Anomalous Expansion

To understand what Anomalous expansion is, we need to first learn what Thermal expansion is.

What is Thermal Expansion?

Thermal expansion, which does not include phase transitions, is the tendency of matter to change its area, shape, density, and volume when there is a change in temperature. Usually increasing the temperature causes the substance to increase in the previously mentioned physical characteristics. It should also be kept in mind that some substances can contract with increasing temperature and this is an unusual property which is not very commonplace. It also only occurs within limited and defined temperature ranges. When substances are heated, its molecules begin to vibrate and move more, resulting in a greater distance between them. The coefficient of linear thermal expansion of a material is defined as the relative expansion divided by the change in temperature, and it fluctuates with temperature. Particles move faster as their energy increases, reducing the intermolecular forces between them and thereby expanding the substance.

What is Anomalous Expansion?

As mentioned in the paragraph above, a substance usually expands with an increase in temperature and contracts when the temperature decreases. For a few substances, something interesting happens when you reduce its temperature. They expand! Yes. This is contradictory to what was explained above. You might think these substances are rare. Well, you would be surprised to find out that Water, is one of these substances. This is what anomalous expansion is. Anomalous means out of the ordinary, deviating from the normal. You will agree with me that substances expand when the temperature increases. Thus for a substance to expand when the temperature decreases, it deviates from the normal. Let us take water as an example and talk more about the anomalous expansion of water.

Anomalous Expansion of water:

The anomalous expansion of water is a property of water that causes it to expand rather than contract as the temperature goes from 4 degree Celsius to 0 degree Celsius, and it gets less dense.The density of water is highest at 4 degrees Celsius and declines below that temperature. Because water molecules generally form open crystal formations when solidified, the density decreases as it freezes.

What this means is that when temperatures reduce, for example, in the oceans, since water is heaviest at 4 degree Celsius, it settles below the layer of ice that forms above. First a layer of ice is formed which floats on top and prevents the water below from reducing in temperature and freezing completely all the way to the bottom. This property of water is why oceans don’t freeze over. 

You might now be wondering why a bottle of water kept in a freezer, freezes completely. Well the answer to that is simple. A freezer blasts cold air from all directions in a freezer and the temperature of a freezer is generally negative 20 degree Celsius. Even so, if you observe a bottle of water kept in a freezer, you will notice that at first, the layer of ice is formed at the top, reaffirming the concept that water at 4 degree Celsius is denser and settles at the bottom. After a while, due to constant exposure to negative 20 degrees celsius temperature, the entire bottle freezes. The anomalous expansion of water is the reason why life exists on earth today. If water did not have this special property which allowed the top layer to freeze and the bottom layers to remain in a liquid state, we might not have been present today. 

The reason for anomalous expansion of Water

One oxygen atom is joined with two hydrogen atoms to form a water molecule. The intermolecular attraction of the water molecules holds the water molecules together in a liquid state at room temperature. The water molecules in a liquid state are continuously moving around in the container and being redistributed.

Hydrogen has a role in intermolecular attraction as well. There is also an attraction between the hydrogen atoms of one water molecule and the oxygen atom of another. In water, the distance between the H-O bond and the O-O attraction is greater. As the water cools, the pace at which the molecules zip around slows down as they run out of energy. As the water molecules freeze more, they begin to squeeze together, increasing the density. At 4 degree Celsius, the density hits its maximum, and the water molecules can no longer squeeze any tighter.

The H-O attraction, not the O-O attraction, keeps the water from freezing over into ice. Ice’s lattice architecture prevents water molecules from moving. However, because the H-O connection is not as tight as the O-O bond, it expands somewhat after the H-O bond takes over..