Modeling water at the pond surface (3 points)

A student draws two water molecules with oxygen (O) and hydrogen (H) atoms. (a) Describe or sketch

the partial charges on each water molecule. (b) Use a dotted line to represent ONE hydrogen bond

between different molecules, or describe where it belongs. (c) Explain how this molecular interaction

helps a water strider remain at the surface.

Water molecules have partial charges due to the electronegativity difference between oxygen and hydrogen, which leads to hydrogen bonding. These bonds contribute to water's surface tension, allowing water striders to walk on its surface without sinking. The interaction of water molecules through hydrogen bonds is crucial for maintaining this ability.

Explanation:

Modeling Water Molecules

The molecular structure of water (H2O) is crucial for understanding its properties. Each water molecule consists of one oxygen atom and two hydrogen atoms. The oxygen atom is more electronegative than hydrogen, which means it pulls the shared electrons closer to itself, resulting in a partial negative charge on the oxygen and partial positive charges on the hydrogen atoms.

(a) Partial Charges

When drawing water molecules, place the oxygen atom at the center and the two hydrogen atoms at an angle of about 104.5 degrees. Label the oxygen with a delta negative charge (δ−) and the hydrogens with delta positive charges (δ+). This can be illustrated as follows:

Oxygen (O) - δ−
Hydrogens (H) - δ+

(b) Hydrogen Bond Representation

A hydrogen bond occurs between the partial positive charge of a hydrogen in one water molecule and the partial negative charge of oxygen in another water molecule. You can represent this bond with a dotted line connecting the hydrogen of one molecule to the oxygen of another, illustrating the attraction between them.

(c) Interaction with Water Striders

These molecular interactions are vital for the water strider, a type of insect capable of walking on water. The hydrogen bonds create a cohesive force at the water's surface, leading to surface tension. This surface tension enables the water strider to distribute its weight over a larger surface area, preventing it from sinking. Thus, the hydrogen bonds allow the strider to remain suspended on the water without breaking through the surface layer.

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