AQA GCSE Combined Science: Trilogy Chemistry Paper 2 (Higher), 2020: Question 3

✅ Correct answers

Agricultural waste water: microbes

Industrial waste water: chemicals

💡 Key knowledge

• Agricultural waste water can contain harmful microbes such as bacteria or pathogens from animal waste.
• Industrial waste water may contain toxic chemicals from factories and manufacturing processes.
• The question is testing whether you know what type of harmful substance needs removing in each case.

🧠 Exam technique

• Read the context first: agricultural links to living things and waste from animals/farming.
• Industrial links to factory processes and chemical contamination.
• Use the exact word the mark scheme wants if possible: microbes and chemicals.

❌ Common errors

• Writing germs instead of microbes can be too vague in some exam settings.
• Mixing up the two: microbes belong with agricultural waste water; chemicals belong with industrial waste water.
• Students sometimes write a process, not the substance, e.g. “filtering” or “screening” — that does not answer the sentence.

✅ Correct answer

Anaerobic digestion

💡 Key knowledge

• Anaerobic means without oxygen.
• Sewage sludge is broken down by microorganisms in the absence of oxygen.
• This produces useful products such as biogas.

🧠 Exam technique

• If a question asks about sludge, think of the treatment stage where microbes break it down.
• Tick the most specific option, not just a general one.

❌ Common errors

• Grit removal and screening are early stages of sewage treatment, not sludge treatment.
• Aerobic biological treatment uses oxygen, so it is not the correct answer here.
• Students often choose the first familiar option rather than the one linked to sludge.

📐 Calculations: step-by-step

1. Find the decrease: 8.0 - 1.3 = 6.7 billion
2. Divide by the original number: 6.7 ÷ 8.0 = 0.8375
3. Convert to a percentage: 0.8375 × 100 = 83.75%
4. Round appropriately: 84%

Percentage decrease = 84%

✅ Correct answer

84%

🧠 Exam technique

• Always use the original value when finding percentage decrease.
• Show the calculation clearly if you want method marks.
• Use the numbers exactly as given: 8.0 and 1.3.
• Write the answer with the % sign.

❌ Common calculation traps

• Using 1.3 ÷ 8.0 × 100 gives the percentage remaining, not the percentage decrease.
• Subtracting the wrong way round can lead to a negative answer.
• Forgetting that the question asks for a decrease.
• Writing 83.75% is close, but the mark scheme allows the rounded answer 84%.

✅ Correct answers

• Respiratory problems
• Acid rain
• Smog

💡 Key knowledge

• Oxides of nitrogen are harmful gases formed in car engines.
• They can irritate the lungs and worsen conditions like asthma.
• They also contribute to acid rain, which can damage buildings and kill aquatic life.

🧠 Exam technique

• Because it is a 1-mark question, a short clear answer is enough.
• If you choose acid rain, that is valid by itself.
• If you choose respiratory problems, you can also name asthma or breathing problems.

❌ Common errors

• Giving a pollution type instead of a problem, e.g. “they are gases” — this does not answer the question.
• Being too vague, e.g. “they are bad for the environment” without saying how.
• Writing a consequence of acid rain that is not linked clearly enough, such as just “damage” with no detail.

✅ Correct answer

Increased: more traffic / more cars

Decreased: improved efficiency of car engines, including catalytic converters, more hybrid or electric cars, lower engine temperatures, more public transport, and more walking/cycling

💡 Key knowledge

• More cars on the road means more fuel burned, so more oxides of nitrogen are produced overall.
• Cleaner technology can reduce emissions from cars.
• The graph shows a rise first, then a fall, so your explanation must match both changes.

🧠 Exam technique

• Use the word because in your explanation to link cause and effect.
• Answer both parts: Increased and Decreased.
• Top answers refer to a change in car use or technology, not just “pollution changed”.

❌ Common errors

• Saying only “people used more fuel” without explaining the link to more traffic or more cars.
• For the decrease, writing something too vague like “cars got better” — the mark scheme wants a clear idea such as improved efficiency or catalytic converters.
• Ignoring the graph trend and giving a general fact about nitrogen oxides instead of explaining the change over time.

💡 Must-know facts

• Agricultural waste water: remove microbes
• Industrial waste water: remove chemicals
• Sewage sludge treatment: anaerobic digestion
• Percentage decrease here: 84%
• Oxides of nitrogen can cause respiratory problems or acid rain

🧠 Final exam tips

• For 1-mark questions, keep answers short and exact.
• For calculations, always show working and use the original value.
• For graph questions, describe the pattern first, then explain it.