Why does water expand when it freezes?

From burst bottles in the freezer to frost damage on the street: When water freezes, it takes up more space than when it is liquid. It is different with most liquids and gases, they contract when they cool down. Why does water of all things behave so unusually?

Most fabrics expand when heated and contract when cooled. This effect is particularly pronounced with gases, but it also applies to liquids and solids. A good example is air in a balloon: in cold weather it shrinks when the window is open, and it can even burst near a heater that is turned up high.

Molecules need space

The reason for this lies in the molecules: the warmer a body or a gas becomes, the more energy you add, the faster the molecules move in it. As a result, they collide much more frequently and more strongly and require more space – the pressure exerted by gas molecules on the envelope of a balloon increases. To maintain the same pressure, a larger volume is required and the material expands.

However, water behaves differently: if you cool it down, its volume decreases up to a temperature of around four degrees Celsius. That is still exactly as one would expect. However, below this temperature the water expands again. So its density is highest at around four degrees. This property is known as the density anomaly of the water.

But where does this anomaly come from? Again, the molecules are responsible: A water molecule consists of two hydrogen atoms and one oxygen atom – hence the chemical formula H2O. However, these atoms attract the electrons in the water molecule to different degrees. This creates a slightly positive center of gravity on hydrogen and a negative one on oxygen. When water molecules meet, the hydrogen atoms of one are attracted to the oxygen atoms of the other and align themselves there – so-called hydrogen bridges are formed.

Bridges in the water need even more space

Due to these bridges, there are still areas in liquid water in which the molecules are regularly aligned with one another, similar to those in ice crystals. These so-called clusters are not as rigid as in a solid crystal: they change very quickly, especially at higher temperatures. However, when the water cools down, more and more clusters are formed and they last longer. However, they also need more and more space: below around four degrees Celsius, the water expands again. Below zero, the crystal precursors take over and the water freezes.

This peculiarity of water is important for many processes in nature, even essential for survival: Because ice has a lower density than cold water, it floats on top. As a result, bodies of water freeze over from top to bottom, and the thickest water with a temperature of four degrees collects at the bottom. Fish and other water dwellers in the garden pond, for example, have a residual liquid water with tolerable temperatures at the bottom of the pond even in winter, and they do not freeze.

17.01.2018 – AKR


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