2024 O Level Chem P1 Q33 — Crude Oil / Fractional Distillation Solution

Answer: C. Fractional distillation of crude oil exploits differences in boiling point. Vaporised crude rises through a column that is hot at the bottom and cooler at the top.

Larger hydrocarbons have stronger intermolecular forces, so higher boiling points. They condense first, lower down (the hot end). Smaller hydrocarbons have lower boiling points and rise higher before condensing in the cooler regions.

Following the upward arrow: temperature decreases, molecule size decreases, boiling point decreases, viscosity decreases.

Row C captures two of these together (size decreases and column temperature decreases). The other rows mix correct and incorrect trends, e.g. claiming size and BP both increase upwards (B), or boiling point increases upwards (D).