You are viewing text created during the EU sponsored project "Kids & Science". More information on this project can be found on the project Web site:


The Anomaly of Water

Water has a number of exceptional properties, which distinguish it from other fluids. One is the anomaly of density.

The Behaviour of Water at Different Temperatures

Usually, substances expand when they are heated. But if you heat water starting at 0°C, you will notice something weird: At first, the volume of the liquid does not get bigger, but smaller! Up to a temperature of 4°C, the water contracts when heated. Once it has reached that temperature, it starts to expand with increasing temperature just like other fluids. That means water has a higher density at 4°C than at any other temperature.

Why does ice need more space than cold water?

When cooling down below 4°C, water expands. When it freezes at 0°C, the volume increases once again. This unusual behaviour has something to do with the structure of solid and liquid water: In the liquid, the molecules can move relatively freely, whereas in ice, they assemble into regular structures with big hollow spaces in between, which require more space.

Since ice has a lower density than water, it floats on the water. You know this from ice cubes in a drink or from icebergs floating in the sea.

The Anomaly Determines Life on Earth!

Thanks to the anomaly, fish can survive in stretches of standing water in winter! Since water with a temperature of 4°C is heavier than colder water, it sinks downwards. Therefore, it is warmer on the bottom of the lake than on the surface. On top it is the coldest. As a result, the lake starts to freeze from the top to the bottom, and the warmer layers of water beneath the sheet of ice cool down considerably more slowly and remain liquid. There, the aquatic animals can spend the winter. If the water froze from the bottom to the top, soon the whole water would have frozen and the fish would have no chance to survive!

The anomaly also has its drawbacks, though:

If water is enclosed in a closed vessel, it still expands while freezing and may develop such forces that it bursts the vessel. Water enclosed in rock cracks is even able to break up the rock, and therefore plays an important role in the erosion process. In the same way, frost damage is caused on the streets when water seeps into the cracks of the road surface and freezes. Water must also be prevented, from freezing in water pipes during winter, since otherwise the pipes burst.