How does the boiling point of the main fractions in crude oil change with respect to the number of carbon atoms?

The boiling point of the main fractions in crude oil increases as the number of carbon atoms increases due to several factors related to molecular size and structure. As the number of carbon atoms in a hydrocarbon increases, the molecular weight of the compound also increases. Larger molecules have greater surface areas, which enhances the van der Waals forces (or London dispersion forces) between the molecules. These intermolecular forces require more energy to overcome, thus resulting in higher boiling points.

In addition, longer hydrocarbon chains exhibit more complexity in their structural arrangements, contributing further to the boiling point increase. This trend can be observed across the various fractions obtained from crude oil: for example, methane (with one carbon) has a much lower boiling point than octane (with eight carbons) or even larger hydrocarbons like dodecane.

The other options do not accurately reflect this relationship. The boiling point does not remain constant regardless of the number of carbon atoms, nor does it decrease with an increase in carbon atoms. The notion that boiling point varies unpredictably also contradicts the established trend observed in hydrocarbon behavior. Hence, the increase in boiling point with the rise in carbon atom number is a well-documented and fundamental property of hydrocarbon compounds.