First of all let us see why is it called anomalous ( meaning unusual ) expansion of water. Raising the temperature of liquid or gas increases the average separation between the molecules because they start vibrating more vigorously, leading to decrease in the density. The same should happen with water , heating it to 373 K (100°Celsius) increases the average separation up to a point where it becomes a gas and lowering it to 273 K (0° Celsius) decreases the average separation and becomes ice. So according to that logic ice should have the highest density, right?

But it is observed that as we cool water, density goes on increasing till 4°C and then starts decreasing again. Weird, right? In-fact this is the reason why ice floats on water. At about 4°C water reaches its maximum density.

A water molecule consists of 2 Hydrogen and 1 Oxygen atom. As we cool the water, the inter-molecular distance decreases and molecules come closer thus increasing the density. At about 4° C the water molecules come close enough that Hydrogen bonding takes place between Hydrogen atom of one molecule and Oxygen atom of other molecule. This type of bonding is not as strong as covalent bonds. Now the H₂O molecule has a specific structure. Due to Hydrogen bonding the atoms begin to arrange themselves in a lattice like structure and at 0°C the state changes to solid when water crystallizes. The water expands after 4°C because the oxygen atoms repel other oxygen atoms of water molecules and hydrogen bonds maintain the lattice structure.

The positive and negative signs in the above figure denote the effective charge on atoms in water molecules. Water molecule is a polar molecule. It means that there in uneven distribution of electron density in the atoms. In one water molecule, the probability of finding the electron of hydrogen atom is more near the oxygen atom due to the bond between them. So effectively hydrogen atom has effective positive polarity and oxygen atom has negative polarity. An electrostatic attraction between the partial positive charge near the hydrogen atoms and the partial negative charge near the oxygen results in the formation of a hydrogen bond between 2 water molecules.

Effects of anomalous expansion

This very strange but interesting property of water was actually very significant in the survival of aquatic life.

As the water has highest density at 4°C, in very frigid and cool atmospheres the lakes used to have liquid water at bottom. Only the top layer of lake would turn into ice, further insulating the water below ice sheet. This helped aquatic life survive in the depths. Same with ocean caps and icebergs. The density of ice is 9% less than water at 4°C so it floats. So this anomalous expansion of water in-fact helps the aquatic animals to live. Even under icebergs or we can think back to the ice-age where ocean layers froze but deep layers were water due to this property of water.

Other such effect is bursting of water pipelines in really cold regions. Due to extreme cold weather, the water freezes inside pipes, expands and due to this immense pressure the pipes burst. You can observe this really easily by filling a plastic water bottle upto the brim and keeping it in the freezer. The bottle expands or in some cases bursts. (If you are really adventurous try freezing a glass bottle full of water. Cleaning is not fun.)