Advanced Placement (AP) Biology Practice Exam

How do hydrocarbons interact with water?

1. They are highly soluble in water

2. They are polar and dissolve easily

3. They are nonpolar and hydrophobic

4. They react with water to form acids

The correct answer is: They are nonpolar and hydrophobic

Hydrocarbons are composed primarily of carbon and hydrogen atoms and are characterized by their nonpolar nature. This nonpolarity arises because the carbon-hydrogen bonds share electrons relatively evenly, meaning there is no significant charge separation within the molecule. As a result, hydrocarbons do not interact favorably with polar molecules like water, which has a partial positive charge on the hydrogen atoms and a partial negative charge on the oxygen atom. The term "hydrophobic" refers to the tendency of nonpolar substances to be excluded from interactions with water. Since hydrocarbons do not form hydrogen bonds with water, they tend to be insoluble in aqueous environments. This explains why hydrocarbons do not mingle well with water, leading to scenarios where they separate from it (such as oil floating on water). In contrast, the other options mischaracterize the interaction of hydrocarbons with water; they imply either solubility or reactivity that does not align with the nonpolar and hydrophobic properties of hydrocarbons.