What is hydrogen bond in water

 From a chemical point of view, water is an amazing substance. If it behaved like its closest chemical relatives, it would have to boil and evaporate at minus 80 degrees. Under these conditions, life as we know it could not have developed.

The reason for the high boiling point of water is the so-called hydrogen bond. It ensures that the water molecules stick together more closely than usual and thus escape from the liquid and pass into the gas phase less easily. The hydrogen bond is a special way of holding molecules together. They are not bound as tightly as with a normal chemical bond, but the cohesion is much stronger than, for example, with methane, which already boils at - 161 ° C.

The hydrogen bond is formed between the oxygen atom of one water molecule and two hydrogen atoms of two foreign water molecules. Thus, every water molecule can enter into four hydrogen bonds: with its hydrogen atoms it binds to two other oxygen atoms, with its own oxygen atom it binds to two foreign hydrogen atoms.


The hydrogen bonds are in this illustration
of liquid water can be recognized by the dashed lines.

The hydrogen bonds arise because the oxygen atom in the water molecule is partially negatively charged, the hydrogen atoms partially positively charged. Since opposing charges attract each other, the phenomenon of hydrogen bonding as described occurs. The angled, tetrahedral structure of the water molecule enables the formation of charge centers. The oxygen atom sits in the middle of the tetrahedron. On the four sides of the tetrahedron sit the two hydrogen atoms and two so-called non-bonding electron pairs of oxygen.


The water molecule forms a tetrahedron. In the middle the oxygen atom,
at the four corners the two hydrogen atoms and the two non-bonding electron pairs.

In order to break the hydrogen bonds - i.e. the bond between the individual water molecules - energy is required. Therefore the boiling point of the water is 100 ° C and not much lower.

The water molecules are only actually present individually as a gas - in the liquid and solid state we always have to deal with large groups of molecules that are connected by hydrogen bridges.


Only in gas form are the water molecules really unbound.