Water: a special substance

You shower with it, you drink it, and more than half of your body is made of it: water is with you all day long. Yet H₂O is anything but an ordinary substance. It dissolves a surprising number of substances, constantly changes its state, and even breaks a rule that almost all other substances follow.

The most important solvent

Water is the most important solvent of all. Sugar in tea, salt in soup, minerals in drinking water: countless substances dissolve in water. That is exactly why your blood, which is mostly water, transports nutrients through your body. Plants, too, only take up their nutrients dissolved in water.

Three states, one cycle

You know water in all three states of matter: as ice in the freezer, as liquid water from the tap, and as water vapour above a cooking pot. At normal air pressure ice melts at 0 °C and water boils at 100 °C. High up on a mountain water boils at less than 100 °C, because the air pressure is lower there. In nature these transitions happen all the time: the sun makes water evaporate, high up it condenses into clouds, falls back as rain or snow, and flows into the sea again. That is the water cycle.

The anomaly: ice floats

Almost all substances are denser as a solid than as a liquid. Not water: when it freezes, the particles arrange into a loose lattice with lots of space in between. A litre of ice therefore weighs only about 920 g while a litre of water weighs 1000 g, and so the ice floats on top. Water is densest at 4 °C. That is why lakes freeze from the top down while the 4 °C water collects at the bottom. For fish this is vital: under the sheet of ice, liquid water remains where they can survive the winter.