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Surface Tension


Origin and Definition of Surface Tension
It was seen during the study of vapour pressure that the molecules at the surface of a liquid experience a net inward pull because of the larger number of molecules towards the liquid side than towards the vapour side. There is a tendency on the part of surface molecules to go in the bulk of the liquid. The surface of the liquid is therefore in some sort of tension and it tends to contract to the smallest possible area in order to have the minimum number of molecules at the surface. It is for this reason that the surface of a liquid is spherically curved, since the surface area is minimum for a given volume in the case of a sphere.
If the area of the surface is to be extended, then one has to bring more molecules from the bulk of a liquid to its surface. This will require expenditure of some energy because work has to be done in bringing molecules from the bulk against the inward attractive forces. The amount of work done in increasing the area by unity is known as the surface energy. The larger the forces of attraction amongst the molecules of a liquid, the larger will be the net inward pull; thus, the amount of work done also increases. Thus, the surface energy can be used to define these net inwards pull. It is customary to define these inward pull in terms of the surface tension. The latter is defined as the force acting along the surface of liquid at right angles to any line of unit length (Table Surface tensions of some common liquids at 20oC.

Liquid

× 103/N m-2

Liquid

× 103/N m-2

Water

72.8

Acetone

23.32

Benzene

28.87

Methanol

22.55

Toluene

28.53

Ethanol

22.30

Chloroform

27.2

Ethyl ether

17.05

Carbon tetrachloride

26.75

Methyl acetate

24.8

 

Unit of Surface Tension
The unit of surface tension is that of force per unit length. In CGS units it is expressed as dyn cm-1 whereas in SI units, it is expressed as N m-1 (newton per metre).

Effect of Temperature on Surface Tension
Since the forces of attraction between the molecules of liquid decrease with and increase in the temperature, it follows, therefore, that the surface tension will decrease when the temperature is increased.




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