AQA GCSE Combined Science: Trilogy Chemistry Paper 1 (Higher), 2021: Question 4

✅ Correct answer

C₆₀

💡 Key knowledge

• Buckminsterfullerene is a fullerene made of 60 carbon atoms.
• It is often called buckyball.

🧠 Exam technique

• For a multiple-choice box question, do not overthink it: choose the exact formula from memory.
• Check the subscript carefully: C₆₀ , not C60 written without a subscript if you are writing by hand.

❌ Common errors

• Mixing up fullerene formulas with other carbon structures.
• Writing C₆ or C₈₀ instead of C₆₀ .

✅ Correct answer

• Graphite has delocalised electrons.
• These electrons move through the structure and carry electrical charge.

💡 Key knowledge

• In graphite, each carbon atom forms 3 bonds, leaving 1 electron per atom delocalised.
• These electrons are free to move along the layers.

🧠 Exam technique

• Use both idea and explanation: what the electrons are and what they do.
• A strong 2-mark answer often looks like: “Graphite has delocalised electrons, which can move through the structure and carry charge.”

❌ Common errors

• Saying “graphite has free electrons” without explaining they are delocalised.
• Saying the electrons “carry heat” instead of electrical charge.
• Confusing graphite with diamond, which has no delocalised electrons.

✅ Correct answer

• Carbon atoms are a different size from iron atoms/ions.
• They distort the layers of iron atoms/ions.
• The layers cannot slide over each other easily, so steel is harder.

💡 Key knowledge

• Steel is an alloy of iron and carbon.
• Pure metals are softer because layers can slide more easily.
• Alloys are usually harder because different-sized atoms disrupt the regular metal lattice.

🧠 Exam technique

• Use the sequence: different size → distortion → layers cannot slide.
• This is a classic AQA mark scheme chain. If you include all three, you are very likely to gain full marks.

❌ Common errors

• Only saying “alloys are harder” without explaining why.
• Talking about stronger bonds in too vague a way.
• Forgetting the key phrase layers cannot slide.

📐 Calculation: step-by-step

1. Find the total mass of the fork using the carbon, chromium and nickel masses and the fact that iron is 71.92% of the fork.
2. First, add the other masses:
   0.05 + 10.44 + 5.80 = 16.29 g
3. These other elements make up:
   100% - 71.92% = 28.08%
4. So, if 28.08% = 16.29 g , then:
   100% = 16.29 ÷ 28.08 × 100 = 58.01 g
5. Now calculate the mass of iron:
   71.92% of 58.01 = 71.92 ÷ 100 × 58.01 = 41.72 g

Answer: X = 41.72 g
Accept 41.7 g depending on rounding.

✅ Correct answer

X = 41.72 g

• Allow 41.7 g for rounded final answer.
• Allow a correct alternative method, such as: total mass − mass of other elements = iron mass.

🧠 Exam technique

• Always write the percentage relationship clearly.
• Show working in stages so you can gain method marks even if your final answer is wrong.
• Keep units on every line: g and %.
• Use sensible rounding at the end, not at every step.

❌ Common errors

• Adding all masses together and stopping there.
• Using 71.92 as a mass instead of a percentage.
• Forgetting to convert percentage into a fraction by dividing by 100.
• Mixing up which percentage belongs to iron and which belongs to the other elements.
• Losing marks by rounding too early.

💡 Remember these facts

• Buckminsterfullerene = C₆₀
• Graphite conducts because of delocalised electrons
• Steel is harder than iron because carbon atoms distort layers

🧠 How to score full marks

• Use the exact science keyword from the mark scheme.
• For explanations, make each sentence do one marking job.
• For calculations, write the percentage relationship, show the arithmetic, and include units.