Water is so common that it seems plain. It is not. Almost every special thing about water comes from the hydrogen bonds you met in The Elements and Bonds of Life.
Key idea
Water is polar, so its molecules stick to each other by hydrogen bonds. That gives water cohesion, adhesion, surface tension, a high specific heat, a high heat of vaporization, great power as a solvent, and ice that floats.
| Property | What hydrogen bonds do | Where life uses it |
|---|---|---|
| Cohesion and surface tension | Water molecules cling to each other | A water strider walks on a pond; a column of water holds together as it is pulled up a tree |
| Adhesion | Water clings to other polar surfaces | Water creeps up the narrow tubes (xylem) of a plant |
| High specific heat | Energy goes into breaking hydrogen bonds before molecules speed up | Lakes, oceans and bodies change temperature slowly |
| High heat of vaporization | Escaping molecules carry a lot of energy away | Sweating cools the skin |
| Solvent | δ+ and δ− ends surround ions and polar molecules | Sugars, salts and many proteins dissolve in blood and cytoplasm |
| Ice floats | In ice, hydrogen bonds lock molecules into an open lattice, about 9% larger in volume | Lakes freeze from the top down, and life survives under the ice |
Why it works
Specific heat is the energy needed to warm 1 gram by 1 °C. For water it is about 4.18 J (1 calorie); for dry sand it is about a fifth of that. When you heat water, much of the energy breaks hydrogen bonds instead of making molecules move faster, so the temperature rises slowly. The heat needed is $q = m \times c \times \Delta T$.
Worked example: Warming a pond sample
How much energy warms 200 g of water from 15 °C to 25 °C? (c = 4.18 J/(g·°C))
- The change in temperature: $\Delta T = 25 - 15 = 10$ °C.
- Multiply: $q = 200 \times 4.18 \times 10 = 8360$ J.
Answer: $8360$ J (about 8.4 kJ)
Worked example: Match the property
Why does sweating cool you more than the same amount of a liquid that evaporates easily, like rubbing alcohol on a table?
- To evaporate, a water molecule must break all its hydrogen bonds.
- That takes a lot of energy, which comes from your skin.
- This is water's high heat of vaporization.
Answer: Water's high heat of vaporization
Common mistake
Thinking that boiling breaks water molecules apart into hydrogen and oxygen. Steam is still H₂O. Boiling breaks the weak hydrogen bonds between molecules so they can fly off as a gas; the strong covalent bonds inside each molecule stay whole. Splitting water into hydrogen and oxygen gas takes a chemical reaction, such as passing electricity through it.