International General Certificate of Secondary Education (IGCSE) Chemistry Practice Exam

Which fraction of crude oil has the lowest boiling point?

1. Diesel

2. Refinery gases

3. Kerosene

4. Bitumen

The correct answer is: Refinery gases

Refinery gases represent the fraction of crude oil with the lowest boiling point. In the process of fractional distillation, crude oil is heated and separated into various components based on their boiling points. The lighter fractions, such as refinery gases, which include gases like propane and butane, boil at much lower temperatures compared to heavier fractions. Refinery gases can typically boil at temperatures below 25 °C, making them the lightest and least dense fractions of crude oil. This low boiling point is due to their smaller molecular size and weaker intermolecular forces, which require less energy (in the form of heat) to overcome. In contrast, fractions like diesel, kerosene, and bitumen have significantly higher boiling points. Diesel is larger and denser than refinery gases, commonly boiling between 200 °C to 350 °C. Kerosene is in between, boiling around 150 °C to 300 °C, while bitumen, being the heaviest fraction, has very high boiling points, often exceeding 500 °C due to its complex mixture of large molecules. Thus, refinery gases are correctly identified as the fraction with the lowest boiling point in crude oil.