Melting

A snapshot of a drop of water at 25°C based on molecular dynamics simulation. There are 432 molecules in the cluster, taken from the center of a box containing 2,000 molecules at thermodynamic equilibrium.

It's important to remember that the model is static. As mentioned above, water is less ordered in the liquid state due to chaotic thermal motion of the molecules.

Melting of ice is a cataclysmic event. At 0.0000°C ice and water co-exist but at 0.0001°C the ice completely disappears. As the ice melts, the average hydrogen bonds length increases slightly. In the reverse direction no cataclysmic event occurs. Liquid water can exist without any ice at temperatures well below 0°C. The coldest temperature at which supercooled water has been shown to exist is -40°C. This reflects the difficulty of disrupting the hydrogen bonds network in the flickering clusters during the liquid-to-ice transition at low temperatures.

There is still considerable hydrogen bonding in water at 100°C. Since all the hydrogen bonds to a water molecule must be broken for it to escape into the gas phase, a great deal of energy is required to convert water to steam.