1.1 Structure of Water & Hydrogen Bonding

Water is a polar molecule

In a water molecule (H₂O), oxygen pulls on the shared electrons more strongly than hydrogen does — oxygen is more electronegative. This unequal sharing makes each O–H bond a polar covalent bond: the oxygen end carries a partial negative charge (δ−) and each hydrogen end a partial positive charge (δ+).

Hydrogen bonds link water molecules

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, but there are so many that together they give water its remarkable properties.

Why it matters

Hydrogen bonding explains cohesion (water sticking to itself → surface tension), adhesion (water sticking to other surfaces → capillary action), a high specific heat (water resists temperature change), evaporative cooling, and why ice floats — solid water forms a spacious lattice that is less dense than liquid water.

Key terms

Polar covalent bond

A bond in which electrons are shared unequally, creating partial positive and negative charges.

Hydrogen bond

A weak attraction between a δ+ hydrogen of one molecule and a δ− atom (like oxygen) of another.

Cohesion

Attraction between molecules of the same substance (water to water) — creates surface tension.

Adhesion

Attraction between water and a different substance — enables capillary action.

Specific heat

The energy needed to raise 1 g of a substance by 1 °C. Water's is unusually high, so it resists temperature change.

Try an AP-style question

Which property of water best explains why it rises in the glass tube?

1. Water's lower density compared with mercury
2. Adhesion of polar water molecules to the charged glass surface
3. Water's high specific heat keeping the column warm
4. Cohesion between water molecules alone pulling water upward