AQA GCSE Combined Science: Trilogy Chemistry Paper 1 (Higher), 2020: Question 1

✅ Correct answers

• Number of electrons = 13
• Number of neutrons = 14

Marks are awarded in this order only: electrons first, then neutrons.

💡 Key knowledge

• In a neutral atom, electrons = protons.
• The atomic number of aluminium is 13, so it has 13 protons and 13 electrons.
• Number of neutrons = mass number − atomic number = 27 − 13 = 14.

🧠 Exam technique

• Read the numbers from the nuclide symbol carefully.
• Use the top number for mass number and the bottom number for atomic number.
• Always check the answer order if the question asks for two values.

❌ Common errors

• Mixing up atomic number and mass number.
• Writing 27 electrons or 27 neutrons.
• Forgetting that atoms are neutral, so electrons match protons.

✅ Correct answer

Loss of oxygen.

💡 Key knowledge

• Reduction can be remembered as removal of oxygen.
• For aluminium oxide, reduction means aluminium oxide loses oxygen.
• The mark scheme also allows: Al³⁺ gains electrons.

🧠 Exam technique

A one-mark definition question needs a short, exact answer. If you know both definitions, use the simplest one: loss of oxygen.

❌ Common errors

• Saying reduction means gain of oxygen — that is the opposite.
• Writing a long explanation but not giving the key term loss of oxygen.

✅ Correct answer

• At high temperature, oxygen reacts with the carbon electrode.
• The positive electrode burns / wears away.
• This produces carbon dioxide.

Equation: C + O₂ → CO₂

💡 Key knowledge

• The positive carbon electrode is the anode.
• Oxygen is formed at the anode during electrolysis.
• Because carbon is reactive at high temperature, it is slowly used up.

🧠 Exam technique

• This is a 3-mark explanation, so give linked points.
• Full-mark answers usually include:
  • oxygen formed at the positive electrode,
  • oxygen reacts with carbon,
  • carbon dioxide is made so the electrode wears away.

❌ Common errors

• Saying the electrode is replaced because it is “used up” without saying how.
• Forgetting that oxygen is formed at the positive electrode.
• Writing CO instead of CO₂.

✅ Correct answers

• Carbon electrode (graphite): delocalised electrons
• Molten mixture of aluminium oxide and cryolite: ions
• Metal wire: delocalised electrons

💡 Key knowledge

• Graphite and metals conduct because they contain delocalised electrons that can move through the structure.
• Molten ionic compounds conduct because the ions are free to move when melted.
• Solid ionic compounds do not conduct because their ions are fixed in place.

🧠 Exam technique

• Use the exact particle name the mark scheme expects.
• For graphite and metals, the safest answer is delocalised electrons.
• For a molten ionic substance, write ions.

❌ Common errors

• Saying “charged particles” instead of naming them.
• Writing atoms for metal wire or graphite.
• Writing electrons for the molten ionic mixture.

✅ Full-mark answers in one glance

• 01.1 13; 14
• 01.2 Loss of oxygen
• 01.3 Oxygen reacts with carbon at high temperature, so the positive electrode burns/wears away and carbon dioxide is formed
• 01.4 Graphite: delocalised electrons; molten mixture: ions; metal wire: delocalised electrons

🧠 What distinguished top responses

• They used the exact scientific terms: loss of oxygen, anode, delocalised electrons, ions.
• For the electrode question, they explained the whole process, not just one phrase.
• They answered in the correct order and with no extra confusion.