Anomalous Expansion of Water

Anomalous Expansion of Water

Ice floats on water, it is a simple fact just like sun rises in the east. A general notion is that gases have lot of empty spaces in between their particles, in liquids particles are crammed together and there aren’t any empty spaces and so is the case with the particles in solid materials. So when a material is cooled down from gaseous to liquid state and further to solid state, the number of particles in a given volume should increase, as more particles are crammed together at lower temperatures, resulting in an increase in the density. This information tells us that the density of water should increase as we go from water vapor, water to ice. From the laws of buoyancy, we know that the objects with high density do not float in a liquid of lower density. Hence ice should not float in water. But why does Ice float in water and what are its repercussions?

Floating of ice on water is a result of the phenomenon called as anomalous expansion of water. The density of water increases as the temperature is reduced till 4oC and cooling beyond this would result in the decrease of density. And as temperature reaches the freezing point of 0oC, the density of water ice decreases and becomes lower than that of the density of liquid water. Hence ice floats on water. The question now is what changes at the molecular level are the causative of this phenomenon.

Two hydrogen atoms and an oxygen atom are bound together by covalent bonds to form water molecule. The molecules of water are in turn bound by hydrogen bonds. A bunch of water molecules, when come together form a cage like structure. When the temperature is reduced, water molecules that are adjacent to each other form a cage like structure increasing the empty spaces between them. This is similar to how when a house of cards is made of card stack, the space occupied by the cards increases. Now when the temperature is increased the cage structure is broken and the empty spaces created during the formation of cages are occupied by the water molecules, thus the number of water molecules present in a given volume is less at lower temperatures. The cage like structure formed as a result of hydrogen bonding is the cause of anomalous expansion of water.

Anomalous expansion of water has vital repercussions for aquatic life. When the temperature dips down to below zero levels, the surface of lakes is covered by ice. Ice formed on the surface acts like a thermal insulator and protects the aquatic forms from the extreme low temperatures. As mentioned during our Physics tuition classes, the bottom of the lakes and rivers are therefore still maintained at about 40C during the winter seasons.

Let’s discuss a simple experiment to observe the anomalous expansion of water, fill a water bottle, with a concave bottom, with water to the brim and leave it a deep refrigerator for long time. One could observe the concave bottom bloats out into a convex shape as a result of the expansion due to the formation of ice.