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Question

The magnitude of surface tension of liquid depend upon the attractive force between the molecules.Arrange the following in increasing order of surface tension.Water,alcohol (C2H5OH) and hexane (CH3(CH2)4CH3)

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Solution

Since the magnitude of surface tension depends on intermolecular attraction, the high will be the attraction higher will be the surface tension. Among the given molecules the intermolecular force of attraction is highest in case of H2O because of hydrogen bonding. The hydrogen bonding also exist in alcohol (ethanol) but the extent of hydrogen bonding decreases with increase of carbon chain. In hexane there is very weak intermolecular attraction.Therefore order of surface tension in given liquids will be as follows: hexane < alcohol < water

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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 60o 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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