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

(a) Surface tension is the property of the free surface of a liquid by which it behaves like a stretched elastic skin (membrane) and tends to contract to occupy the smallest possible area.

In terms of intermolecular forces: A molecule deep inside the liquid is attracted equally in all directions by neighbouring molecules, so the net force on it is zero. A molecule at the surface has no liquid molecules above it, so it experiences a net inward (downward) cohesive pull. This inward pull on all surface molecules makes the surface behave as though it were under tension.

(b) Experiment: Tie a loop of thread loosely across a wire ring and dip the ring into soap solution to form a film. The thread lies in an irregular shape. Now break the film inside the loop (with a hot wire). The thread is at once pulled outward into a perfect circle, showing that the remaining film pulls equally on all sides - a demonstration of surface tension.

(c) Effects of surface tension:

• Small insects (e.g. pond skaters) can walk on the surface of water.
• Falling raindrops and small liquid drops are spherical.
• A greased needle can float on the surface of water although it is denser than water.

(d) Water wets clean glass because the force of adhesion between water and glass molecules is greater than the force of cohesion between water molecules, so the water spreads over the glass (concave meniscus). For mercury the cohesive force between mercury molecules is greater than the adhesive force between mercury and glass, so mercury does not wet the glass and instead forms rounded beads (convex meniscus).

(e) Methods of reducing surface tension:

• Raising the temperature of the liquid.
• Adding an impurity such as soap or detergent.