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2022 O Level Chem P1 Q32 — Fractional Distillation of Petroleum Solution

Fractional Distillation of Petroleum 2022 O Level Pure Chemistry · Paper 1 · Q32

Question

Petroleum is separated by fractional distillation. Which row best describes a fraction collected near the top of the fractionating column?

Ahigh flammability, low boiling point, viscous
Blong chain molecule, low flammability, viscous
Cshort chain molecule, high flammability, low boiling point
Dshort chain molecule, low flammability, high boiling point

Correct answer: C

Video Solution

Explanation

Answer: C. The fractionating column separates by boiling point. The hottest fractions (long chains, low volatility, high boiling point) come out at the BOTTOM. The coolest fractions (short chains, high volatility, low boiling point) come out at the TOP.

So a fraction collected near the top is:

• Short chain molecule — fewer carbons, lower \(M_r\)
• High flammability — short-chain hydrocarbons ignite easily
• Low boiling point — that's why they vapourised at the top of the column

This matches option C exactly.

A wrongly calls a low-b.p. fraction "viscous" — viscosity goes with long chains/high-b.p. fractions, not the top. B and D describe heavy fractions ('long chain, viscous' or 'low flammability'), which come out near the bottom, not the top.

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