Why Water Is Strange
- Chemistry
- Ages 14–18 · Grades 9–12
- Hands-on experiment
What you’ll find out
- Water is polar, and hydrogen bonds between its molecules are far stronger than the attractions between molecules of similar size.
- Boiling separates whole molecules from each other; it does not break them apart.
- Hydrogen bonding explains surface tension, the high boiling point and heat capacity, and why ice floats.
No sign-up and no real names. In the lesson you check your understanding as you read, run the experiment yourself, and practise until it sticks.
A small molecule that holds on
By size alone, water should be a gas. Methane molecules are about the same size, and methane boils at about −162 °C. Water does not boil until 100 °C. The difference is how strongly the molecules hold on to each other. In water, the oxygen atom pulls the shared electrons towards itself, so the oxygen end is slightly negative and the hydrogen ends slightly positive. The molecule is polar. The positive hydrogen of one molecule is pulled towards the negative oxygen of the next: a hydrogen bond. It is much weaker than the bonds inside a molecule, but there are huge numbers of them. Boiling does not split water into hydrogen and oxygen. It pulls whole molecules away from their neighbours, and that takes a lot of energy. The same pull gives water surface tension. A molecule at the surface is pulled sideways and inward, so the surface acts like a stretched skin. That is why dozens of drops pile into a dome on a coin, and why detergent, which weakens the pull, makes the dome collapse. Rubbing alcohol, whose molecules attract each other less, holds only a few drops.
Ice floats, and why that matters
Almost every substance shrinks when it freezes, so its solid sinks in its own liquid. Water does the opposite, and hydrogen bonds are the reason. As water freezes, the hydrogen bonds lock the molecules into a regular, open, honeycomb-like pattern with more space between them than in the liquid. Ice takes up about nine per cent more space than the water it came from, so it is less dense and floats. Liquid water is densest at about 4 °C. This has big effects. When a lake cools in winter, the coldest water rises to the top and freezes there, and the ice then protects the water below. Lakes freeze from the top down, so fish survive under the ice. Freezing also pushes: water that freezes in a crack widens it, which splits rocks, breaks up roads and can burst pipes. Hydrogen bonds also give water a high heat capacity. It takes a lot of energy to warm water, and it releases that energy slowly, which is why coastal places have milder climates than places inland.
Curriculum links: NGSS HS-PS1-3