Structure of water and hydrogen bonding

What the exam asks

• Multiple Choice (Section I): Expect visual representations of water molecules or biological macromolecules where you must identify correct orientations of hydrogen bonds, or experimental sets comparing water to nonpolar or less polar liquids (like oil or alcohol) in drop-count, capillary tube, or heating rate assays.
• Free Response (Section II): Often appears as a conceptual part in Q1/Q2 (experimental results on plant transpiration or thermal tolerance) or Q5 (Analyze a Model, asking you to draw or identify intermolecular interactions). When asked to explain a property of water, you must connect three levels: molecular polarity $\rightarrow$ hydrogen bonding $\rightarrow$ macroscopic biological function.

Core ideas

Polar covalent bonds and molecular polarity

A single water molecule ($\text{H}_2\text{O}$) consists of two hydrogen atoms joined to one oxygen atom by polar covalent bonds (intramolecular bonds).

• Oxygen has a much higher electronegativity than hydrogen, so it pulls the shared electrons closer to its nucleus.
• This unequal sharing of electrons, together with the molecule’s bent (V) shape, creates an asymmetrical distribution of charge (a dipole): the oxygen region carries a partial negative charge ($\delta^-$), and each hydrogen atom carries a partial positive charge ($\delta^+$).

Hydrogen bonding

Because water molecules are polar, they attract one another and other polar or charged molecules.

• A hydrogen bond is a weak, transient electrostatic attraction (an intermolecular attraction) between the partially positive hydrogen ($\delta^+$) of one molecule and the partially negative atom ($\delta^-$, typically $\text{O}$ or $\text{N}$) of another molecule or a different region of the same macromolecule.

Emergent properties of water

The collective strength of millions of hydrogen bonds gives water emergent properties critical to living systems:

Worked examples

Exam technique

• Link structure to function on FRQs: Naming a buzzword like “cohesion” or “high specific heat” rarely earns the point on its own when the prompt says explain or justify. Always state what the hydrogen bonds are doing (e.g., “absorbing thermal energy to break hydrogen bonds before kinetic energy increases”).
• Distinguish bond types: When a question asks what happens when water boils or evaporates, state that intermolecular hydrogen bonds break. The intramolecular polar covalent bonds within $\text{H}_2\text{O}$ do not break during phase changes.

Common mistakes

Quick recap

• Unequal electron sharing in $\text{O–H}$ polar covalent bonds makes water a polar molecule with $\text{O}^{\delta-}$ and $\text{H}^{\delta+}$ regions.
• Opposite partial charges between adjacent polar molecules form weak intermolecular hydrogen bonds.
• Cohesion (water-to-water H-bonding) and (water-to-polar-surface H-bonding) support upward water transport through plant xylem, and cohesion at the water’s surface creates .