AQA GCSE Chemistry Chemistry Paper 2 (Foundation), June 2025: Question 9

✅ Correct Answer

fractional distillation

💡 Key Knowledge

Crude oil is a finite mixture of hydrocarbons with different chain lengths and boiling points. It is separated industrially by fractional distillation inside a fractionating column.

❌ Common Errors

Writing just "distillation" or "cracking". Plain distillation does not account for separating a mixture into multiple fractions; cracking is a breakdown reaction, not a physical separation.

✅ Correct Answer

• Heater: evaporation (or boiling / liquid to gas) [1]
• Column: condensation (or gas to liquid) [1]

💡 Key Knowledge

Crude oil enters the heater where hydrocarbons turn from liquid to gas. As hot vapours rise up the column (which is hot at the bottom and cooler at the top), they cool and turn back from gas to liquid at their boiling points.

🧠 Exam Technique

Always specify physical change names cleanly. Pair them logically: evaporation/boiling requires heat input (heater), whereas condensation occurs upon cooling (column).

✅ Correct Answer

petrol

🧠 Step-by-Step Deduction

1. Note the temperature gradient: cool at the top, hot at the bottom.
2. Arrange fractions from lowest to highest boiling point:
   A: Petroleum gases (< 30 °C)
   B: Petrol (40 – 110 °C)
   C: Kerosene (180 – 260 °C)
   D: Diesel oil (260 – 320 °C)
   E: Heavy fuel oil (320 – 400 °C)
3. Position B directly corresponds to petrol.

❌ Common Errors

Selecting petroleum gases by forgetting that fraction A exits at the very top, or confusing the top of the column as the hottest point.

✅ Correct Option

☑ heavy fuel oil → diesel oil → kerosene

💡 Key Trend Rules

• Shorter chains: lower boiling point → higher volatility → more flammable.
• Longer chains: higher boiling point → lower volatility → less flammable.
• Increasing flammability means: least flammable → most flammable.

❌ Common Errors

Reading the trend backwards! Selecting kerosene → diesel oil → heavy fuel oil gives the order of decreasing flammability.

✅ Correct Answers

• 09.5 Molecular formula: C₄H₁₀ [1]
• 09.6 Name: butane [1]

💡 Key Knowledge

In standard ball-and-stick models:
• Black spheres = Carbon (C) → count = 4
• White/grey spheres = Hydrogen (H) → count = 10
General formula for alkanes: C_nH_2n+2 → C₄H₂(₄)+₂ = C₄H₁₀ .

❌ Common Errors

• Writing incorrect letter case or bad formatting (e.g., c4h10 or C4H10 without subscripts).
• Spelling errors in naming: writing butene (an alkene) instead of butane. Examiners specifically underline the a .

✅ Balanced Equation

Coefficients: 3 before CO₂ and 4 before H₂O.

📐 Step-by-Step Balancing Method

1. Balance Carbons (C):
   Reactants have 3 C atoms in C₃H₈.
   Therefore, you need 3 CO₂ .
2. Balance Hydrogens (H):
   Reactants have 8 H atoms in C₃H₈.
   Water contains 2 H atoms per molecule → 8 ÷ 2 = 4 .
   Therefore, you need 4 H₂O .
3. Check Oxygen (O):
   Products: (3 × 2) + (4 × 1) = 6 + 4 = 10 O atoms.
   Reactants: 5 O₂ = 10 O atoms. (Balanced!)

❌ Common Traps

Attempting to change the given coefficient of 5 O₂ or writing subscripts instead of large balancing numbers in front of formulas.