surface tension


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Presentation Transcript

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The force between the molecules of the same substance is known as Cohesive force.

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The force between the molecules of different substance is known as Adhesive force.

Surface Tension : 

Surface Tension The cohesive forces between liquid molecules are responsible for the phenomenon known as surface tension. Surface tension is typically measured in dynes/cm, the force in dynes required to break a film of length 1 cm. equivalently, it can be stated as surface energy in ergs per square centimeter. Water at 20°C has a surface tension of 72.8 dynes/cm compared to 22.3 for ethyl alcohol and 465 for mercury.

Cohesion and Surface Tension : 

Cohesion and Surface Tension The cohesive forces between molecules down into a liquid are shared with all neighboring atoms. Those on the surface have no neighboring atoms above and exhibit stronger attractive forces upon their nearest neighbors on the surface. This enhancement of the intermolecular attractive forces at the surface is called SURFACE TENSION.

Surface Tension of Water : 

Surface Tension of Water The surface tension of water is 72 dynes/cm at 25°C. It would take a force of 72 dynes to break a surface film of water 1 cm long. The surface tension of water decreases significantly with temperature as shown in the graph. The surface tension arises from the polar nature of the water molecule.

Surface Tension Examples : 

Surface Tension Examples Walking on water Small insects such as the water strider can walk on water because their weight is not enough to penetrate the surface.

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Don't touch the tent! Common tent materials are somewhat rainproof in that the surface tension of water will bridge the pores in the finely woven material. But if you touch the tent material with your finger, you break the surface tension and the rain will drip through.

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Clinical test for jaundice Normal urine has a surface tension of about 66 dynes/cm but if bile is present (a test for jaundice), it drops to about 55. In the Hay test, powdered sulfur is sprinkled on the urine surface. It will float on normal urine, but sink if the S.T. is lowered by the bile.

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Surface tension disinfectants Disinfectants are usually solutions of low surface tension. These allow them to spread out on the cell walls of bacteria and disrupt them. One such disinfectant, S.T.37, has a name which points to its low surface tension compared to the 72 dynes/cm for water.

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Floating a needle If carefully placed on the surface, a small needle can be made to float on the surface of water even though it is several times as dense as water. If the surface is agitated to break up the surface tension, then needle will quickly sink.

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Soaps and detergents Help the cleaning of clothes by lowering the surface tension of the water so that it more readily soaks into pores and soiled areas.

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Washing with cold water The major reason for using hot water for washing is that its surface tension is lower and it is a better wetting agent. But if the detergent lowers the surface tension, the heating may be unnecessary.

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