3a. What is surface tension? Explain the phenomenon in terms of intermolecular forces. b. Describe a simple experiment to demonstrate the surface tension of...

Answer Details

a. Surface tension: Surface tension is a property of a liquid whereby its surface behaves as if it were covered by an elastic skin. Or it is the tangential force of a liquid.

b. The surface “skin” of a liquid exists because of the attractive forces between its molecules. A molecule inside the liquid is pulled equally in all directions by surrounding molecules, so the net force on it is zero. However, a molecule at the surface has more neighboring molecules below than above, creating a net inward (downward) pull. This inward force tends to minimize the surface area, causing the surface to behave like a stretched elastic skin. This effect is known as surface tension.

See diagram above. 

Aim: To demonstrate surface tension in a liquid

Apparatus: A needle, water, a beaker, and a blotting paper

Procedure: The beaker of water is filled as shown above. A needle is now placed on a blotting paper and then pushed gently to the middle of the water surface. The paper wets and sinks, but the needle remains floating on the water's surface.

Conclusion: The surface acts like a skin covering the liquid, which can support the weight of the needle.

c. (i). When mercury spills on a glass sheet, it forms a spherical droplet (ii). A flowing needle on water (iii). A toy duck on liquid.

d. The cohesion force between molecules of water is less than its adhesion to glass, therefore when water is allowed to spill on glass, it wets. Conversely, the cohesion between the molecules of mercury is greater than its adhesion to glass. Hence, it does not wet glass.

e. (i). By adding detergents, (ii). By heating the liquid (ii). By addition of camphor