How is crude oil separated into fractions?

• A. By filtration
• B. By fractional distillation based on boiling points
• C. By electrolysis
• D. By chemical reaction with acid

Answer: (B)

Separating crude oil into fractions relies on differences in boiling points. Crude oil is a mixture of many hydrocarbons, each with its own boiling point. When you heat the crude oil in a fractionating column, the lighter, more volatile components (those with lower boiling points) vaporize first and rise up the column. The column has a temperature gradient: hotter at the bottom, cooler as you go higher. As the vapour rises and cools, different hydrocarbons condense at different heights where the temperature matches their boiling point. Each section collects a fraction containing hydrocarbons with similar boiling points. Heavier components don’t vaporize as easily and stay lower in the column or condense into heavier fractions, while the lighter ones are collected at the top.

This process is called fractional distillation because the column provides many tiny distillations in one go, improving separation so you can obtain distinct fractions like gases, petrol, kerosene, diesel, and heavier oils, rather than a single mixed distillate.

Filtration wouldn’t work here because it separates solids from liquids, not mixtures of liquids. Electrolysis or reacting with acid would chemically alter the substances and isn’t how you separate components by their boiling points.