PHENOMENON OF CAPILLARY ACTION

  

 Capillary action is very important in nature, particularly in the transport of fluids in plants and through the soil.

An effect due to intermolecular forces most easily seen in liquids in narrow, open tubes. If the affinity between the liquid and the tube molecules is greater than between the liquid molecules themselves then the liquid will form a positive meniscus and it will climb the tube slightly (e.g. water in glass). If on the other hand the affinity is greatest between the liquid molecules then a negative meniscus will form and the liquid level will drop slightly (e.g. mercury in glass).

 

When a capillary tube is dipped in liquid,there occurs either rise or fall in liquid level in the capillary tube.This phenomenon is

known as capillary action.Two types of force bring about the capillary action.One is the intermolecular force of attraction between

the molecules liquid.This is known as COHESIVE force.Another is attraction force between the solid surface and liquid molecules .

This is known as A adhesive force.Water being polar molecule, the attraction between the glass and water molecule is quite strong.

This adhesive force is stronger than cohesive force in case of water surface interface.So water level rises up the glass capillary

tube with concave meniscus upward .The water level rises up until the adhesive force is balanced by the weight of water in the tube.

The tendency of water to spread over the glass surface is also due to the strong adhesive force between the glass and water molecules.

On the other hand water droplets in the waxed paper are spherical in shape because the adhesive force between polar water molecules and

non-polar wax is weaker than cohesive force.This cohesive force which gives rise to surface tension dominates over the adhesive force.

Therefore water droplets over the waxed paper are spherical in shape.  

If the capillary tube is dipped into mercury,a depression of the mercury level occurs with a convex meniscus upward.This happen because

the adhesive force between non-polar mercury and glass surface is relatively weak compared to the cohesive force in mercury.So mercury doesn't

spread over the glass surface,rather it tries to have minimum surface area and thus mercury meniscus in the glass tube is convex upward.