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Capillary Action

Insects are a...

Question

Insects are able to run on the surface of water because :

A

insects have less weight

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B

insects swim on water

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C

of the Archimede's upthrust

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D

surface tension makes the surface behave as elastic membrane.

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Solution

The correct option is D surface tension makes the surface behave as elastic membrane.
The surface of the water behaves like an elastic film: the insect's feet cause indentations in the water's surface, increasing its surface area. Surface Tension balances the weight of the insect.

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Q. Water forms drop owing to attendance of its surface to contract. The surface of water acts like a stretched membrane by virtue of an effect called Surface Tension. For a small quantity of water, the effect of surface tension is greater than the weight of water; its surface contracts so as to achieve the smallest surface area. This implies a spherical shape.The phenomenon of surface tension is common to all liquids. The nature of this phenomenon can be understood on the basis of molecular forces. Consider a liquid with its surface open to the atmosphere. Some molecules of the liquid will escape from the surface and exist as a vapor above the surface. In fact, molecules on the surface are in a dynamic state. In equilibrium, some molecules are leaving the surface at any time to convert the vapor state and an equal number of molecules convert from the vapor state to the liquid state on the surface. Consequently, the number of molecules in the surface layer is smaller than in the layers lying below. Average separation between the surface molecules is slightly greater and attractive forces between them have a tendency to pull them together. This explains how the surface behaves like a stretched membrane and also the origin of surface tension. Inside the liquid, average intermolecular force is zero. Surface tension depends on temperature. As the temperature is increased, the exchange rate of molecules between the liquid and the vapor also increases. You are familiar with the quantitative definition of surface tension as the force acting per unit length on an imaginary line that separates adjacent sections of the surface of the liquid. It is possible, due to surface tension, that tiny objects of density more than the density of water can float on the surface of the water.
Consider an insect with eight legs standing on water. The end of each leg has a spherical shape of radius 1 mm. A surface of water makes an angle 60

^{o}

with the vertical at its contact with the leg and surface tension of water at the given temperature is 72 dyne/cm.
Answer the following questions:
As temperature is increased

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