AQA GCSE Combined Science: Trilogy Chemistry Paper 1 (Higher), 2023: Question 3

✅ Correct answer (1 mark)

Mark scheme note: multiples are allowed (e.g. 8 Na + 2 O₂ → 4 Na₂O).

🧠 Exam technique: quickest way to balance

1. Start with oxygen: O₂ means an even number of O atoms.
2. Make 2 O on the right by putting 2 in front of Na₂O → 2 Na₂O .
3. Now you have 4 Na on the right, so put 4 in front of Na on the left.

❌ Common errors (what loses the mark)

• Changing subscripts (e.g. writing NaO or Na₂O₂) instead of using coefficients.
• Balancing Na but forgetting oxygen is diatomic (O₂).
• Leaving it as 2 Na + O₂ → Na₂O (oxygen not balanced).

✅ Full-mark answer (hits all 4 marking points)

Each sodium atom loses one electron to form a Na⁺ ion. Each oxygen atom gains two electrons to form an O²⁻ ion. Therefore two sodium atoms are needed for one oxygen atom (two electrons transferred in total), forming an ionic compound with full outer shells.

💡 Key knowledge (what the mark scheme is looking for)

• 1 mark: “Sodium atom loses one electron”.
• 1 mark: “Oxygen atom gains two electrons”.
• 1 mark: “So ratio is 2 sodium atoms to 1 oxygen atom”.
• 1 mark: Any one of:
  • forms Na⁺ and O²⁻
  • forms sodium ions and oxide ions
  • forms ions with full outer shells/levels

🧠 Exam technique: write it like “electron bookkeeping”

• Use exact phrases: loses / gains electrons (not “shares”).
• State charges clearly: Na⁺ and O²⁻ .
• Link to the formula Na₂O: the 2 shows two Na for each O.

❌ Common errors (examiner-style warnings)

• Saying oxygen gains “one” electron (it gains two to become O²⁻).
• Describing covalent bonding (“sharing electrons”) instead of ionic transfer.
• Mixing up charges (e.g. Na²⁺ or O⁻).
• Forgetting the 2:1 ratio and not linking it to electron transfer.

✅ Correct observations (2 marks total)

• Yellow flame (allow: orange flame) [1 mark]
• Any one of:
  • Sodium melts [1 mark]
  • White smoke / white solid / white powder forms [1 mark]

💡 Key knowledge: what counts as an “observation”

• Observations are what you can see (colour, state change, solid/smoke produced).
• “Sodium oxide forms” is not as good as describing it as a white solid/powder/smoke.
• You only need two separate observations for full marks.

❌ Common errors

• Writing vague comments like “it reacts” or “it burns” (not specific enough).
• Giving non-visual statements like “exothermic” unless you link to something visible (e.g. bright flame).
• Wrong flame colour (must be yellow/orange for sodium).

🧠 Exam technique: bankable 2-mark pair

Write one about the flame colour and one about the product/state change:
yellow/orange flame + white solid/smoke forms .

✅ Correct answers (2 marks)

• Lilac flame (allow: different colour flame) [1 mark]
• Burns faster / more vigorous reaction [1 mark]

💡 Key knowledge: why potassium is different

• Potassium is below sodium in Group 1 → it has a more reactive outer electron (further from nucleus, more shielding).
• More reactive → reaction is faster/more vigorous in oxygen.
• Flame test colours: sodium = yellow, potassium = lilac.

🧠 Exam technique: ensure they are “differences”

• Make it comparative: “potassium burns faster” (not just “it burns”).
• Give the specific flame colour “lilac” to guarantee the mark.

❌ Common errors

• Repeating sodium observations without changing them (question asks for differences).
• Using the wrong flame colour (e.g. red/green).
• Stating “more reactive” without linking to an observation like “burns faster/more vigorous”.

💡 Must-remember facts

• Balanced: 4 Na + O₂ → 2 Na₂O
• Na loses 1 e⁻ → Na⁺
• O gains 2 e⁻ → O²⁻
• Sodium flame: yellow/orange
• Potassium flame: lilac and reacts faster

🧠 30-second exam structure

• (a) Write coefficients only.
• (b) “Na loses 1e⁻; O gains 2e⁻; so 2 Na per O; Na⁺ and O²⁻.”
• (c) Flame colour + white solid/smoke OR melting.
• (d) Lilac + faster/more vigorous.