What does each fraction of crude oil primarily contain?

The correct answer is that each fraction of crude oil primarily contains molecules with a similar number of carbon atoms. Crude oil is a complex mixture of hydrocarbons that can be separated into different fractions through a process called fractional distillation. Each fraction obtained during this process has a specific boiling point range and tends to contain hydrocarbons that have similar molecular weights and structures, particularly relating to the number of carbon atoms they possess.

For example, lighter fractions contain shorter-chain hydrocarbons, whereas heavier fractions consist of longer-chain hydrocarbons. This similarity in the number of carbon atoms influences the physical and chemical properties of the fractions, such as volatility, viscosity, and boiling points, making it easier to refine crude oil into useful products.

In contrast, options that suggest the presence of varying functional groups or pure hydrocarbons only do not accurately reflect the nature of the fractions. Crude oil fractions predominantly consist of hydrocarbons (compounds containing only hydrogen and carbon) and do not include functional groups typical of other organic compounds. Additionally, while fractions may contain mixtures of hydrocarbons, they are not purely composed of only solid hydrocarbons, as they typically include gases and liquids as well. Therefore, the defining characteristic of each fraction is that it primarily contains molecules with a similar number