PROPERTIES OF MATTER 25 



When a non-volatile material, one of low vapor pressure, is dissolved 

 in an ordinary liquid, the vapor pressure of the resulting system is due 

 essentially to the liquid alone. Since the concentration of the liquid 

 in the solution is reduced when compared to the pure state, the vapor 

 pressure of the solution is also reduced. Hence the vapor pressure of 

 blood is due to the water present but is less than that of pure water at 

 the same temperature because in blood the water molecides are diluted 

 by all the other materials present. 



Surface Tension 



The non-uniformity of the force field acting on the molecules in the 

 surface of a liquid has been discussed in the preceding section. This 

 lack of symmetry leaves the surface molecules with forces on the 

 surface side that can be satisfied only as extra attractions between 

 neighboring molecules in the surface. Such extra attractions draw the 

 molecules somewhat closer together, increasing the density of the 

 surface layer. As a consequence the surface layer behaves like a film 

 and is capable of supporting small objects of greater density than the 

 liquid itself, provided these materials are not wet by the liquid. Thus 

 a greasy needle or razor blade or certain insects float on water. 



The extra force between neighboring surface molecules is called 

 surface tension. It leads to the film-like character of the surface of a 

 liquid. Moreover, it represents a tendency for the surface to contract 

 to the minimimr area compatible with other forces acting on the 

 material. Hence small droplets are spherical, this form having the 

 lowest possible area to volume ratio. Drops of larger mass are subject 

 to correspondingly greater gravitational force and when placed on an 

 unwetted solid are flattened more or less, depending whether they are 

 large or small. 



When a liquid contacts a porous solid that it does not wet, surface 

 tension minimizes the area of liquid surface as usual and prevents 

 entrance of liquid into the pores. This type of behavior is encouraged 

 by waterproofing textiles to keep water from entering small spaces but 

 is objectionable when attempting to pass water into and through 

 certain thoroughly dried soils. In both cases the water wets the solid 

 with difficulty or not at all and is excluded from pores and cavities. 



On the other hand, a liquid that wets a solid is drawn into small 

 openings in the solid. In this situation the solid behaves as though 

 it is completely covered by a very thin liquid film. The liquid-gas 

 surface is then minimized by movement of liquid in the small capillary 

 spaces. These openings may be completely filled or not, depending 

 on the balance struck between the opposing tendencies of surface 



