AQA GCSE Chemistry Chemistry Paper 2 (Foundation), November 2021: Question 9

✅ Correct Answer

• Plot 5 points accurately:
• (4, 0)
• (5, 36)
• (6, 69)
• (8, 126)
• (9, 151)

🧠 Exam Technique

• Check the scale: On the y-axis, 10 small squares = 20 °C, so 1 small square = 2 °C.
• For 36 °C: count 3 small squares above 30 °C (or 8 above 20 °C).
• Use a sharp pencil and neat small crosses (×), not large round blobs. Large dots often exceed the ± ½ square tolerance!

✅ Correct Answer

X = 98 °C

🧠 Exam Technique & Line of Best Fit

• Draw a smooth, continuous curved line of best fit through your plotted points.
• Do not use a ruler to join points dot-to-dot—alkane boiling points follow a smooth curve!
• Go to 7 on the x-axis, trace vertically up to your curve, then trace left horizontally to read the temperature on the y-axis.

❌ Common Errors

Guessing the midpoint between 69 and 126 (which gives 97.5) without consulting the trend line, or reading the y-axis scale incorrectly (remembering each square is 2 °C, not 1 °C).

✅ Correct Answer

The boiling point is lower than 0 °C (it is a negative temperature).

💡 Key Knowledge

Looking at the trend, fewer carbons = lower boiling point. Butane (4 carbons) boils at 0 °C. Therefore, propane (3 carbons) must boil at a temperature below 0 °C. The y-axis starts at 0 °C and has no negative scale.

✅ Correct Answer

Gas (or g)

💡 Key Knowledge: State Rule

• If temperature is below melting point → Solid
• If temperature is between melting & boiling point → Liquid
• If temperature is above boiling point → Gas
• Butane has a boiling point of 0 °C. Since 20 °C is above 0 °C, it has already boiled into a gas.

❌ Common Errors

Writing "liquid" because students assume room temperature (20 °C) implies liquids, forgetting to check the boiling point of butane given in the table (0 °C).

✅ Correct Answer

C₉H₂₀ (write 20 in the space provided)

📐 Calculation Step

1. General formula for alkanes: CₙH₂ₙ₊₂
2. Here, n = 9:
3. Number of H atoms = (2 × 9) + 2 = 18 + 2 = 20

❌ Common Errors

Confusing alkanes with alkenes (CₙH₂ₙ) and writing C₉H₁₈.

✅ Correct Answer

• Mark 1: Nonane has a higher boiling point (than the other alkanes in the table).
• Mark 2: So it condenses where the column is hotter / has a higher temperature (near the bottom).

💡 Key Knowledge: How the Column Works

• Temperature Gradient: The fractionating column is hottest at the bottom and coolest at the top.
• Vapours rise up the column until they reach a level where the temperature falls below their boiling point.
• Molecules with longer chains have stronger intermolecular forces, giving them higher boiling points, so they condense early at high temperatures near the base.

❌ Common Errors

• Stating "nonane is heavier so it sinks" — condensation is caused by temperature and boiling point, not gravity!
• Forgetting to mention the temperature gradient inside the column, even though the question explicitly prompted for it.