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

Which of the following fractions generally has the highest viscosity?

1. Kerosene

2. Gasoline

3. Bitumen

4. Refinery gases

The correct answer is: Bitumen

The fraction that generally has the highest viscosity is bitumen due to its molecular structure and composition. Viscosity refers to a fluid's resistance to flow, which is influenced by the size and interaction of its molecules. Bitumen is a thick, sticky substance primarily composed of large, complex hydrocarbon molecules. These heavy molecules have high molecular weight, leading to strong intermolecular forces that increase the thickness and flow resistance of the substance. The higher the molecular weight and complexity, the more viscous the substance will be. In contrast, kerosene and gasoline consist of smaller hydrocarbons that are more volatile and less complex, allowing them to flow more easily. Refinery gases typically consist of light gases (like propane and butane), which have very low viscosity due to their small molecular size. Therefore, among the options, bitumen stands out as having the highest viscosity, attributed to its larger molecules and strong intermolecular interactions.