Surface Tension

Surface tension is the ability of a liquid surface to act like stretched elastic skin.  Surface tension occurs due to the force of attraction between molecules of liquid

How Surface Tensions Occur?

Particles at the surface have no neighbor molecules which tend to possess strongest attraction force up on their nearest neighboring on the surface, this result elastic nature of the liquids surface.

If an object placed on surface of liquid its weight pushes downward cause a deformation, which tends to increase the surface area of the liquid. The surface tension resists that increase in area by pushing upward on the object 

Physical Phenomenon Show Presence of Surface Tension

i)        Walking on water: Small insects such as the water strider can walk on water because their weight is not enough to penetrate the surface.

ii)       Pond skater (water strider) floats in pond due to surface tension

iii)     Floating a needle: A carefully placed 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.  Pin floats over water due to surface tension

iv)     Razor floats over water due to surface tension formation of soap bubbles due to surface tension

v)       Formation of drops of water on leaves of a plant due to surface tension

vi)     Don't touch the wet 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.

vii)   Clinical test for jaundice: Normal urine has a surface tension of about 66 dynes/centimeter 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 will sink if the surface tension is lowered by the bile.

viii)  Surface tension disinfectants: Disinfectants are usually solutions of low surface tension. This allows them to spread out on the cell walls of bacteria and disrupt them.

ix)     Soaps and detergents: These help the cleaning of clothes by lowering the surface tension of the water so that it more readily soaks into pores and soiled areas.

x)       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.

xi)     Why bubbles are round: The surface tension of water provides the necessary wall tension for the formation of bubbles with water. The tendency to minimize that wall tension pulls the bubbles into spherical shapes.

xii)   Surface Tension and Droplets: Surface tension is responsible for the shape of liquid droplets. Although easily deformed, droplets of water tend to be pulled into a spherical shape by the cohesive forces of the surface layer

xiii)  Glass house leaks water inside during the starting of the rain season, when glass roof partial wetted have poor/not surface tension of glass roof in which water leaks inside but when glass roof total wetted experience surface tension which prevent leaks of water inside the glass house

Factor Affect Surface Tension

i)        Nature of the liquid

Different liquid has different surface tension, for Example, mercury has higher surface tension than water

ii)       Contamination/impurities

Impurities in liquid lower the surface tension. The substance which lowers surface tension is called surfactants (acronym for surface active agent). Example, of surface tension lower is detergents

iii)     Temperature

Surface tension of a liquid decreases with increase in temperature

 

Application of Surface Tension

i)        Cleaning action of soap, e.g. detergent lower surface tension between particles of closes and dirty

ii)       Mosquitoes normally lay their eggs in water. When small amount of oil is poured on the water, it lowers the surface tension which results sinking of eggs

iii)     Surfactants are also used to make emulsion of two liquids like oil and water

iv)     It used in extraction of impurities in laboratory

v)       Hot soup has lower surface tension as a result soup spread over a large area of the tongue, hence hot soup is tastier that cold one