Water questions are hydrogen-bond questions. Ask what forms or breaks between molecules; the O-H bonds inside stay intact.
Oxygen hogs the shared electrons in water, so each molecule has slightly charged ends. Opposite ends of neighboring molecules attract. Those attractions are hydrogen bonds, and they explain why water sticks together, resists temperature change, and freezes into ice that floats.
Key terms
- Polar covalent bond — electrons are shared, but unequally. In water, they spend more time near oxygen than near hydrogen.
- Hydrogen bond — a weak attraction between a partly positive hydrogen, bonded to O or N, and a partly negative O or N nearby.
- Cohesion — water molecules hydrogen bonding to each other. It creates surface tension and keeps a column of water from breaking.
- Adhesion — water hydrogen bonding to a different polar surface, such as the cellulose wall of a xylem vessel.
- Specific heat — the energy to warm 1 gram by 1 °C. Water's is high: part of the heat goes into breaking hydrogen bonds.
How it works
- Oxygen is more electronegative than hydrogen. It pulls the shared electrons closer, leaving oxygen partly negative and each hydrogen partly positive.
- Water is bent, not straight, so those charges do not cancel out. The molecule as a whole is polar.
- The partly positive hydrogen of one molecule is drawn to the partly negative oxygen of a neighbor. Each molecule can form up to four hydrogen bonds.
- In liquid water, these bonds break and re-form constantly, so molecules slide past each other and pack closely.
- In ice, each molecule locks onto four neighbors at fixed angles. That open lattice takes about 9% more space, so ice floats.
- Heat added to water goes partly into breaking hydrogen bonds, not only into speeding molecules up. So water warms and cools slowly.
Why it matters
Water questions arrive as data or scenarios: boiling points, temperature rise, a frozen lake, a tall tree. Each one asks you to trace the result back to hydrogen bonds between molecules. Hydrogen bonds also hold DNA's two strands together and help proteins fold. Polarity decides what dissolves: charged and polar substances mix with water, while nonpolar ones, like oils, do not.
Exam traps
| Trap | Actually |
|---|
| A hydrogen bond links two hydrogen atoms. | It links a partly positive hydrogen on one molecule to a partly negative oxygen or nitrogen on another. |
| Any molecule with an oxygen can hydrogen bond to others like it. | Oxygen alone is not enough. The molecule also needs a hydrogen bonded to O or N, like an O-H group. |
| Freezing breaks water's hydrogen bonds. | Freezing locks them in place. Each molecule holds four neighbors in an open lattice, so ice is less dense and floats. |
| Boiling breaks the bonds inside each water molecule. | Boiling only pulls molecules apart. The hydrogen bonds between them break; the covalent O-H bonds inside stay intact. |
Worked example
A student stands three narrow tubes upright in a dish of water. Water climbs 30 mm up a clean glass tube 1 mm wide and 60 mm up a clean glass tube 0.5 mm wide. In a 1 mm glass tube coated inside with wax, the water ends up several millimeters below the level in the dish. What best explains the result in the waxed tube?
- Name what lifts water in a thin tube. Adhesion pulls water up the wall; cohesion drags the rest of the column along behind it.
- Find what changed. The water is the same in every tube, so cohesion is the same. Only the wall differs: polar glass versus nonpolar wax.
- Water can hydrogen bond to glass but not to wax. Without that grip on the wall, nothing lifts the column.
- Check the other result. A narrower tube gives each drop of water more wall to cling to, so it climbs higher. That fits.
Wax is nonpolar, so water cannot hydrogen bond to it. With almost no adhesion at the wall, the column does not climb.
Capillary rise needs both cohesion and adhesion, and only adhesion depends on the wall. The tempting pick is 'wax repels water.' Wax does not push water away. Water molecules attract each other much more than they attract wax, so cohesion wins at the wall, the surface curves down, and the level sits low.