AQA GCSE Combined Science: Trilogy Chemistry Paper 2 (Higher), 2023: Question 1

✅ Correct answer

CH₄ + 2O₂ → CO₂ + 2H₂O

💡 Key knowledge

• Combustion of a hydrocarbon produces carbon dioxide and water.
• You balance equations by changing big numbers in front of formulas, not the small numbers inside formulas.
• CH₄ has 4 H atoms, so you need 2H₂O to make 4 H atoms on the right.

🧠 Exam technique

• Count atoms on each side carefully: C, then H, then O.
• Leave CH₄, CO₂ and H₂O formulas unchanged.
• For a 1-mark balancing question, one wrong number means no mark.

❌ Common errors

• Writing CH₄ + O₂ → CO₂ + 2H₂O and forgetting oxygen is not balanced.
• Changing a formula, for example writing O₄ or H₄O₂.
• Not checking the total oxygen atoms at the end.

✅ Correct answers

Any two valid comparisons from the table:

• Hydrocarbon molecules in petrol have fewer carbon atoms than those in diesel.
• Petrol has a lower boiling point range than diesel.

💡 Key knowledge

• Petrol range: 4 to 12 carbon atoms, 40 to 205 °C.
• Diesel range: 12 to 20 carbon atoms, 250 to 350 °C.
• Longer hydrocarbon molecules usually have higher boiling points.
• The mark scheme also allows: petrol is more flammable and petrol is less viscous than diesel.

🧠 Exam technique

• Use the wording compare: mention both fuels.
• Lift data from the table and turn it into a sentence.
• For 2 marks, make two separate comparison points.
• Best answers use phrases like lower than , higher than , fewer than , more than .

❌ Common errors

• Only describing one fuel without comparing it to the other.
• Giving unrelated facts not from the table.
• Saying petrol has “more carbon atoms” than diesel.
• Missing the idea of range and writing just one number.

✅ Correct answer

Oxygen

💡 Key knowledge

• Oxides of nitrogen contain oxygen.
• Carbon monoxide contains oxygen.
• Water vapour is H₂O, so it contains oxygen too.

❌ Common errors

• Choosing hydrogen because of water vapour — but carbon monoxide and nitrogen oxides do not contain hydrogen.
• Choosing nitrogen because of nitrogen oxides — but carbon monoxide and water do not both contain nitrogen.

✅ Correct answer

From the air.

💡 Key knowledge

At the high temperatures inside an engine, nitrogen and oxygen from the air can react to form nitrogen oxides.

🧠 Exam technique

This is a short recall question. Give the exact source, not a long story.

❌ Common errors

• Saying the nitrogen comes from diesel itself.
• Answering just “from burning” without naming air or atmosphere.

✅ Correct answer

Global dimming

💡 Key knowledge

Particulates in the atmosphere can block or reflect sunlight, so less sunlight reaches the Earth's surface. This is called global dimming.

❌ Common errors

• Giving a health effect such as breathing problems when the question asks for an environmental effect.
• Writing global warming instead of global dimming.

✅ Correct answer

Carbon monoxide is colourless or odourless .

💡 Key knowledge

Carbon monoxide is dangerous because people may not notice it is present.

🧠 Exam technique

Only one reason is needed. Keep it precise and use scientific vocabulary.

❌ Common errors

• Writing “clear” — the mark scheme does not credit this.
• Talking about toxicity only. The question is about why it is hard to detect.

✅ Correct answer

A full 2-mark answer needs both ideas:

1. Diesel burns at a high temperature / at a temperature greater than 100 °C.
2. This temperature is above the boiling point of water, so the water produced is a gas (water vapour), not a liquid.

💡 Key knowledge

• Boiling point of water = 100 °C.
• If water is formed at temperatures above this, it will be water vapour.
• This question is about state change, not about how much oxygen is present.

🧠 Exam technique

For a 2-mark explain question, do not stop at “it is hot”. Add the scientific link: above boiling point.

Top answers clearly chain the idea: high temperature → above 100 °C → water is vapour .

❌ Common errors

• Saying “because water is made as a gas” with no reason.
• Talking about evaporation after the reaction instead of the combustion temperature.
• Forgetting to mention 100 °C or boiling point.

✅ Correct answer

A full-mark answer should include this chain:

1. When diesel burns, sulfur produces sulfur dioxide.
2. Sulfur dioxide causes acid rain.
3. Acid rain causes damage, for example:
   • damage to buildings, statues or bridges
   • damage to trees or plants
   • damage to aquatic life
   • acidification of lakes, rivers or soil
   • respiratory problems

💡 Key knowledge

Sulfur is an impurity, so it is not part of the useful fuel. When it burns, it forms a pollutant. The key pollutant named by the mark scheme is sulfur dioxide.

🧠 Exam technique

This is a classic 3-mark “explain why” question. Build a clear cause-and-effect sequence:

sulfur in diesel → sulfur dioxide → acid rain → environmental damage

Students often lose the third mark by not giving the final effect.

❌ Common errors

• Saying sulfur causes global warming — not the mark scheme answer here.
• Mentioning sulfur dioxide but not acid rain.
• Mentioning acid rain but not a consequence.
• Giving vague effects like “it is bad for the environment” without an example.

💡 What strong answers did well

• Used exact scientific terms from the specification: oxygen, air, global dimming, sulfur dioxide, acid rain.
• For comparison questions, made direct comparisons rather than separate descriptions.
• For explanation questions, linked ideas in the correct order to secure all available marks.

🧠 Best strategy for similar questions

• On 1-mark questions, give the precise keyword and move on.
• On 2- or 3-mark questions, write one point per mark.
• If a table is given, use it directly.
• If the question says explain, include a reason or consequence, not just a fact.

❌ Most common misconceptions

• Confusing global dimming with global warming.
• Thinking nitrogen oxides come from the fuel rather than the air.
• Giving health effects when the question asks for environmental effects.
• Stopping too early in longer explanations and missing the final mark.