What happens to the boiling point of crude oil fractions as you go up the distillation column?

• A. Decreases
• B. Increases
• C. Remains the same
• D. Varies unpredictably

Answer: (A)

In a distillation column, components separate by boiling point, and the column has a temperature gradient from hot at the bottom to cooler at the top. The vaporized oil rises, and as it moves upward it encounters cooler regions; when its boiling point is reached, that fraction condenses and is drawn off at that level. Heavier, higher-boiling-point substances stay lower in the column, while lighter, lower-boiling-point substances rise to the top. So the fractions collected higher up have progressively lower boiling points. In short, as you go up the column, the boiling point of the fractions decreases.