Advanced Placement (AP) Biology Practice Exam

What property of water is primarily due to its molecular structure and polarity?

1. Hydrophobicity

2. Cohesion

3. Solubility

4. Density

The correct answer is: Cohesion

The property of water that is primarily due to its molecular structure and polarity is cohesion. Cohesion refers to the attraction between water molecules, which is a direct result of hydrogen bonding. Water molecules have polar covalent bonds, where the oxygen atom has a slight negative charge and the hydrogen atoms have slight positive charges. This polarity allows water molecules to be attracted to each other, causing them to stick together. This cohesive property is essential for various biological processes, such as the movement of water in plants during transpiration and the formation of water droplets. It also contributes to surface tension, allowing small organisms to walk on water and enabling plants to transport water through capillary action. While other properties like hydrophobicity, solubility, and density can also be influenced by water's molecular structure, they arise from different characteristics or interactions. For instance, hydrophobicity refers to the tendency of nonpolar substances to repel water, while solubility pertains to the ability of substances to dissolve in water, which involves interactions with the polar water molecules. Density in water varies with temperature, particularly its unusual behavior when it freezes, but this does not directly stem from the same cohesive forces that characterize cohesion.