Surface Tension

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Have you ever wondered why few insects like water striders walk in water? or why do small droplets of rain stick to our window?.

water droplets on a window

The scientific explanation of this phenomenon is associated with the term “Surface Tension”. Surface Tension is the tendency of the surface of a liquid to contract until it reaches the smallest possible surface area.The intermolecular cohesive forces are responsible for this phenomenon.Molecules in the interior of a liquid interacts with other molecules from all directions,but molecules on the surface are only affected by molecules near and below them.Therefore they experience greater force of attraction than the molecules present in it.

Factors affecting Surface Tension


The surface tension of a liquid decreases with increasing temperature. As the temperature rises, the liquid molecules become more active and begin to move in random directions, reducing the strength of intermolecular bonds.The value of surface tension continues to decrease with increasing temperature and reaches zero, since at the critical temperature the intermolecular forces in the liquid and gaseous states are equal and the liquid expands freely.

Eg:- Hot soup is better than cold soup.Because surface tension of hot soup is less than the cold soup.

hot soup


Presence of sparingly soluble impurities decreases the surface tension.This is due to the fact that the adhesive force between liquid molecules and impurity molecules is less than the cohesive force between liquid molecules, and the intermolecular attraction between solvent and solute molecules is weak, so the surface tension decreases.

On the other hand, highly soluble impurities increase the surface tension.It is due to the strong intermolecular force of  attraction between a soluble compound and a solute.


3.Surface Active agents

Surface active agents reduce the surface tension of water.

Eg:Soap increases the cleaning action of water.

Examples of Surface Tension

  • Soap and Detergents:- Soaps and detergents reduce the surface tension of water. So it can soak into dirty areas.
  • Shape of bubble:- Surface tension is the reason for the spherical shape of the bubbles
shape of bubble
  • Needle or paper clip floating on water:- Needles which are denser than water can float on water due to surface tension.
paper clip in water

Read more : DNA

Check your knowledge

Surface tension is the tension of the surface film of a liquid caused by the attraction of the particles in the surface layer by the bulk of the liquid, which tends to minimise surface area

Temperature,Impurities,Surface active agents

Soap and Detergents:- They lower the surface tension of water.So it can soak into pores and dirty areas.

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