What does it mean when we say that each fraction obtained from fractional distillation contains molecules with a similar number of carbon atoms?

It means that the hydrocarbons in each fraction have a comparable chain length, which affects their physical properties such as boiling point and viscosity. This similarity allows for the separation of hydrocarbons based on their molecular size during the distillation process.

This question tests the understanding of the concept of fractional distillation and the significance of molecular size in separating hydrocarbons. A strong answer demonstrates knowledge of how fractions are categorized and their implications on properties.