AQA GCSE Chemistry Chemistry Paper 1 (Foundation), June 2024: Question 9

✅ Correct Answer

• Similarity: Both have 1 electron in their outer shell (or energy level) [1 mark]
• Difference: Sodium has 3 electron shells, whereas potassium has 4 electron shells (or potassium has more shells than sodium) [1 mark]

💡 Key Knowledge

• Group number = Number of electrons in the outer shell (both are in Group 1 → 1 outer electron).
• Period number = Number of occupied electron shells (Na is in Period 3 → 2,8,1; K is in Period 4 → 2,8,8,1).

🧠 Exam Technique

Be specific! State "outer shell" or "outer energy level". Simply writing "they both end in 1" is too vague. When stating differences, clearly name both elements or state the exact number of shells.

❌ Common Errors

Confusing electronic structure with chemical properties (e.g. answering "both form +1 ions" or "both react with water" scores 0 marks because these are not features of electronic structure).

✅ Correct Answers (Choose any TWO)

• Effervescence / bubbling / fizzing [1 mark]
• Floats on the water [1 mark]
• Moves rapidly across the surface [1 mark]
• Melts into a ball / sphere [1 mark]
• Catches fire / produces a lilac flame [1 mark]
• Decreases in size / disappears / dissolves [1 mark]
• Explodes / pops [1 mark]

❌ Common Errors

• "Hydrogen gas produced": This is a chemical deduction, NOT an observation! You must state what you see (e.g. fizzing, effervescence, or bubbles).
• Writing just "lilac": Colour alone is rejected; you must mention a flame (or lilac flame).
• "Steam produced": Often confused with white smoke or mist; not creditworthy.

✅ Correct Answer

• Colour: Blue / purple / violet [1 mark]
• Reason: The solution is alkaline / basic (or contains OH⁻ / hydroxide ions) [1 mark]

💡 Key Knowledge

Alkali metal + water → metal hydroxide + hydrogen
2K + 2H₂O → 2KOH + H₂

The potassium hydroxide dissociates into K⁺ and OH⁻ ions. Hydroxide ions (OH⁻) make the solution strongly alkaline (pH 12–14), turning universal indicator purple.

🧠 Exam Technique

Always ensure your colour matches the stated reason. Writing "green" (neutral) or "red" (acidic) immediately forfeits the first mark. Never say "potassium is an alkali metal" as your reason—it is the hydroxide ions in solution that cause the indicator change.

📐 09.4: Plotting the Data [2 marks]

Check the grid scale carefully before placing points:

• x-axis (Atomic number): 10 small squares = 20 → 1 small square = 2 units.
• y-axis (Density in mg/cm³): 10 small squares = 2 → 1 small square = 0.2 units.

Points to plot from Table 4:

• Helium: (2, 0.2) → 1 square across, 1 square up
• Neon: (10, 0.8) → 5 squares across, 4 squares up
• Argon: (18, 1.6) → 9 squares across, 8 squares up
• Xenon: (54, 5.4) → 27 squares across, 27 squares up
• Radon: (86, 9.1) → 43 squares across, 45.5 squares up

✅ 09.5: Estimating Density of Krypton (X) [1 mark]

Atomic number of Krypton = 36.

• Find 36 on the x-axis (18 small squares along).
• Read up to your line/curve of best fit.
• Read across to the y-axis.

Expected value: 3.4 mg/cm³
(Mark scheme accepts any value in range: 3.0 to 3.8 mg/cm³)

❌ Graph Plotting Pitfalls

• Using blunt pencils or large "blobs" that exceed the ± ½ square tolerance. Use neat crosses ( × ).
• Misreading the scale: thinking each square on the x-axis is 1 unit instead of 2.

✅ 09.6: Displacement Reaction [1 mark]

Correct Box to Tick:

Why: Chlorine is more reactive than bromine, so it displaces bromide:
Cl₂ + 2KBr → 2KCl + Br₂

✅ 09.7: Group 7 Trends [1 mark]

Correct Box to Tick:

💡 Group 7 Summary Rules

• Reactivity: Decreases down the group (F > Cl > Br > I). A higher halogen displaces a lower halogen from its halide salt.
• Molecular size: Increases down the group (more electron shells → larger relative molecular mass, Mᵣ).
• Boiling points: Increase down the group because larger molecules have stronger intermolecular forces requiring more energy to break.

🧠 Exam Tip for Displacement

Compare the free halogen element to the halide ion in the salt:

• Cl₂ + KF → No reaction (Cl is less reactive than F)
• Cl₂ + KBr → Reaction occurs (Cl is more reactive than Br)
• Br₂ + KF → No reaction (Br is less reactive than F)
• Br₂ + KCl → No reaction (Br is less reactive than Cl)