In the bottom-to-top order of fractions, which is the heaviest fraction at the bottom?

• A. Bitumen
• B. Heavy fuel oil
• C. Lubricating oil
• D. Diesel

Answer: (A)

In fractional distillation, fractions are separated by boiling points. Heavier, high-boiling-point compounds need more energy to vaporize, so they tend to stay in the liquid phase and condense lower in the column. The bottom of the column is where the hottest, most condensed liquids collect, so the heaviest fraction forms the residue there. Bitumen is the heaviest, most viscous fraction of crude oil, with the highest boiling range, so it remains as a thick residue at the bottom. The lighter fractions—heavy fuel oil, lubricating oil, and diesel—have lower boiling points and condense higher up in the column, making bitumen the bottom-most fraction. Bitumen’s extreme heaviness also explains its use as asphalt rather than as a fuel.