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Sci Tech
Density of oil
Why is it that even if oil is thicker than water, it is less dense?
VARSHA SHANKAR
Chennai
A substance is seen to be thicker if it does not flow easily against the walls of a container or against any other surface. This can happen because of variety of microscopic reasons. A thin liquid flows easily and does not stick to the surface on which it flows.
The molecules constituting the material do not stick to each other and on to the solid surface. However, if the temperature is increased, in most cases, the liquid flow is easier and we experience that liquid to be thin.
The oil molecules are made up of long carbon chains containing a large number of carbon atoms in a line and in specific cases these are branched. An aggregate of such molecules is the oil. When it flows, the molecules have to move against each other. Understandably, the flow characteristics have to be different in different oils.
This behaviour should also be different from that of the thin liquids like water, alcohol, petrol etc where the molecules are relatively simpler and so do not resist flow against each other.
On the other hand the density of a substance is decided by the amount of matter mass packed into a given volume. The long chain molecules, when packed into a volume of space, leave a large fraction of gaps.
The situation can be likened to the case of keeping a given mass of paper pins into a match box where a large fraction of the volume would remain unfilled. But if the same match box is filled with small iron balls, it would weigh much more while the overall volume remains the same. The comparison of the oil and water is similar to the above case and water turns out to be denser than oil. The long chains entangle among each other and stick to the containers owing to several types of physical forces. Such processes are absent in the case of water. Thus, oil is thicker than water.
Dr. H. K. SAHU
Scientific Officer, IGCAR
Kalpakkam, Tamil Nadu, India
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