Water & Its Properties

You’ll be able to

Explain how polarity and hydrogen bonding arise from water’s structure
Connect hydrogen bonding to cohesion, adhesion, and thermal stability
Predict how water behaves as the universal biological solvent

A bent, lopsided molecule

Water is just H₂O, but its shape is everything. Oxygen holds the shared electrons much more tightly than hydrogen does, and the molecule is bent (about 104.5°) rather than straight. The result is a polar molecule: the oxygen end carries a partial negative charge (δ⁻) and each hydrogen end a partial positive charge (δ⁺). One molecule with a plus side and a minus side is the seed of nearly every property that follows.

Hydrogen bonds: weak alone, mighty together

Because water is polar, the δ⁺ hydrogen of one molecule is attracted to the δ⁻ oxygen of a neighbor. This attraction is a hydrogen bond. Any single hydrogen bond is weak — roughly a twentieth the strength of the covalent bond inside the molecule — but each water molecule can hydrogen-bond to up to four neighbors at once. Billions of these fleeting bonds, constantly breaking and reforming, give liquid water its remarkable behavior.

Hydrogen bond

δ⁺H···O δ⁻

The dotted line is the hydrogen bond (an intermolecular attraction), not the covalent O–H bond within a molecule.

Four life-supporting consequences

Hydrogen bonding explains water’s signature traits. Cohesion — water sticks to itself, pulling columns of sap up a tree and creating surface tension. Adhesion — water sticks to other polar surfaces, helping it climb narrow tubes (capillary action). High specific heat — heat first goes into breaking hydrogen bonds rather than speeding molecules up, so water resists temperature change and buffers organisms against swings. Evaporative cooling — the fastest, hottest molecules escape as vapor, carrying heat away and cooling the surface (how sweat works).

Worked example

Water climbs a thin glass tube on its own, rising against gravity. Which properties explain this, and how?

1.Adhesion: polar water molecules hydrogen-bond to the polar –OH groups on the glass, so water is pulled upward along the wall.
2.Cohesion: water molecules hydrogen-bond to each other, so as the edge climbs it drags the molecules behind it along too.
3.Together these produce capillary action — the water column rises until the upward pull is balanced by the weight of the water lifted.

Answer: Capillary action: adhesion pulls water up the glass and cohesion drags the rest of the column along — both are consequences of hydrogen bonding.

Checkpoint

On a hot day, sweating cools you down because water evaporating from your skin carries away heat. Which property of water is responsible?

Watch out

Do not confuse cohesion with adhesion. Cohesion is water sticking to water (like to like); adhesion is water sticking to a different surface. A meniscus and capillary action need both at once.

The universal solvent

Because water is polar, it surrounds charged and polar solutes: the δ⁻ oxygens face cations and the δ⁺ hydrogens face anions, forming a hydration shell that pulls the solute apart and keeps it dissolved. Substances that dissolve in water are hydrophilic ("water-loving"); nonpolar substances like oils cannot form these attractions and are hydrophobic ("water-fearing"). This single distinction shapes how membranes form and how molecules move through cells.

Checkpoint

Table salt (NaCl) dissolves readily in water, but vegetable oil does not. Why?

On the exam

On the AP exam, almost every water question traces back to one root cause: polarity → hydrogen bonding. When asked to explain a property, name that chain explicitly rather than just stating the property — the mechanism earns the point.

Answer the 2 checkpoints as you read.