Question

explain the phenomena of surface tension on the basis of molecular theory​

Answer 1

Answer:

The phenomenon of suface tension arises due to the cohesive forces acting between the molecules of a liquid. The layer of the liquid surface of thickness equal to the range of moleeular attraction is called surface film. The cohesive forces on the liquid molecules within the surface film differ from those acting on molecules deep in the interior of the liquid

Consider three molecules of a liquid : A molecule A well inside the liquid, and molecules B and C lying within the surface film, as shown in the figure. The figure also shows their spheres of influence of radius R.

(1) The sphere of influence of molecule A is entirely inside the liquid and the molecule is surrounded by its nearest neighbours on all sides. Hence, molecule A is equally attracted from all sides, so that the net cohesive force acting on it is zero. Hence, it is free to move anywhere within the liquid.

(2) For molecule Ba part of its sphere of influence is outside the llquid surface. This part contains air molecules whose number is negligible compared to the number of molecules in an equal volume of the liquid Therefore, molecule B experiences a net cohesive force downward.

(3) For molecule C, the upper half of fts sphore of influenoe is outside the liquid surface Therefore, the net cohesive force acting on molecule C in the dowanward direction is maximum.

(4) Thus, all molecules lying within a surfiee film of thickness equal to Rexperience a net cohosive force direcied into the liquid.

(5) The surface area is proprtional l to the numbor of molectiles on the surface. To increase the surface arwa, molbeules must be brought to the surface trim within the liquid. For this, work must be done againsl t cohesive surces. This work is wrored in the liquid surface In the form potential energy. With a tendency to have minimum potential energy, the liquid tries to reduce the number of molecules on the surface so as to have minimum surface area. This is the reason why the surface of a liquid behave like a stressed elastic membrance.