Question

Insects are able to run on the surface of water because:
A: insects have less weight
B: insects swim on water
C: of Archimedes upthrust
D: surface tension makes the surface behave as elastic membrane

Answer 1

Hint: The insect in moving in the surface of water unlike a fish. Only a tiny volume of its body is immersed in water. The density of an insect is usually higher than that of water.
The surface tension causes the water surface to form a strong bond within the water molecules, and the surface acts like an elastic membrane.

Complete answer:
We might think that lighter objects float on water whereas heavier objects tend to be immersed in it. Because when we look at the world around us lighter objects like a bird feather would be seen floating on still water, but a heavy rock would sink in.
But if we look closely we can understand that this phenomena is not actually based on the mass of the object but the density, a small coin, no matter how big the water body is, would sink into the water despite the fact that the weight of the coin is far less than that of the water body. But a ton of feather would most likely float on water due to its low density.
So, we can discard our first option because though insects are usually small and lighter, they necessarily have relatively greater density than water in most cases.
And insects do not use the flow of water or direct the water around its body to move in the water like a fish or human (who can swim). An insect never enters the bulk of the water body, it simply floats on the surface, so we cannot claim that an insect is swimming.
As per Archimedes principle, the buoyant force might be the reason for the insect to float. But then again, this force is proportional to the volume of water displaced by the insect. But, if we consider an insect like a small water strider, and consider the portion of its body underwater, the amount of water displaced is very low. And might not be sufficient for the insect to float and move around comfortably.
The surface of water acts like an elastic membrane due to its hydrogen bonding property. This bond between the molecules is considerably strong to make the surface appear similar to a stretched membrane. The insect’s feet would cause indentations on the water's surface which would result in an increase in the surface area. And the weight of the insect is balanced by the surface tension of the water.
Therefore, the insect is able to run on the surface of water because surface tension makes the surface behave as an elastic membrane.

So, the correct answer is “Option D”.

Note:
The buoyant force is equal to the weight of the volume of water displaced by the body which experiences the buoyant force.
Substances with lower density than water float on water whereas objects with higher density sinks. Ice has a lower density due to which it floats on water.