Anomalous expansion of water - By Aditya Abeysinghe

The expansion of water when cooled from four degrees centigrade to zero degrees centigrade is known as the "anomalous expansion of water."

Before examining the anomalous expansion of water let’s examine how other liquids react with changing volume and density above 273K.

When the temperature of a system, in which the liquid is stored, increases due to the volumetric expansion of the system, the volume of the liquid also increases.

However, by definition, density is the mass per unit volume or according to popular equation d = m /v. Thus, there exists an inverse relationship between the density of a liquid and its volume, when the mass is constant.

Meanwhile, when the volume of a liquid increases with increasing temperature, above 273K, the density of the liquid should decrease.

But, what differ water from other liquids is the presence of hydrogen bonds in water.

Though ice is supposed to expand when it is converted into water, formation of hydrogen bonds causes it to contract (the contraction caused due to the formation of hydrogen bonds is greater than the actual expansion of ice). Therefore, during melting, though the temperature remains the same, the volume of water decreases. This effect continues until 277K when all the hydrogen bonds are formed.

For example see the diagram below.

(Photo credit: User blogger - Aditya Abeysinghe)

Hydrogen bonds contract the liquid, hence reducing the volume (When bonds are formed, distance between elements are reduced, hence the volume occupied by elements is reduced, thus the volume occupied by the liquid is decreased).

Since this effect stops at 277K water starts to behave as to how a usual liquid behaves. Thus, when the temperature increases beyond 277K, the volume of water increases and its density decreases.

Thus, water records its highest density and lowest volume at 277K.

Therefore, the graphs obtained for the change in volume and density for a sample of water with increasing temperature are as follows:

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