Final answer:
The essential property for the fractional distillation of two hydrocarbons is the difference in their boiling points. This allows the compounds to vaporize and condense at different temperatures, enabling their separation.
Step-by-step explanation:
The property that is essential for the separation process of fractional distillation of two straight-chain hydrocarbons is their boiling points. Fractional distillation works because different substances will have different boiling temperatures, allowing them to vaporize at different times. The substance with the lower boiling point will vaporize first, and as it is condensed, it can be collected separately from the substance with the higher boiling point.
In the context of the distillation of hydrocarbons, such as in the refining of crude petroleum, the lighter and more volatile components with lower boiling points will rise to the top of the distillation column and can be drawn off earlier. Conversely, heavier components with higher boiling points will condense at the bottom.