CBSE Class 11-science Answered

To understand what causes the surface tension in the liquids refer the accompanying figure in which two molecules of the liquid, A and B, are considered. The molecule A is completely inside the fluid and it is attracted by cohesive forces of the surrounding molecules in all directions. The resultant force acting on molecule is zero. Similarly, the resultant forces acting on all the molecules located below the surface of the fluid is zero.

Now let us consider the molecule B located at the surface of the fluid. There are downward forces of attraction on this molecule due to other molecules. Thus there is net resultant force acting on molecule B in the downward direction. In a similar manner all the molecules on the surface of the fluid experience net downward force. Thus the free surface of the fluid acts like a very thin film under tension of the surface of the liquid acts as though it is an elastic membrane under tension, thus causing the surface tension.

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