Every liquid, when taken in a container, has the property that, its free surface behaves like a stretched membrane with a tendency to contract and acquire the _______ surface area.

Maximum

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Infinite

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Minimum

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None of these

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Solution

The correct option is C Minimum

The cohesive forces between liquid molecules are responsible for the phenomenon known as surface tension. The molecules at the surface do not have other like molecules on all sides of them and consequently, they cohere more strongly to those directly associated with them on the surface. This forms a surface "film" which makes it more difficult to move an object through the surface than to move it when it is completely submerged.

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Q. The top surface of a liquid behaves like stretched membrane because

________is the property of liquid responsible for its tendency to form a spherical drop on a flat surface.

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

Column A	Column B
1. The phenomenon due to which top surface of a liquid behaves like stretched membrane	(a) Gas pressure
2. A substance which cannot be subdivided into two or more simpler substances by any chemical means	(b) Intermolecular force.
3. The force of attraction between the molecules of a given substance	(c) Adhesive force
4. The force exerted per unit area by the molecules of a gas on the sides of containing vessel	(d) Surface Tension
5. The force due to which ink sticks to a piece of paper	(e) Element

Explain the statement ," electric lines of force have a tendency to contract in length like a stretched string "

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