Capillary Action

Capillary action is the ability of a liquid to rise upwards against the force of gravity. It is an important natural phenomenon. Several natural and artificial processes are functioning on this principle.

Capillary action is the ability of a liquid to flow in narrow spaces without the assistance of any forces, and most of the time in opposition to, external forces like gravity.

Perform the following experiment with water to understand Capillary Action.

This is a simple experiment:

Materials Required:

A trough with water

Glass tubes of different diameters

Procedure: Collect water in the trough from the tap and place it on a table. Place empty glass tubes of different diameters into the trough containing water. Observe.

Observation: Water level rises in the tubes. Water level is greater in the tubes with lesser diameter. Water level is lesser in the tubes with greater diameter.

Explanation:

Water molecules on the surface of the trough is attracted to one another. Each molecule of water is attracted to another molecule of water by cohesive forces. This inter-molecular attraction between the molecules present at the surface of a liquid is known as surface tension. Surface tension results in the formation of a strongly bonded layer of molecules on the surface of water. Thus water molecules experience two types of attractive forces. They are:

Attractive forces between the molecules of water termed as cohesion.

Cohesive forces on the surface of water, due to surface tension.

When a glass tube, is placed on the surface of water, there is attraction between the glass molecules and the water molecules. This is known as adhesion. The attractive force between unlike molecules is known as adhesive forces.

In a glass tube with lesser diameter, the adhesive forces exerted by the water molecules are greater than the cohesive forces acting between the water molecules on the surface, which causes the water to rise inside the tubes. 

Thus the tendency of water to rise in a thin tube is called capillary action. In this experiment both the types of forces are made to work. The thinner the tube, the higher the water level. Thinner the diameter, greater is the force of adhesion.

Examples of Capillary Action:When we dip a paint brush in paint, the paint is drawn by the bristles of the paint brush into a thin tube due to capillary action.

• Paper towels have small minute pores in them. When they come in contact with a liquid, capillary action allows the liquid to move up into the towel. Thus paper towel will be soaked with the liquid.
• Sponges have minute pores present in them. These pores act as capillaries when they come in contact with liquids and absorb large quantity of liquids.
• A candle burns due to capillary action. When the candle burns, the heat of the flame causes the wax of the candle to melt. The molten wax is then pulled up through the wick by capillary action and supplies the fuel needed to sustain the fire and to keep the candle burning. 
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• The roots of the plants draw water by capillary action from the soil, and thus, supply it to various parts of the plant including the stems, branches, and leaves.
• Thus capillary action in plants is one of the most essential processes which helps the plants to survive.
• Capillary action is essential for the drainage of tears from the eyes. Two tiny- tubes of least diameter called the lacrimal ducts, are present in the inner corner of the eyelid. These ducts secrete tears into the eyes. The tear fluid from within the eyes rise into lachrymal ducts until they get expelled.
• The nib of the fountain pen is split into two. Ink moves from the body of the fountain pen through the slit. Gravity is responsible for the ink moving out of the pen tip when we start writing, but capillary action is needed to keep the ink flowing onto the paper.
• Chromatography works on the basis of capillary action.

Thus, capillary action is a property of all liquids. The liquids are able to travel through narrow spaces or capillaries in solids, without the assistance of any external force. It is an important natural phenomenon which has several applications in nature. Capillary action is also useful in many man made applications. 

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