The unique physical and chemical behaviour of water is due to its hydrogen bonds. These bonds account for the extremely high boiling point of water in comparison to other liquids. The hydrogen sulfide molecule (), for example, is more massive than the water molecule, has nearly the same geometry, and sulfide is in the same group as oxygen in the table of elements. However hydrogen sulfide is a gas at room temperature and is not capable of making hydrogen bonds. If water were a gas at room temperature, life as we know it would be impossible. The fact that water has its density maximum not in the solid but in the liquid phase (at Celsius) results from the fact that the network is not static; rather it is a fluctuating network of flickering hydrogen bonds. If this were not so -- if ice sank rather than floated, -- many of the earth's oceans would be permanently frozen solid.
The hydrogen bond is not a normal bond; it is a shared electron bond (see also section 3.1 on Bonds). It can be regarded as a weak covalent bond, meaning that the energy needed to break it is low - about (compared with to break one H--O bond in H--O--H). This is the reason why thermal agitation is sufficient to cause hydrogen bonds to ``flicker'' on and off rapidly in liquid water.
In this section we explore the influence that heating and cooling have on the dynamics and orientation of the hydrogen bond network.
Start with the NVT ensemble. Set the temperature to 273 K and fix the density at . Slowly raise the temperature in increments of 10 K and view the changes that occur. Pay attention to the ``flickering'' of the hydrogen bonds and notice the increase in speed at which the water molecules rotate and move about.
on the hydrogen bond network?
on the dynamics of the system?
on the orientation of the molecules?