[Science] Surface Tension, Cohesive Force & Adhesive Force

[Science] Surface Tension, Cohesive Force & Adhesive Force

• Surface Tension : It is the property of a liquid by virtue of which it has the tendency to have the area of its free surface minimum as if it were under tension like a stretched elastic membrane.

• If a force F acts on an imaginary line of length L then T = F/L

• Surface tension of a liquid decreases with the increase of temperature and becomes zero at critical temperature.

• Some examples of surface tension :

• If a needle is very slowly kept on the surface of water, it floats due to surface tension.

• The addition of detergent or soap decreases the surface tension of water and thus increases the cleaning ability.

• Bubbles of soap solution are big because addition of soap decreases the surface tension of water.

• When kerosene oil is sprinkled on water, its surface tension decreases. As a result the larva of mosquitoes floating on the surface of water die due to sinking.

• Warm soup is tasty because at high temperature its surface tension is low and the soup spreads on all parts of the tongue.

• The smaller drops of mercury are spherical.

• Cohesive force :The force of attraction between the molecules of same substance is called cohesive force.This force is maximum in solids.This is why solids have a fixed shape. Cohesive force is negligible in case of gases.

• Adhesive force : Force of attraction between the molecules of different substances is called adhesive force. Due to adhesive force, one body sticks to other.

• If the cohesive force among the molecules of liquid is less than the adhesive force between the liquid and the vessel then that liquid wets the walls of the vessel. Example: water wets the walls of the vessel. On the other hand if the cohesive force among the molecules of liquid is greater than the adhesive force between the liquid and the vessel then that liquid does not wet the walls of the vessel. Example: mercury does not wet the walls of the vessel.

• The adhesive force between ink and paper is more than the cohesive force of the ink so at the time of writing, the ink sticks to the paper and writing is possible.

• We can not clean the glass by silk. For this purpose, we use cotton.The reason is that the cohesive force of silk is greater than the adhesive force between water and glass.

• The adhesive force between oil and water is less than the cohesive force of the molecules of the water but is more than the cohesive force of the oil so the water drops are spherical on the surface of oil while the oil drops expand on the surface of the water.

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