AQA GCSE Chemistry Chemistry Paper 1 (Foundation), June 2023: Question 7

✅ Correct Answer

• Bottom-front-left corner: Na⁺ (open white circle)
• Bottom-back-left corner: Cl⁻ (filled black circle)
• Bottom-front-right corner: Cl⁻ (filled black circle)
• Bottom-back-right corner: Na⁺ (open white circle)

💡 Key Knowledge

Ionic compounds form a giant 3D ionic lattice where positively charged ions (Na⁺) and negatively charged ions (Cl⁻) alternate in all directions to maximise electrostatic attractions and minimise repulsions.

Rule of thumb: No two identical ions ever touch directly along an edge!

🧠 Exam Technique: Diagram Guide

Look directly at the ion sitting directly above each empty vertex on the top face:

• Top-front-left is Cl⁻ (black) → bottom-front-left must be Na⁺ (white)
• Top-back-left is Na⁺ (white) → bottom-back-left must be Cl⁻ (black)
• Top-front-right is Na⁺ (white) → bottom-front-right must be Cl⁻ (black)
• Top-back-right is Cl⁻ (black) → bottom-back-right must be Na⁺ (white)

❌ Common Errors

• Mixing up the key (shading in Na⁺ circles instead of Cl⁻).
• Placing identical ions next to each other along an edge.

✅ Correct Answer

The ions are free to move (and carry charge throughout the liquid).

💡 Key Knowledge

• In solid NaCl: ions are fixed in position within the lattice vibrating about fixed points, so it cannot conduct.
• In molten or aqueous NaCl: the giant lattice breaks down, so the ions are free to move.

❌ Critical Misconception (Examiner Warning)

DO NOT mention electrons!

The mark scheme explicitly states: "do not accept reference to moving / delocalised electrons". Metals and graphite conduct via delocalised electrons; ionic liquids conduct via mobile ions.

💡 Key Knowledge: Molten Electrolysis Rules

In a molten binary ionic compound, there is no water, so only two types of ions exist:

• Positive metal ions (cations) are attracted to the negative cathode → reduced to form the metal.
• Negative non-metal ions (anions) are attracted to the positive anode → oxidised to form the non-metal element.

❌ Common Errors

• Writing "Bromide" instead of the elemental name Bromine.
• Writing "Chloride" instead of Chlorine.

✅ Correct Answer

Tick box 1: The mixture has a lower melting point than pure aluminium oxide.

💡 Key Knowledge

• Pure aluminium oxide (Al₂O₃) has an extremely high melting point (over 2000 °C).
• Dissolving it in molten cryolite reduces the required working temperature to around 950 °C.
• This saves huge amounts of thermal energy and significantly lowers processing costs.

✅ 07.5 Source of Gases [1 mark]

Tick box 3: Water

Water partially ionises into H⁺ and OH⁻ ions: H₂O ⇌ H⁺ + OH⁻ . These ions discharge to form H₂ gas and O₂ gas.

✅ 07.6 Why Hydrogen is Produced [1 mark]

Tick box 1: Hydrogen is less reactive than sodium.

🧠 Cathode Rule for Aqueous Solutions

At the negative electrode (cathode), H⁺ and metal ions compete:

• If the metal is more reactive than hydrogen (e.g. Na, K, Mg), hydrogen gas is produced.
• If the metal is less reactive than hydrogen (e.g. Cu, Ag), the metal is produced.

❌ Common Errors

• Thinking oxygen comes from the sulfate ion (SO₄²⁻) — sulfate ions do not discharge; hydroxide ions from water produce oxygen.
• Confusing the reactivity rule and ticking "hydrogen is more reactive than sodium". Always remember sodium is high up in the reactivity series!

✅ Correct Answer (Any one of the following)

• The volume of hydrogen is twice the volume of oxygen. (Best response)
• The volume of hydrogen is greater than the volume of oxygen.
• The volume of hydrogen is directly proportional to the volume of oxygen.

📐 Graph Evidence & Calculation

Pick clear data points along the line of best fit:

1. When Volume of Oxygen = 2 cm³, Volume of Hydrogen = 4 cm³
2. When Volume of Oxygen = 4 cm³, Volume of Hydrogen = 8 cm³
3. Ratio: 8 cm³ / 4 cm³ = 2 : 1

This matches the chemical formula of water: 2H₂ + O₂ → 2H₂O, where 2 volumes of H₂ are formed for every 1 volume of O₂.

🧠 Top Tip for "Conclusion" Questions

While stating "hydrogen is greater than oxygen" gets the mark, giving the exact numerical relationship ("twice as much" or "2:1 ratio") proves precise data interpretation and guarantees the mark across all exam boards.