On the basis of molecular theory, explain the phenomenon of surface tension.

Answer:

Terms: The maximum distance up to which a molecule exerts an appreciable cohesive force on other molecules is its range of molecular attraction (about \(10^{-9}\) m). A sphere of that radius drawn around the molecule is its sphere of influence. A layer of the liquid at the surface of thickness equal to this range is the surface film.

[Figure: a liquid in a container with three molecules A (deep inside), B (just below the surface, inside the surface film) and C (on the free surface), each with its sphere of influence drawn.]

• Molecule A well inside the liquid: its sphere of influence is wholly inside the liquid, so it is attracted equally in all directions and the net force on it is zero.
• Molecule B in the surface film: part of its sphere of influence lies outside the liquid (in air, where there are very few molecules). The lower part has more liquid molecules than the upper part, so B experiences a net downward force.
• Molecule C on the surface: half its sphere of influence is in air, so the net downward force on it is maximum.

Thus every molecule in the surface film is pulled inward. To bring a molecule from the interior to the surface, work must be done against this inward force, so surface molecules have extra potential energy. A system tends to have minimum potential energy, so the liquid surface contracts to have the least possible number of molecules on it, i.e. minimum surface area. The surface therefore behaves like a stretched elastic membrane. This tendency of the free surface to contract is surface tension.