AQA GCSE Combined Science: Trilogy Chemistry Paper 2 (Foundation), June 2025: Question 1

✅ Correct Answer

Plankton

💡 Key Knowledge

Crude oil is a finite fossil fuel formed over millions of years from the biomass of ancient, microscopic marine organisms (mainly plankton) that were buried in mud under high pressure and heat in the absence of oxygen.

❌ Common Errors

• Ticking Microbes: Students confuse the decomposition process with the original organic material itself.
• Ticking Water: While oil forms under marine sediments, water is an inorganic compound, not the biological source.

🧠 Exam Technique

Always watch the tick instruction: "Tick (✓) one box." If you make a mistake, cross out the whole wrong box completely and tick the correct one clearly.

✅ Correct Answer

Hydrocarbons

💡 Key Knowledge

A hydrocarbon is a compound consisting exclusively of hydrogen and carbon atoms only. Crude oil is a complex mixture of very many hydrocarbons, most of which are alkanes.

❌ Distractors Explained

• Halogens: Group 7 elements (fluorine, chlorine, bromine, iodine), not organic compounds.
• Fertilisers: Compounds rich in nitrogen, phosphorus, and potassium (NPK) used to aid plant growth.

🧠 Exam Technique

Break the word down: Hydro- (hydrogen) + -carbon (carbon). The question explicitly specifies compounds containing "carbon and hydrogen atoms only".

Also allowed: "The main component in area A is petrol, whereas in area B it is heavy fuel oil."

🧠 How to Structure Comparative Answers

• Use comparative language: more than, less than, or higher percentage of.
• Clearly name the area you are describing (e.g., "Area A has more diesel oil than Area B").
• Cover 3 different fractions on lines 1, 2, and 3 to ensure you collect all 3 distinct marks.

❌ Where Marks Were Lost

• Quoting numbers without comparison: Writing "Area A diesel is 18%, Area B diesel is 13%" does not directly state the difference unless comparative words like "which is higher" are included.
• Incomplete statements: Stating "contains more petrol" without identifying which area has more.

✅ Mark Breakdown

• Mark 1: Correct calculation setup: (12 / 100) × 160
• Mark 2: Correct intermediate answer: 19.2
• Mark 3: Final answer rounded to 2 sig figs: 19

❌ Common Calculation Traps

• Forgetting significant figures: Leaving the answer as 19.2 loses the final mark. Always re-read the question for formatting requirements!
• Reading the wrong table: Using kerosene data from Table 1 (Area A: 12% or Area B: 10%) instead of Table 2 (Area C: 12%). While A and C share 12%, using petrol or diesel from Table 1 is a common mix-up.

✅ Mark Scheme Requirements

• Mark 1 (y-axis scale): Evenly spaced numbers on the major gridlines: 20, 30, 40 (and optionally 50). 0 and 10 are already given.
• Mark 2 (Kerosene bar): Height plotted accurately at exactly 12% (tolerance of ±0.5 small square) and clearly labelled "Kerosene".
• Mark 3 (Petrol bar): Height plotted accurately at exactly 31% (tolerance of ±0.5 small square) and clearly labelled "Petrol".

🧠 How to Plot with Precision

• Work out the scale: If 10 major subdivisions equal 10 units, then 1 small square = 1 unit.
• Kerosene (12%): Draw the top of the bar exactly 2 small squares above the 10 line.
• Petrol (31%): Draw the top of the bar exactly 1 small square above the 30 line.
• Keep bar widths uniform: Match the bar width and spacing of the existing bars (Diesel and Heavy fuel oil).