Advanced Placement (AP) Biology Practice Exam

Which type of bond is responsible for the polar nature of water molecules?

1. Ionic bonds

2. Polar covalent bonds

3. Hydrogen bonds

4. Van der Waals forces

The correct answer is: Polar covalent bonds

Water molecules are characterized by a polar covalent bond, which occurs between the hydrogen and oxygen atoms within each water molecule. In this type of bond, there is an unequal sharing of electrons, where the oxygen atom, being more electronegative, pulls the shared electrons closer to itself. This creates a partial negative charge around the oxygen atom and a partial positive charge around the hydrogen atoms, resulting in the polar nature of water. This polarity is crucial for water's properties, including its ability to dissolve many substances (making it a universal solvent) and its high surface tension. It also leads to the formation of hydrogen bonds between water molecules, which contributes to water's unique characteristics, such as high specific heat and cohesion. The distinction is important as hydrogen bonds themselves are weak attractions between polar molecules and do not create the molecule's polarity.