AQA GCSE Combined Science: Trilogy Chemistry Paper 2 (Foundation), 2024: Question 2

✅ Correct answer

Substance B

Reason: B has a specific melting point of 61 °C.

💡 Key knowledge

• A pure substance melts at one exact temperature.
• Impure substances melt over a range of temperatures.
• From the table, A, C and D all melt over a range, so they are not pure.

🧠 Exam technique

• For the first mark, name the correct substance.
• For the second mark, give the scientific reason: specific melting point.
• Use the data from the table in your explanation.

❌ Common errors

• Saying “highest melting point” is not enough.
• Saying “it melts at 61” without explaining why that means pure may lose the reason mark.
• Writing “a narrow range” is weaker than “a specific melting point”.

✅ Correct answer

The colour changes from white to blue.

💡 Key knowledge

• Anhydrous copper sulfate is white.
• Hydrated copper sulfate is blue.
• This is used as a test for water.

🧠 Exam technique

• The question asks for from and to.
• Give both colours in the correct order to gain both marks.

❌ Common errors

• Mixing up the colours: copper sulfate starts white, not blue.
• Writing “green” or “yellow” from the options.
• Giving only one colour instead of both.

✅ Correct answer

It uses the reversible symbol ⇌ .

💡 Key knowledge

• A reversible reaction can go in both directions.
• The symbol ⇌ means the reaction can go forwards and backwards.

🧠 Exam technique

• State the symbol clearly if asked “how does it show?”.
• Either ⇌ or arrows both ways can be accepted.

❌ Common errors

• Writing only a single arrow → .
• Saying “it changes back” without mentioning the symbol.
• Using the wrong direction arrow.

✅ Correct answer

6

📐 Calculation / counting steps

1. Count the atoms in CuSO₄ .
2. Cu = 1 atom
3. S = 1 atom
4. O₄ = 4 atoms
5. Total = 1 + 1 + 4 = 6

💡 Key knowledge

• Small numbers after an element symbol show how many atoms there are.
• If there is no number after the symbol, there is 1 atom.

❌ Common errors

• Counting CuSO₄ as 4 atoms because of the 4 on oxygen only.
• Forgetting that Cu and S each count as 1.
• Writing 5 by missing one atom.

✅ Correct answer

Test: Use damp blue litmus paper (or damp red litmus paper).

Result: The litmus paper is bleached white.

💡 Key knowledge

• Chlorine is an acidic, bleaching gas.
• It turns damp blue litmus red first, then bleaches it white.
• The mark scheme also accepts: turns white or is bleached.

🧠 Exam technique

• The question wants both the test and the result.
• Be specific: say damp litmus paper.
• Full marks need the correct paper and the correct observation.

❌ Common errors

• Saying only “litmus paper changes colour” without naming chlorine’s bleaching effect.
• Writing “turns red” as the final answer, which is incomplete.
• Forgetting that the paper must be damp.

✅ Answers in one place

• 02.1: B; because it has a specific melting point.
• 02.2: white to blue.
• 02.3: the symbol ⇌ .
• 02.4: 6.
• 02.5: damp litmus paper; bleached white / turns white.

🧠 How top answers scored full marks

• They used precise science words: specific melting point, reversible, bleached.
• They answered exactly what the question asked, not extra unrelated detail.
• They used the data table properly in 02.1.

❌ Biggest traps

• Confusing pure and impure melting points.
• Mixing up anhydrous copper sulfate colours.
• Forgetting the damp litmus in the chlorine test.