Surface tension is one of the most important and interesting propertie

1.	Surface tension is one of the most important and interesting properties of the liquids. It is the force acting downwards on the surface of the liquid which tends to pull the surface downwards so that it has minimum surface area. The value of surface tension of a liquid depends upon the intermolecular forces and hence on the nature of the liquid. When a liquid comes in contact with another liquid or some surface, the behavior observed depends upon the adhesive forces between them. This property of liquids has a no. of applications in daily life. Now, answer the following questions: (i) How the property of surface tension helps in the clinical test for jaundice? (ii) Why washing of clothes and dishes is more effective with hot water than with cold water? (iii) Why rain water drops falling on the waxed car smoothly slide down to the ground? (iv) Why water sticks to a glass surface but mercury does not?
Answer» (i) 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 dynes/cm. In this test, sulphur is sprinkled on the urine surface. If it floats, the test is negative but if it sinks, it is positive. (ii) Hot water is more effective because surface tension is lowered and it becomes a better wetting agent. (iii) Waxing changes the surface properties of the car. Water tends to adhere weakly to the wax and strongly to itself. (iv) In case of water taken on the glass surface the adhesive force between water and glass are greater than cohesive forces among water molecules but in case of mercury taken in glass tube, it is the reverse.

Surface tension is one of the most important and interesting properties of the liquids. It is the force acting downwards on the surface of the liquid which tends to pull the surface downwards so that it has minimum surface area. The value of surface tension of a liquid depends upon the intermolecular forces and hence on the nature of the liquid. When a liquid comes in contact with another liquid or some surface, the behavior observed depends upon the adhesive forces between them. This property of liquids has a no. of applications in daily life. Now, answer the following questions: (i) How the property of surface tension helps in the clinical test for jaundice? (ii) Why washing of clothes and dishes is more effective with hot water than with cold water? (iii) Why rain water drops falling on the waxed car smoothly slide down to the ground? (iv) Why water sticks to a glass surface but mercury does not?

Answer»

(i) 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 dynes/cm. In this test, sulphur is sprinkled on the urine surface. If it floats, the test is negative but if it sinks, it is positive.

(ii) Hot water is more effective because surface tension is lowered and it becomes a better wetting agent.

(iii) Waxing changes the surface properties of the car. Water tends to adhere weakly to the wax and strongly to itself.

(iv) In case of water taken on the glass surface the adhesive force between water and glass are greater than cohesive forces among water molecules but in case of mercury taken in glass tube, it is the reverse.

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