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Surface Tension

Cohesive Forces- Cohesive forces are the intermolecular forces, for example; hydrogen bonding and Van der Waals forces which cause a tendency in liquids to resist separation. These forces which are attractive forces exist between molecules of the same material. For example, rain falls as droplets, and not as fine mist, since water has strong cohesion hence its molecules pulls tightly together and droplets are formed. The cohesive force tends to bring closer molecules of a liquid, by making them comparatively large clusters which owes to molecules' dislike for its surrounding.

Adhesive Forces- Adhesive forces are the attractive forces which apply among unlike molecules. These forces originate from force acting between two substances, for example mechanical forces and electrostatic forces etc.

In the case of a liquid wetting agent, adhesion causes the liquid to cling to the surface on which it rests. When water is poured on clean glass, it tends to spread, forming a thin, uniform film over the glass surface. This happens because the adhesive forces between water and glass are strong enough to pull the water molecules out of their spherical formation and hold them against the surface of the glass, thus avoiding the repulsion between like molecules.

Cohesive Forces and Surface Tension- The cohesive forces between liquid molecules are responsible for the phenomenon known as surface tension. The molecules at the surface do not have other like molecules on all sides of them and consequently they cohere more strongly to those directly associated with them on the surface. This forms a surface "film" which makes it more difficult to move an object through the surface than to move it when it is completely submersed.

Surface tension is generally expressed in dynes/cm; also it is expressed ergs per square centimeter.

Water at 20°C has a surface tension of 72.8 dynes/cm compared to 22.3 for ethyl alcohol and 465 for mercury.

Examples of Surface Tension
Here given some examples of surface tension, how this naturally occurring phenomenon affects our surrounding.

•    Walking on water - Small insects such as the water strider can walk on water because their weight is not enough to penetrate the surface.    
•    Floating a needle- If a small needle carefully placed on the surface, it made to float on the surface of water even though it is several times as dense as water. If the surface is frantic to break up the surface tension, then needle will quickly sink.

•    Don't touch the tent- Common tent materials are somewhat rainproof in that the surface tension of water will bridge the pores in the finely woven material. But if you touch the tent material with your finger, you break the surface tension and the rain will drip through.

•    Soaps and detergents- The soaps and detergents help the cleaning of clothes by lowering the surface tension of the water so that it more readily soaks into pores and soiled areas.

•    Clinical test for jaundice- Normal urine has a surface tension of about 66 dynes/cm but if bile is present (a test for jaundice), it drops to about 55. In the Hay test, powdered sulfur is sprinkled on the urine surface. It will float on normal urine, but sink if the surface tension is lowered by the bile.    

•    Washing with cold water- The major reason for using hot water for washing is that its surface tension is lower and it is a better wetting agent. But if the detergent lowers the surface tension, the heating may be unneccessary.

•    Disinfectants- Disinfectants are usually solutions of low surface tension. This allows them to spread out on the cell walls of bacteria and disrupt them.

Capillary Tube – A capillary tube is a tube of small internal diameter which holds liquid by capillary action. They can be made out of glass, plastic, metal, etc. Glass capillary tubes are probably the most common and are frequently used by biologists and chemists.

The natural phenomenon that makes capillary tubes so useful is called 'capillary action.' Because of capillary action if you stick the open end of a capillary tube into water, the water will draw itself right up into the tube. The reason for this is that water has natural tendency to stick to things like glass or plastic. So the water pulls itself up the inside of the plastic, sticking to it as depicted in figure here:

Image result for capillary tube

Some Examples of Capillary Action

•    A piece of blotting paper soaks ink because the pores of the blotting paper act as capillary tubes.
•    The oil in the wick of a lamp rises because of capillary action of threads in the wick.
•    The root hairs of plants draw water from the soil because of capillary action in them.
•    The prevent loss of water due to capillary action the soil is loosened by farmers for agriculture.

•    If a capillary tube is dipped in water in an artificial satellite, water rises up to other end of tube because of its zero apparent weight, this is independent of length of the capillary tube.

•    Towel soaks water from the body due to capillary action of cotton of which towel is made.
•    Melted wax in a candle rises up to wick due to capillary action.


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