Which fraction is the lightest in the bottom-to-top order?

• A. Bitumen
• B. Refinery gases
• C. Petrol
• D. Diesel

Answer: (B)

In fractional distillation of crude oil, fractions separate by boiling point: the lower the boiling point, the more volatile the substance, and it rises to the top of the column, while heavier, higher-boiling fractions stay toward the bottom. Bitumen is the heaviest and has a very high boiling point, so it sits at the bottom. Petrol is lighter than diesel and much heavier than the lightest gases. Diesel sits below petrol in volatility. Refinery gases have the lowest boiling points, making them the most volatile and thus the lightest, collected at the top. So the lightest in bottom-to-top order is refinery gases.