|University Federico II
of Naples, Italy
Have you observed that ice floats on liquid water? A very strange event, since the contrary happens for almost all other substances (commonly, the density of the solid is higher than that of the corresponding liquid). Ice is characterized by a very rigid three-dimensional structure, determined by the presence of particular interactions, called hydrogen bonds. The structure of ice is a "void" structure, meaning that there are many spaces. These spaces are lost when ice transforms in liquid water, where molecules are closer.
Structure of ice
Ice presents many void spaces, then it has a lower density as respect to liquid water, on which it floats. Many of the hydrogen bonds present in the ice are maintained in liquid water: that determines the peculiarity of many of its properties. Liquid water attains its highest density about 4 degrees (at 3.98 °C) above its melting temperature, i.e. 0 °C. When a mass of water cools below 3.98 °C, the density decreases and allows water to rise to the surface, where freezing occurs. The layer of ice formed on the surface does not sink and it acts as a thermal insulator, thus protecting the biological environment beneath it. That is why the hunter can catch fishes - alive! - under layers of ice, even at the Northern Polar Circle.