Definition Of Surface Tension

Definition Of Surface Tension

Surface tension is a contractive tendency of the surface of a liquid that allows it to resist an external force

More About Surface tension

  • The cohesive forces between liquid molecules are responsible for the phenomenon known as surface tension
  • A molecule in the bulk of liquid experiences equal intermolecular forces from all sides
  • The molecule, therefore does not experience any net force
  • But for the molecule on the surface of liquid, net attractive force is towards the interior of the liquid due to the molecules below it
  • The molecules on the surface experience a net downward force and have more energy than the molecules in the bulk, which do not experience any net force
  • Therefore, liquids tend to have minimum number of molecules at their surface
  • If surface of the liquid is increased by pulling a molecule from the bulk, attractive forces will have to be overcome
  • This will require expenditure of energy
  • The energy required to increase the surface area of the liquid by one unit is defined as surface energy
  • Its dimensions are Jm-2
  • Surface tension is defined as the force acting per unit length perpendicular to the line drawn on the surface of liquid
  • It has dimensions of kgs-2 and in SI unit it is expressed as Nm-1. The lowest energy state of the liquid will be when surface area is minimum
  • Spherical shape satisfies this condition, which is why mercury drops are spherical in shape
  • This is the reason that sharp glass edges are heated for making them smooth
  • On heating, the glass melts and the surface of the liquid tends to take the rounded shape at the edges, which makes the edges smooth
  • This is called fire polishing of glass.
  • The magnitude of surface tension of a liquid depends on the attractive forces between the molecules. When the attractive forces are large, the surface tension is large
  • Increase in temperature increases the kinetic energy of the molecules and effectiveness of intermolecular attraction decreases, so surface tension decreases as the temperature is raised

Applications:

  • Liquid tends to rise (or fall) in the capillary because of surface tension
  • Liquids wet the things because they spread across their surfaces as thin film
  • Moist soil grains are pulled together because surface area of thin film of water is reduced
  • It is surface tension which gives stretching property to the surface of a liquid
  • On flat surface, droplets are slightly flattened by the effect of gravity; but in the gravity free environments drops are perfectly spherical
 

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