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

✅ Correct Answer

fractional distillation

💡 Key Knowledge

Crude oil is a finite mixture of many different hydrocarbons (mostly alkanes). Because these molecules have different chain lengths, they have different boiling points, allowing them to be separated by fractional distillation.

❌ Common Errors

• Writing just "distillation" — at GCSE, the word fractional is required because it separates a mixture into multiple fractions.
• Confusing separation with chemical breakdown: writing "cracking". Cracking is a thermal decomposition reaction, not a physical separation.

✅ Correct Answer

Heater: evaporation (allow boiling / liquid to gas) [1 mark]

Column: condensation (allow gas to liquid) [1 mark]

💡 How the Column Works

• In the heater: Crude oil is heated strongly until most of it turns from liquid into vapour (evaporates).
• In the column: Vapours rise up the column, which is hot at the bottom and cooler at the top. When each fraction reaches a height where the temperature drops below its boiling point, it turns back into liquid (condenses) and is tapped off.

🧠 Exam Technique

Always name the standard physical state change. Writing descriptions like "turns into steam" risks losing marks. Use formal scientific terms: evaporation and condensation.

✅ Correct Answer

petrol

🧠 Step-by-Step Column Matching

1. Look at Figure 1: The arrow shows increasing temperature downwards. That means A is the coldest zone (top) and E is the hottest zone (bottom).
2. Order Table 2 from lowest to highest boiling point:
   • A: Petroleum gases (below 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 corresponds directly to petrol.

❌ Common Errors

Reading the column upside down! Remember: Small molecules with low boiling points condense near the cool top; large molecules with high boiling points condense near the hot bottom.

✅ Correct Answer

[✓] Heavy fuel oil → diesel oil → kerosene

💡 Hydrocarbon Properties Trend

• Shorter carbon chains: Lower boiling point, lower viscosity (runnier), higher flammability (ignites easily).
• Longer carbon chains: Higher boiling point, higher viscosity (thick), lower flammability (hard to ignite).

❌ Common Trap: "Increasing" vs "Decreasing"

"Increasing flammability" means you must start with the least flammable (hardest to burn) and end with the most flammable (easiest to burn):

Heavy fuel oil (least) → Diesel oil → Kerosene (most)

Many students choose the reverse order because they confuse high boiling point with high flammability.

✅ Correct Answers

02.5 Molecular Formula: C₄H₁₀ [1 mark]

02.6 Hydrocarbon Name: butane [1 mark]

🧠 Counting from 3D Models

• Count the black spheres (central carbon chain): 4 Carbons.
• Count the attached grey/white spheres: 10 Hydrogens.
• Check general formula for alkanes (C_nH_2n+2): for n = 4, 2(4) + 2 = 10. Formula is C₄H₁₀ .
• Recall alkane prefixes: Meth- (1), Eth- (2), Prop- (3), But- (4). Because it is an alkane with only single bonds, the name ends in -ane → butane.

❌ Common Errors

• Writing lowercase letters or wrong subscripting: c4h10 or C4H10 . Chemical symbols must be capitalised with subscripts: C₄H₁₀ .
• Spelling as "butene" (an alkene) instead of butane. The mark scheme specifically highlights the a in but a ne.

✅ Balanced Equation

C₃H₈ + 5 O₂ → 3 CO₂ + 4 H₂O

📐 Step-by-Step Balancing Method

1. Balance Carbon (C) first:
   Left side: 3 C in C₃H₈.
   Right side: Needs 3 CO₂.
2. Balance Hydrogen (H) second:
   Left side: 8 H in C₃H₈.
   Right side: Each H₂O has 2 H, so 8 ÷ 2 = 4 H₂O.
3. Check Oxygen (O):
   Right side has: (3 × 2) + (4 × 1) = 6 + 4 = 10 O atoms.
   Left side has: 5 O₂ = 10 O atoms. Equation is fully balanced!

❌ Common Errors

• Attempting to change chemical subscripts instead of balancing coefficients (e.g. writing C₃O₂).
• Forgetting that each H₂O contains 2 hydrogens and putting 8 in front of H₂O.