As pointed out earlier, all molecules in a liquid are subject to attractive forces from neighboring molecules. For molecules in the body of the liquid, these forces balance each other (See fig. 2.1l), but for molecules at the surface there is resultant attractive force acting downwards. (Fig. 2.12). These forces which tend to pull the surface molecules downwards are responsible for the surface tension. It is designated by γ. The dimensions of surface tension are dynes/cm or ergs/cm² (numerically the values in both the units are equal).Hence the surface tension may be regarded as the force per unit length or energy per unit area of the surface of liquid. Table 2.2 gives the surface tension of some liquids at 20°C.
Table 2.2 — Surface tension (dynes/cm or erg/cm²) at 20ºC.
This table shows that the surface tension of water is higher than most of the organic liquids because there are stronger intermolecular attraction in water (Hydrogen bands).
It is interesting to note that the surface tension tends to reduce the surface area of the liquids. For this reason, a freely falling drop of liquid is spherical (Fig. 2.13), since sphere has the smallest ratio of surface area to volume.
It is our common observation that a liquid always rises in a capillary tube (fig. 2.14), the effect is referred to as the ‘capillary action’. Capillary action of liquids is also due to their surface tension. The surface area to volume ratio of a capillary of small bore is much smaller than the container in which liquid is placed.
Like viscosity, the surface tension of a liquid also decreases with the rise of temperature owing to the weakening of the inter molecular attractions.