As the size of a hydrocarbon molecule increases, what is the expected change in physical properties?

As the size of a hydrocarbon molecule increases, the physical properties related to melting and boiling points generally exhibit an increasing trend. This can be attributed to the larger molecular size leading to a greater surface area, which allows for increased van der Waals forces or London dispersion forces between molecules.

In smaller hydrocarbons, these intermolecular forces are weaker, resulting in lower melting and boiling points. However, as the hydrocarbon chain grows in size, the number of carbon atoms increases, thereby enhancing the intermolecular interactions. As a result, more energy is required to overcome these forces when transitioning from solid to liquid (melting) or liquid to gas (boiling), leading to higher melting and boiling points.

Thus, the relationship between molecular size and melting/boiling points aligns with the observed trend that larger hydrocarbons tend to have higher melting and boiling points compared to their smaller counterparts.