As the size of a hydrocarbon molecule increases, which of the following properties also increases?

As the size of a hydrocarbon molecule increases, both boiling point and viscosity tend to increase. This is because larger hydrocarbon molecules have more atoms, leading to larger van der Waals forces between the molecules. As a result, more energy is required to overcome these forces when the hydrocarbon is heated, thus leading to a higher boiling point.

Viscosity, which refers to a substance's resistance to flow, also increases with larger hydrocarbons. Larger molecules have a greater surface area and more complex structures, resulting in increased intermolecular interactions. This means that as the size of the molecule increases, it becomes more difficult for them to slide past one another, leading to a thicker or more viscous fluid.

Flammability and explosiveness do not necessarily correlate with the size of hydrocarbon molecules in the same way. While the ability to combust may also depend on molecular structure and other factors, the relationship is not as direct or consistent as that of boiling point and viscosity. Smaller hydrocarbons can be highly flammable, and as they increase in size, they don’t inherently become more flammable or explosive.

Thus, the answer indicates that the inherent properties of boiling point and viscosity increase with larger hydrocarbon molecules due to increased intermolecular forces.