Question

Explain Laplace’s molecular theory of liquid & hence define surface tension. 

Answer 1

Consider three molecules A, B & C of the liquid. A sphere of influence is drawn as shown in fig.The sphere of influence of molecule ‘A’ is completely inside the liquid, so it is equally attracted in all directions by the other molecules lying within its sphere. Hence the resultant force acting on it is zero. The part of the sphere of influence of molecule ‘B’ lies outside the liquid & the major part lie inside the liquid. Therefore resultant force acting on it is directed downward. For Molecule ‘C’ half of its sphere of influence lies inside the liquid and half lies outside the liquid. So, the maximum resultant downward force is acting on molecule ‘C’ Thus molecule A experiences zero resultant force, B experience downward resultant force, C experience more downward resultant force. In short molecules below imaginary line PQ experience zero resultant force and molecules about line PQ experience some or more downward resultant force. Thus molecules which lie on the surface of liquid (surface film) experience downward resultant force and are being pulled inside the liquid. To balance this downward force, molecules come closer to each other. This reduces the surface area of liquid. This gives rise to surface tension. It is the contraction force which decreases the surface area of the liquid. Definition:- It is defined as property of liquid by virtue of which the surface of liquid is under constant tension due to the tendency to contract and occupy minimum surface area.