AQA GCSE Combined Science: Trilogy Chemistry Paper 2 (Higher), June 2025: Question 2

✅ Mark Scheme Model Answer

• Test: Insert a glowing splint into the gas. [1 mark]
• Result: Splint relights (re-ignites). [1 mark]

❌ Common Errors & Lost Marks

• Writing a burning or lit splint – strictly 0 marks. A burning splint is used for hydrogen (squeaky pop).
• Getting the result mark without the correct test: the mark scheme specifies MP2 is dependent upon MP1 being awarded.

💡 Key Knowledge: Explaining the Changes

• Carbon dioxide decreased because:
  • Dissolved in the newly formed oceans.
  • Reacted with dissolved minerals to form insoluble carbonate sediments and sedimentary rocks (e.g. limestone / CaCO₃).
  • Algae and green plants evolved and absorbed CO₂ for photosynthesis.
  • Locked up in fossil fuels (coal, oil, gas) upon burial of dead plant and animal remains.
• Oxygen increased because:
  • Algae first produced oxygen around 2.7 billion years ago through photosynthesis.
  • As more green plants evolved, oxygen levels rose continuously over time.
• Nitrogen increased because:
  • Volcanic activity continuously erupted and released nitrogen gas.
  • Denitrifying bacteria released nitrogen into the air.
  • Nitrogen is very unreactive (stable triple bond), so it built up over billions of years without being removed.

🧠 Level 3 (5–6 Marks) Strategy

This is a 6-mark level-of-response question. To achieve Level 3:

• You must cover all three gases named in Table 1. If you leave out nitrogen, you cannot get into Level 3.
• Give specific, scientifically accurate reasons for each gas rather than vague statements like "plants used the gases".
• Structure your answer with sub-headings or clear paragraphs: one for Carbon Dioxide, one for Oxygen, and one for Nitrogen.

❌ Common Misconceptions

• Confusing respiration and photosynthesis (saying "plants breathe in CO₂ and breathe out O₂"). Use the chemical term photosynthesis.
• Stating nitrogen decreased or forgetting to mention how nitrogen built up.
• Forgetting to mention the dissolution of CO₂ into oceans and formation of carbonate sedimentary rocks.

✅ Model 6-Mark Structure

"First, carbon dioxide decreased from 95% to 0.04% because water vapour condensed to form oceans where CO₂ dissolved to form carbonate precipitates and sedimentary rock (limestone). Additionally, algae and plants evolved, taking in CO₂ for photosynthesis, and carbon became trapped in fossil fuels. Second, oxygen increased from <1% to 21% because algae and green plants produced it as a waste product of photosynthesis. Finally, nitrogen increased from 3% to 78% due to volcanic eruptions releasing nitrogen over time, along with denitrifying bacteria; because nitrogen is unreactive, it accumulated in the atmosphere."

✅ Mark Scheme Answers (Any TWO)

• Reduce livestock farming (e.g. fewer cattle/cows/sheep or eat less meat/dairy). [1 mark]
• Reduce rice cultivation / paddy fields (or reduce wetland areas used for crops). [1 mark]
• Reduce organic waste sent to landfill / increase recycling / composting. [1 mark]
• Collect and burn methane from landfill as an energy source / fuel. [1 mark]
• Reduce fossil fuel extraction/use (reduces natural gas leaks). [1 mark]
• Reduce the destruction/drainage of peat bogs. [1 mark]

🧠 Exam Technique: Be Specific

Avoid overly generic environmental answers such as "stop pollution" or "drive electric cars". Methane comes from distinct biological and industrial processes:

• Digestive systems of livestock (ruminant animals belching).
• Anaerobic decay of organic waste in landfill sites and flooded rice paddy fields.

✅ Mark Scheme Model Answer

1. Fossil fuels (such as coal or crude oil) contain sulfur impurities and are burnt. [1 mark]
2. The sulfur reacts with oxygen / is oxidised:
   S + O₂ → SO₂ . [1 mark]

❌ Common Errors

• Saying "sulfur dioxide is burnt" – sulfur dioxide is the product, not the fuel!
• Missing the mention of oxygen or oxidation; just stating "burning coal makes sulfur dioxide" only secures 1 mark out of 2.

✅ Accepted Answers (Give ONE)

• Causes acid rain (which corrodes limestone buildings, damages trees/forests, and kills aquatic life by acidifying lakes). [1 mark]
• Causes respiratory problems / triggers asthma in humans. [1 mark]

❌ Major Misconceptions

• Do NOT confuse with the greenhouse effect / global warming: SO₂ causes acid rain and respiratory issues, not global warming. (Carbon dioxide and methane are greenhouse gases).
• Do NOT confuse with carbon monoxide: SO₂ does not bind to haemoglobin.

✅ Accepted Answers (Give ONE)

• Less burning of fossil fuels containing sulfur / shift to natural gas, renewables, or nuclear energy. [1 mark]
• Sulfur is removed from fuels before burning (e.g. low-sulfur petrol and ultra-low sulfur diesel). [1 mark]
• Flue gas desulfurisation: sulfur dioxide is removed from waste chimney gases (e.g. using calcium oxide/hydroxide) after combustion. [1 mark]
• Stricter government legislation, Clean Air Acts, and emissions targets. [1 mark]

🧠 Top-Grade Tip

Notice the two distinct engineering approaches: removal before combustion (desulfurisation of fuels) vs removal after combustion (scrubbing flue gases with bases like CaO). Stating either demonstrates outstanding scientific detail.