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

How do the boiling points of fractions in crude oil generally change?

1. Boiling point decreases with more carbon atoms

2. Boiling point remains constant

3. Boiling point increases as the number of carbon atoms increases

4. Boiling point is the same for all fractions

The correct answer is: Boiling point increases as the number of carbon atoms increases

The boiling points of fractions in crude oil generally increase as the number of carbon atoms increases due to the properties of hydrocarbons. Crude oil is made up of a mixture of hydrocarbons with varying chain lengths. As the carbon chain length increases, the molecules become larger and the van der Waals forces (induced dipole interactions) between them also increase. This results in stronger intermolecular forces that require more energy to break when transitioning from a liquid to a gas, resulting in higher boiling points. For example, lighter fractions with smaller carbon chains, such as methane or ethane, have lower boiling points due to weaker intermolecular forces. In contrast, heavier fractions with longer carbon chains, like those found in diesel or lubricating oils, display significantly higher boiling points. This trend is essential in processes like fractional distillation, where different fractions of crude oil are separated based on their boiling points. Thus, the correct answer highlights the direct relationship between carbon chain length and boiling point in hydrocarbons.