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What is Surface Tension ?

Surface tension is the tendency of the fluid surfaces to shrink into the minimum surface area possible. It can be determined by the difference in the interactions between the molecules of the fluid with the molecules of the flask or beaker or the molecules of the storage wall.

According to the definition of surface tension, it is the phenomenon that occurs when the surface of a liquid is in contact with another phase (it can be a liquid as well). Liquids tend to acquire the least surface area possible. The surface of the liquid behaves like an elastic sheet.

Surface tension property : 

“It is defined as the force acting along the surface of a liquid at right angles to any line of 1 unit length.” It is numerically and dimensionally equal to surface energy. It has dimensions kg. 

s^{-2} and n SI unit.

If we consider a molecule in the bulk of the liquid it experiences equal intermolecular forces in all directions. Hence there is no net force acting on it. But a molecule at the surface has intermolecular forces from inside only. Therefore there is a net attractive force on it towards the interior of the liquid. Due to this the surface area of the liquid tends to minimize. That is the molecules experience a downward force and have more energy than the molecules in the bulk.

It decreases with the increase in temperature because of an increase in the K.E. of molecules and a decrease in intermolecular forces.

Below is a table is the surface tension of various liquids:

Liquid Surface Tension (N/m)
Hydrogen 2.4
Helium 0.16
Water 0.072
Ethanol 22.0
Sodium Chloride 114

How to Calculate Surface Tension?

Compute the surface tension of a given liquid whose dragging force is 7 N and the length in which the force acts is 2 m.

Solution:

Given,

  • F = 7 N
  • L = 2 m

According to the formula,

T = F/L

 T = 7/2

 T = 3.5 N/m