What is the reason for the anomalous behaviour of water?  

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The anomalous behavior of water, sometimes called the density anomaly, is due to strong intermolecular attractions between water molecules called hydrogen bonds. The large electronegativity difference between oxygen and hydrogen causes the hydrogen-oxygen bonds to be polar. The oxygen ends of water molecules are slightly negative and the hydrogen ends are slightly positive. The positive and negative ends are attracted to oppositely charged ends of other water molecules.

Here are some of the behaviors caused by hydrogen bonding:

High surface tension: The hydrogen bonds between molecules at the surface of a container of water cause the surface to behave like an invisible skin. Water will mound up when a container is overfilled before it spills over. Water skippers (insects) are able to walk on water because of the high surface tension.

Expands when freezing: Most substances are more dense in the solid state than in the liquid state. Water shows the opposite behavior. As water approaches its freezing point and the molecules get close together, hydrogen bonding causes a hexagonal crystal structure that has a lot of open space. The fact that ice floats on water is what keeps lakes from completely freezing and aquatic life dying off in the winter.

Higher than expected boiling point: Boiling point is a periodic trend. Water is an exception to the trend. In the diagram below you can see that the boiling point of analogous compounds increases with atomic number within a group of the periodic table. For example, CH4 has a lower boiling point than SiH4 and the tetrahydride boiling points continue to increase going down the group. However, water has a higher boiling point than either H2S, H2Se or H2Te. It takes more energy to overcome the hydrogen bonds and separate the molecules.


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