1.1: STRUCTURE OF WATER AND HYDROGEN BONDING

Polarity and Hydrogen Bonding in Water

Water is the most abundant molecule in living systems, and its unique properties arise from its polarity. Oxygen is more electronegative than hydrogen, so the shared electrons in each O--H bond spend more time near the oxygen atom. This creates a polar covalent bond [a covalent bond in which electrons are shared unequally], giving the oxygen end a partial negative charge (δ⁻) and each hydrogen end a partial positive charge (δ⁺).

Because of this charge separation, the partially positive hydrogen of one water molecule is attracted to the partially negative oxygen of a neighboring water molecule. This attraction is called a hydrogen bond. Although a single hydrogen bond is relatively weak compared to a covalent bond, water molecules collectively form vast networks of hydrogen bonds. These networks give water its remarkable physical properties — properties that living systems depend on to sustain life. Hydrogen bonding also occurs between and within biological molecules such as proteins and nucleic acids, meaning water's polarity has consequences far beyond the water molecule itself.

Exam TipOn the AP exam, always use dashed lines (not solid) to represent hydrogen bonds, distinguishing them from covalent bonds.

MisconceptionStudents sometimes think hydrogen bonds form between the two hydrogen atoms of different water molecules. In fact, a hydrogen bond forms between a hydrogen of one molecule and the oxygen of another. The attraction is between opposite partial charges.

Exam TipWhen asked to draw or identify hydrogen bonds, always connect H (δ⁺) to O (δ⁻) on the adjacent molecule.

Thermal Properties of Water

Water has an unusually high specific heat capacity [the amount of energy required to raise the temperature of 1 gram of a substance by 1°C]. Because hydrogen bonds between water molecules absorb a large amount of heat energy before they break, water resists rapid temperature changes. For living organisms, this property is critical for the maintenance of homeostatic body temperature — organisms composed largely of water can buffer themselves against sudden environmental temperature shifts.

Water also has a high heat of vaporization [the amount of energy required to convert a liquid to a gas]. Breaking the extensive hydrogen bond network so that water molecules can escape into the gas phase requires considerable energy input. When water evaporates from a surface, it carries that energy away, producing evaporative cooling. In living organisms, this mechanism helps maintain body temperature. Sweating in mammals and transpiration in plants are both examples of evaporative cooling driven by water's high heat of vaporization.

Both properties — high specific heat capacity and high heat of vaporization — trace back to the same root cause: the extensive hydrogen bonding among water molecules. The exam frequently asks students to connect a water property back to hydrogen bonding as the underlying explanation.

Examiner InsightAP free-response questions often present a scenario (e.g., an organism in a hot environment) and ask you to connect a water property to a biological outcome. Always follow this chain: hydrogen bonding → specific property (e.g., high heat of vaporization) → biological function (e.g., evaporative cooling maintains body temperature).

Exam TipUse the phrase "due to hydrogen bonding between water molecules" as the mechanistic root of every water-property explanation.

Cohesion, Adhesion, and Surface Tension

The hydrogen bonds between adjacent polar water molecules produce three interrelated physical properties: cohesion, adhesion, and surface tension.

Cohesion is the attraction of water molecules to other water molecules. Because each water molecule can form hydrogen bonds with its neighbors, water "sticks to itself." This property is essential in plants, where a continuous column of water is pulled upward through xylem vessels from roots to leaves during transpiration. If the column broke, water delivery to leaves would fail. Cohesion keeps the column intact.

Adhesion is the attraction of water molecules to other polar surfaces. Water molecules form hydrogen bonds with the polar molecules lining the walls of narrow xylem vessels or glass tubes, allowing water to "climb" surfaces. Adhesion works together with cohesion to move water against gravity in plants.

Surface tension results from the net inward pull on water molecules at the air-water interface. Molecules at the surface have no water neighbors above them, so they hydrogen-bond more strongly with their lateral and lower neighbors, creating a "film" that resists disruption. Surface tension allows small organisms, such as water striders, to walk on water without breaking through.

MisconceptionStudents often confuse cohesion and adhesion. Cohesion is water-to-water attraction; adhesion is water-to-another-surface attraction. Both involve hydrogen bonding, but they differ in what water is bonding to.

Exam TipIf asked about water transport in plants, mention both cohesion (holds the water column together) and adhesion (pulls water along vessel walls).