
• For oxygen, mentioning respiration without saying photosynthesis increased oxygen.
• Giving only one gas change when both are asked.

✅ Correct answer points

• Carbon dioxide is used during photosynthesis.
• This happens in trees / plants.
• The trees die and are compressed.
• This happens over millions of years.

💡 Key knowledge

• Plants take in carbon dioxide from the atmosphere.
• Carbon from the plants becomes part of plant material.
• Dead plant remains can be buried and compressed over time.
• Coal is a fossil fuel made from ancient plant remains.

🧠 Exam technique

• Use a time sequence: photosynthesis → death → compression → millions of years.
• Use the word compressed; this is a key mark point.
• Link the idea back to atmospheric carbon dioxide being removed by plants.

❌ Common errors

• Saying coal formed directly from carbon dioxide with no plant stage.
• Writing about burning coal instead of forming coal.
• Leaving out the time scale of millions of years.

✅ Correct answer

Coal has a higher proportion / percentage of carbon.

💡 Key knowledge

• More carbon in the fuel means more carbon dioxide can be made when it burns.
• Coal is less pure than natural gas? Not the key point here — the exam wants carbon content.

🧠 Exam technique

• Use the phrase higher proportion of carbon.
• You do not need a long explanation for a 1-mark suggest question.

❌ Common errors

• Saying coal makes more CO₂ because it has “more mass” without linking to carbon.
• Talking about energy output instead of carbon content.
• Writing about incomplete combustion.

✅ Correct answers

• It uses bacteria.
• The bacteria produce solutions containing copper compounds (or leachate solutions).
• Copper is then obtained from the solution by displacement or electrolysis.

💡 Key knowledge

• Bioleaching is used for low-grade ores where traditional extraction may not be economical.
• Bacteria help release copper compounds into solution.
• The copper is then recovered from the solution.

🧠 Exam technique

• Write in order: bacteria → solution/leachate → copper recovered.
• If you say “copper compounds” or “leachate”, that matches the mark scheme.
• Mention how copper is finally obtained: displacement or electrolysis.

❌ Common errors

• Describing phytomining instead of bioleaching.
• Forgetting the final recovery stage.
• Just saying “bacteria help extract copper” without saying what the bacteria produce.

📐 Calculations: step-by-step

1. Find the mass of nickel in the plants.
   0.792% of 1000 kg = 0.792 ÷ 100 × 1000 = 7.92 kg
2. Use the nickel percentage in the ash.
   4.80% of the ash is nickel, so:
   mass of ash = 7.92 × 100 ÷ 4.80
   = 165 kg
3. Convert kg to g.
   165 kg = 165 000 g
4. Write in standard form.
   1.65 × 10⁵ g

✅ Correct answer

Mass of ash = 1.65 × 10⁵ g

🧠 Exam technique

• Always identify the percentage of metal in the original plant and the percentage of metal in the ash.
• Use a ratio idea: if ash is 4.80% nickel, then total ash mass must be bigger than the nickel mass.
• Show units at every step: kg then g.
• Check your final answer is in standard form with the power of 10.

❌ Common traps

• Using 0.792 as if it were 0.792 kg instead of 0.792%.
• Forgetting to divide by 4.8 or using 4.80 incorrectly.
• Giving the answer only in kg, not grams.
• Writing 1.65 × 10⁵ kg instead of g .

🧠 Best strategy

• Learn the key atmospheric changes: nitrogen up, oxygen up.
• Remember the fossil fuel formation story: plants take in CO₂, then are compressed over time.
• For resource extraction, know the simple steps of bioleaching.
• In calculations, always write percentages, units, and final standard form clearly.

💡 Mark scheme habits

• Short factual answers are enough for 1-mark questions.
• For 3- and 4-mark questions, give linked steps in order.
• Use the exact GCSE keywords: photosynthesis, volcanoes, compressed, bacteria, leachate, displacement, electrolysis.