Which properties of hydrocarbons change with the size of the molecule?

The correct choice highlights properties of hydrocarbons that are influenced by the size of the molecule, specifically boiling point, viscosity, and flammability.

As the size of the hydrocarbon molecule increases, the boiling point tends to rise. This is due to larger molecules having greater van der Waals forces (a type of intermolecular force) that require more energy to overcome when transitioning from a liquid to a gas. This means that larger hydrocarbons will generally have higher boiling points than smaller ones.

Viscosity, which refers to a fluid's resistance to flow, also increases with the size of the hydrocarbon molecules. Larger molecules entangle more easily, resulting in a thicker, more viscous substance. For example, short-chain hydrocarbons like propane are gases at room temperature, while longer-chain hydrocarbons can be liquids or even solids.

Flammability can also vary with molecule size; typically, smaller hydrocarbons are more flammable due to their lower boiling points, which allows them to vaporize easily and form ignitable mixtures with air. As the size increases, the flammability generally decreases, although this can depend on other factors such as the presence of functional groups.

In contrast, the other answer choices contain properties that do not correlate with