What happens to the viscosity of the main fractions in crude oil as the number of carbon atoms increases?

As the number of carbon atoms in the main fractions of crude oil increases, the viscosity of the substances also increases. This is primarily due to the larger molecular size and weight of the hydrocarbons present in these fractions.

Larger molecules have greater surface area, which allows them to interact more strongly with one another through van der Waals forces. This results in thicker, more viscous substances because more energy is required to move the molecules past each other smoothly. As a result, lighter fractions with fewer carbon atoms tend to be less viscous, while heavier fractions with more carbon atoms exhibit higher viscosity.

In essence, as the chain length of the hydrocarbons grows, the intermolecular interactions strengthen, and this leads to increased resistance to flow, characterizing the increased viscosity observed in heavier fractions of crude oil.