Chemistry – Organic chemistry - Fuels | e-Consult

The observation that the boiling point of a fraction increases as you move from the bottom to the top of the column is directly related to the properties of the hydrocarbon molecules present in each fraction. Fractions at the bottom of the column contain larger hydrocarbon molecules with longer carbon chains. These longer chains have stronger London dispersion forces between the molecules. Because stronger intermolecular forces require more energy to overcome, the boiling point of these fractions is higher. Conversely, fractions at the top of the column contain smaller hydrocarbon molecules with shorter carbon chains, resulting in weaker London dispersion forces and lower boiling points.

Here's a table illustrating the relationship between chain length and boiling point: