AQA GCSE Combined Science: Trilogy Chemistry Paper 2 (Foundation), June 2025: Question 5

✅ Correct Answer

C 12 H 26

📐 Calculation Steps

1. Identify general formula: C_nH_2n+2
2. Given number of carbon atoms: n = 12
3. Calculate hydrogen atoms: (2 × 12) + 2 = 24 + 2 = 26
4. Combined formula: C₁₂H₂₆

🧠 Exam Technique

Always show small, neat subscripts. Remember that alkanes are saturated hydrocarbons, meaning they follow the "double the carbons and add two" rule.

❌ Common Errors

• Forgetting to add 2 (writing C₁₂H₂₄, which is an alkene).
• Writing numbers as normal size or superscripts instead of subscripts.

✅ Correct Answer (Any one from)

• Safety: Dodecane is flammable / prevents dodecane catching fire (no open flame).
• Control: A water bath allows temperature to be controlled / provides even, steady heating.

💡 Key Knowledge

Hydrocarbons are organic fuels and catch fire easily. When heating flammable liquids, never use a naked Bunsen flame; use an electric heater or a water bath.

❌ Common Errors

• Vague answers like "it is safer" without explaining why (must state dodecane is flammable or catches fire).
• Saying "to heat it faster" (a Bunsen burner is actually faster, but too dangerous and hard to regulate).

🧠 Examiner Insight

Whenever an exam question asks why a water bath or electric mantle is chosen over a Bunsen burner for an organic substance, the mark is almost always awarded for stating that organic compounds are flammable.

✅ Correct Answer

Volume of dodecane

Also acceptable: Volume of dodecane added to the cup (100 cm³) / volume collected in the measuring cylinder (40 cm³).

💡 Key Knowledge: Variable Types

• Independent variable: Temperature of dodecane (the one being changed).
• Dependent variable: Time taken to collect 40 cm³ (the one being measured).
• Control variable: Total volume used (100 cm³), volume collected (40 cm³), size of hole in the cup.

❌ Common Errors

• Confusing the control variable with the dependent variable (writing "time taken").
• Saying "amount of dodecane" instead of the precise scientific term volume.

✅ Mark Scheme Breakdown

• Mark 1: Line of best fit smoothly extrapolated to 75 °C following the curve trend.
• Mark 2: Value at 75 °C correctly read from candidate’s own line (allowed tolerance: ± ½ a small square). Typical expected range: 4 s to 6 s.

🧠 Exam Technique for Extrapolation

1. Keep the curve smooth: Continue the existing downward curve with a single, unbroken pencil stroke. Do not use a ruler to finish a curve!
2. Do not level off unnaturally or turn upward: The curve continues to flatten gradually toward the x-axis.
3. Read accurately: Find 75 °C on the x-axis, draw a dashed vertical line to your curve, and read directly across to the y-axis.

❌ Common Errors

• Drawing a straight horizontal line after 65 °C.
• Letting the line plunge straight into the x-axis at zero.
• Reading the graph correctly from their drawn line, but losing Mark 1 because they forgot to actually draw the extension on the exam paper!

✅ Correct Answer (2-Step Explanation)

1. As the temperature increases, the time taken (to collect 40 cm³) decreases [1 Mark]
2. (Since a shorter time means it flows faster,) the viscosity decreases [1 Mark]

💡 Understanding Viscosity

• High viscosity: Thick, sticky, flows slowly (e.g. honey or cold oil).
• Low viscosity: Runny, flows quickly (e.g. water or warm oil).
• Heating liquids provides particles with kinetic energy to overcome intermolecular forces, making them flow more easily.

🧠 Examiner Insight

The question prompt explicitly tells you: "The higher the viscosity, the slower dodecane flows." Top students use this directly to bridge the link:

Higher Temp → Less Time Taken (Flows Faster) → Lower Viscosity

❌ Common Errors

• Only stating that viscosity decreases without referencing the data from Figure 5 (time decreasing).
• Discussing "rate of reaction" instead of rate of flow (this is a physical property, not a chemical reaction).
• Confusing viscosity with density or thickness without linking it back to the question.