AQA GCSE Combined Science: Trilogy Chemistry Paper 2 (Foundation), November 2020: Question 1

✅ Correct Answer

Crude oil is the remains of plankton [1 mark].

Millions of years ago biomass was buried under mud [1 mark].

💡 Key Knowledge

Crude oil is a finite fossil fuel formed over millions of years from:

• Ancient biomass, mainly microscopic sea organisms called plankton.
• Burial under layers of mud and sediment on sea beds.
• High pressure, high temperature, and the absence of oxygen.

❌ Common Errors

• Choosing trees: Coal is formed from ancient trees and terrestrial plants, not crude oil.
• Choosing air: Crude oil formation requires anaerobic conditions (no air/oxygen).

✅ Correct Answers

01.2 Ordering Stages [1 mark]:

• First stage: B (Crude oil is heated)
• Second stage: A (Hydrocarbons evaporate)
• Third stage: C (Vapours condense)

01.3 Process Name [1 mark]:

[✓] Fractional distillation

💡 Key Knowledge

Fractional distillation separates a mixture of liquids with different boiling points:

1. Boiling/Heating: Oil is vaporised before entering the column.
2. Evaporation & Rising: Hydrocarbon vapours rise up a temperature-gradient column (hot at base, cool at top).
3. Condensation: Vapours condense when they reach temperatures at or below their boiling points and are collected.

🧠 Exam Technique

Remember the simple cause-and-effect rule: you cannot condense a vapour until you have made it (evaporation), and you cannot evaporate oil until you apply energy (heat).

✅ Correct Answers

• 01.4 Formula [1 mark]: C₃H₈
• 01.5 Bond X [1 mark]: [✓] Covalent bond
• 01.6 General Formula [1 mark]: [✓] CₙH₂ₙ₊₂

💡 Key Knowledge

In ball-and-stick models:

• Dark spheres = Carbon (C). There are 3 carbons.
• White spheres = Hydrogen (H). There are 8 hydrogens.
• The sticks represent shared pairs of electrons, which are covalent bonds between non-metal atoms.
• All alkanes follow the general formula CₙH₂ₙ₊₂ where n is the number of carbon atoms (e.g. if n = 3, H = 2(3) + 2 = 8).

❌ Common Errors

• Incorrect subscripts: Writing C3H8 with normal-sized text or mixing up capitals. Always write elements in standard chemical formula format: C₃H₈.
• Confusing general formulae:
  • CₙH₂ₙ is for alkenes.
  • CₙH₂ₙ₋₂ is for alkynes.
• Confusing bond types: Choosing ionic or metallic bonding. Both Carbon and Hydrogen are non-metals, so bonding is always covalent.

📐 Step-by-Step Calculation

1. Identify original quantity: 8.0 billion
2. Identify new quantity: 1.3 billion
3. Calculate decrease:
   8.0 - 1.3 = 6.7
4. Check units on line: The answer space ends with "billion", so write just 6.7 .

✅ Accepted Answers

• 6.7 (billion) [1 mark]
• Also allowed: 6 700 000 000

❌ Common Errors & Traps

Writing too many zeros: Because "billion" is already printed after the answer line, writing 6.7 billion billion or adding extra zeros unnecessarily can create confusion. If you write 6.7 , it matches the unit provided on the paper perfectly.