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

✅ Correct Answer

• Test: Insert a glowing splint into the gas. [1 mark]
• Result: The splint relights (or burns brightly). [1 mark]

🧠 Exam Technique

The second mark is strictly dependent on getting the test right. If you write "lit splint", you score 0 for the entire question because a lit splint will already be burning, so "relights" makes no chemical sense.

❌ Common Errors

• Writing "lit splint" or "burning splint" — that is the test for hydrogen (squeaky pop).
• Confusing the results: stating "turns cloudy" (limewater test for CO₂) or "bleaches damp litmus" (chlorine test).

✅ Model Level 3 Response (5–6 Marks)

"The percentage of carbon dioxide decreased dramatically because water vapour condensed to form oceans, in which CO₂ dissolved. Algae and green plants evolved and absorbed CO₂ for photosynthesis, and marine organisms used dissolved carbonates to form shells, which became sedimentary carbonate rocks (such as limestone). Carbon was also locked up into fossil fuels."

"Oxygen increased significantly because algae and green plants produced oxygen as a waste product of photosynthesis."

"Nitrogen increased because volcanoes released nitrogen gas and ammonia over millions of years. As nitrogen is very unreactive and stable, it accumulated in the atmosphere."

🧠 Level of Response Breakdown

• Level 3 (5–6 marks): Explains changes for all three gases (CO₂, O₂, and N₂) with clear, logically linked scientific causes.
• Level 2 (3–4 marks): Relevant reasons given for at least two gases, but links are incomplete.
• Level 1 (1–2 marks): Simple statements or descriptions of the data without giving scientific reasons.

❌ Common Errors

• Only describing the table values (e.g. "CO₂ fell by 94.96%") without explaining how or why.
• Forgetting to explain nitrogen entirely.
• Saying humans caused these initial changes — these historical changes occurred hundreds of millions of years before humans existed.

✅ Correct Answers (Choose Any Two)

• Reduce cattle/livestock farming (e.g. eat less beef/dairy, switch to plant-based diets). [1 mark]
• Reduce the amount of rubbish/organic waste sent to landfill sites (e.g. recycling, composting). [1 mark]
• Capture and burn methane from landfill sites as a fuel/energy source. [1 mark]
• Reduce rice cultivation / reduce paddy fields / wetland flooding. [1 mark]
• Reduce the extraction/use of fossil fuels. [1 mark]
• Stop the destruction/draining of peat bogs. [1 mark]

🧠 Exam Technique

Make sure your answers refer specifically to methane (CH₄), not carbon dioxide. Planting trees or driving electric cars reduces CO₂, but does not directly reduce methane emissions.

❌ Common Errors

• Vague answers like "stop pollution" or "drive less" (fossil fuels release mainly CO₂).
• Suggesting "banning cows" instead of accepted phrasing like "reducing cattle/livestock farming" or "eating less meat".

✅ Correct Answer

• Fossil fuels (such as coal or crude oil) contain sulfur impurities. [1 mark]
• When these fuels are burned (combusted), the sulfur reacts with oxygen to form sulfur dioxide. [1 mark]

💡 Key Knowledge

Most fossil fuels (especially coal) contain sulfur as an impurity. During combustion in power stations or internal combustion engines, the sulfur is oxidised:

sulfur + oxygen → sulfur dioxide

❌ Common Errors

• Confusing sulfur dioxide with nitrogen oxides (NOₓ). NOₓ comes from nitrogen in the air reacting at high temperatures, whereas sulfur comes directly from impurities in the fuel.
• Failing to mention oxygen or combustion (e.g. just writing "it comes out of coal").

✅ 06.5: One Harmful Effect [1 Mark]

• Causes acid rain (damages trees/plants, kills aquatic life, corrodes limestone buildings and metals).
• Causes respiratory problems / triggers asthma in humans.

✅ 06.6: Reason for Decrease Since 1980 [1 Mark]

Any one from:

• Reduced use of fossil fuels containing sulfur (e.g. shift from coal to natural gas, nuclear, or renewables).
• Desulfurisation: sulfur is removed from fuels before combustion (e.g. ultra-low sulfur diesel).
• Flue-gas desulfurisation: sulfur dioxide is removed from chimney emissions using an alkali (e.g. CaO/CaCO₃) after burning.
• Tighter government environmental legislation / Clean Air Acts.

❌ Major Misconceptions to Avoid

• Pollutant Confusion: Never state that sulfur dioxide damages the ozone layer (CFCs do that) or causes global warming (CO₂/methane do that).
• Vague answers for 06.6: Avoid simply saying "people cared more about the environment" — give a concrete technological, practical, or regulatory reason.