AQA GCSE Chemistry Chemistry Paper 1 (Foundation), June 2025: Question 5

✅ Correct Answers

Molten compound	Product at negative electrode (cathode)	Product at positive electrode (anode)
Copper chloride	copper [1]	Chlorine
Potassium bromide	Potassium	bromine [1]

💡 Key Knowledge

• Molten ionic compounds contain only two elements: the metal cation and the non-metal anion (no water/no H⁺/OH⁻).
• Negative electrode (cathode): attracts positive metal ions → forms metal atoms.
• Positive electrode (anode): attracts negative non-metal ions → forms non-metal molecules.

❌ Common Errors

• Writing bromide instead of bromine. The product is the elemental halogen (bromine gas/liquid), not the ion.
• Writing chemical formulas when names are used (e.g. Br₂ instead of bromine). Stick to the naming format established in the table.

✅ Correct Answer

Tick: O₂ [1 mark]

💡 Key Knowledge

• Oxygen gas exists naturally as a diatomic molecule: two oxygen atoms covalently bonded together ( O₂ ).
• Do not confuse the elemental molecule O₂ with an oxide ion ( O²⁻ ) or individual oxygen atoms ( O ).

🧠 Exam Technique

Remember diatomic elements using the mnemonic "I Have No Clever Or Brilliant Friends": I₂, H₂, N₂, Cl₂, O₂, Br₂, F₂.

✅ Correct Answer

• Aluminium ions are positively charged (or are Al³⁺ ions) [1 mark]
• (So) they are attracted to the negative electrode / cathode (allow: opposite charges attract) [1 mark]

💡 Key Knowledge

• PANIC rule: Positive Anode, Negative Is Cathode.
• Opposite charges always attract: positively charged cations migrate to the cathode; negatively charged anions ( O²⁻ ) migrate to the anode.

❌ Common Errors

• Vaguely stating "aluminium is negative" – aluminium atoms have no charge; it is the aluminium ions ( Al³⁺ ) that carry a positive charge.
• Stating only that "opposites attract" without explicitly stating that the aluminium ion is positive.

✅ Correct Answer

• The mixture has a lower melting point (than pure aluminium oxide / lowers the operating temperature from 2045 °C to 950 °C) [1 mark]
• (So) less energy is required / reduces energy costs [1 mark]

💡 Key Knowledge

• Aluminium oxide has a giant ionic lattice with extremely strong electrostatic attractions between Al³⁺ and O²⁻ ions, giving it a melting point over 2000 °C.
• Dissolving it in molten cryolite drastically lowers the melting point to around 950 °C.

🧠 Exam Technique

Always pair property change with practical/economic consequence: Lower melting point → Less energy needed / cheaper to run.

✅ Correct Answer

Tick: Carbon reacts with the oxygen produced. [1 mark]

💡 Key Knowledge

• Oxygen gas is liberated at the positive carbon (graphite) anode: 2O²⁻ → O₂ + 4e⁻
• At 950 °C, the carbon anodes react with the hot oxygen: C (s) + O₂ (g) → CO₂ (g)
• This causes the anodes to burn away slowly, meaning they must be replaced regularly.

❌ Common Errors

Confusing the reaction with aluminium. Carbon does not react with aluminium metal under these conditions; aluminium forms at the bottom/negative electrode.

✅ Correct Answer

Tick: 3.4 cm³ [1 mark]

🧠 Exam Technique: Downward Displacement

• Notice the scale on the inverted cylinder: 0 is at the closed top, increasing downwards ( 2, 4, 6, 8, 10 cm³ ).
• Between 2 and 4 , there are 10 small divisions → each small division represents 0.2 cm³ .
• The liquid level is 7 marks below 2.0: 2.0 + (7 × 0.2) = 3.4 cm³ .

❌ Common Errors

Reading from the bottom up! Students who misread the inverted scale read 4.6 cm³ or 2.7 cm³. Always check which end has the zero mark on collected gas apparatus.

✅ Correct Answer

Tick: Hydrogen is less reactive than magnesium. [1 mark]

💡 Key Knowledge: Cathode Rule for Solutions

• In an aqueous solution, water provides H⁺ and OH⁻ ions.
• At the negative cathode, H⁺ and metal ions compete for electrons.
• The less reactive element is discharged:
  • If the metal is more reactive than hydrogen (e.g. Mg, Na, K), hydrogen gas is produced.
  • If the metal is less reactive than hydrogen (e.g. Cu, Ag), the metal is deposited.

❌ Common Errors

Choosing "Hydrogen is more reactive than magnesium" – remember magnesium is high in the reactivity series; hydrogen is discharged because it is less reactive.

✅ Correct Answer

Tick: The volume of hydrogen collected depends on both the current and the time. [1 mark]

🧠 Exam Technique: Evaluating Graphs

• Effect of time: For each curve, volume increases linearly as time increases from 0 to 5 min → volume depends on time.
• Effect of current: At any chosen time (e.g. 5 min), a current of 0.10 A yields ~3.8 cm³, while 0.05 A yields ~1.9 cm³ → volume depends on current.
• Therefore, volume depends on both variables.

Mark Scheme Allowance: Any correct matching pair of values from the graph is accepted (e.g. using the origin: 3.8 / 5.0 = 0.76 ). If volume or time is read incorrectly, error carried forward (ecf) applies for working and final division.

🧠 Exam Technique

• Always check the grid scale carefully: on the y-axis, 10 small squares = 1.0 cm³ (each square = 0.1 cm³); on the x-axis, 10 small squares = 1 min (each square = 0.1 min).
• Where an exam question prints "Change in volume" and "Change in time" arrows directly on the graph, use the exact coordinates indicated by those construction lines!

❌ Calculation Traps to Avoid

• Inverting the gradient: Dividing time by volume ( 2.5 / 1.9 = 1.32 ) gives zero marks for the final two steps. Remember: gradient = y / x = rise / run .
• Unit mismatches: The answer space states cm³/min , so keep time in minutes – do not convert minutes into seconds.