Advanced Placement (AP) Biology Practice Exam

Why does water have high surface tension?

1. Because of its low density

2. Due to polar covalent bonds

3. Because of hydrogen bonding between molecules

4. As a result of evaporation

The correct answer is: Because of hydrogen bonding between molecules

Water exhibits high surface tension primarily due to hydrogen bonding between its molecules. In liquid water, each molecule forms hydrogen bonds with several neighboring molecules, leading to a cohesive effect. This cohesion causes water molecules at the surface to be pulled inward by the cohesive forces of the molecules below, creating a "skin" on the surface. This is why objects denser than water, like a needle, can sometimes rest on the surface without sinking, as the hydrogen bonds provide resistance to external forces. The correct understanding of water's surface tension hinges on the unique properties of hydrogen bonds. These transient interactions among water molecules are strong enough to create the surface tension observed. The intermolecular forces create a scenario where the surface molecules experience a net inward force, resulting in a minimized surface area and a high level of tension compared to most other liquids.