24 INTERFACIAL PHENOMENA 



it in all directions. The pull exerted by any molecule upon surrounding 

 molecules, however, diminishes veiy rapidly with increasing distance from 

 that particular molecule, reaching a maximum at distances not exceeding one 

 or two molecular diameters. Liquids possess a definite volume and distinct 

 boundaries because the molecules are close enough together to attract each 

 other with forces of considerable magnitude. The important property of 

 cohesion in liquids is due to these forces. The magnitude of the cohesive 

 force between the molecules is greater in water than in most liquids. The 

 molecules of a solid are so closely bound together by intermolecular attrac- 

 tions that it has lost ability to change its form, and possesses not only definite, 

 but fixed boundaries. The molecules of both liquids and solids are kinetically 

 active, but their movements are limited more or less rigidly by cohesive forces 

 between the molecules. This limitation upon the freedom of movement of 

 molecules is much greater in solids than in liquids. In gases the distance 

 between the molecules is relatively so great that the intermolecular attractive 

 forces are of negligible magnitude. 



Molecules at the surface of a liquid or a solid are affected differently by 

 the cohesive forces between molecules than those within the interior. The 



further more detailed discussion of this point 

 will be in terms of water. In a vessel of 

 water each of the individual molecules in the 

 body of the liquid is attracted in all direc- 

 tions by the cohesive forces exerted upon it 

 by the surrounding molecules. Any indivi- 

 FiG. 3. A diagrammatic rep- ^j^^j molecule is influenced in this way by a 



resentation of the direction of the ■, 1 r v • ll • 11 



, . . large number of its neighbormg molecules. 



torces of cohesion acting upon a i • 11 



water molecule: {A) in the body ^^ long as any given water molecule is well 

 of the liquid, (B) at its surface. within the body of the liquid the attractive 



forces which it sustains will balance each 

 other, so that the molecule is subjected to an equal pull in all directions. At 

 the surface, however, the molecules are not surrounded on all sides by other 

 water molecules. Every molecule in the surface layer is pulled strongly toward 

 the interior by the water molecules beneath it. It is for this reason that drops 

 of water falling freely through space acquire approximately a spherical shape. 

 Since there are no outwardly directed attractions which balance these internally 

 directed forces the surface layer of molecules in any liquid is constantly under 

 a tension. These conditions are illustrated diagrammatically in Fig. 3. 



The strained condition of the surface layer of liquid molecules at liquid- 

 gas boundaries is known as surface tension. Solid surfaces also manifest 

 surface tensions, but gases, having enormous intermolecular distances and no 



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