c11 JWBS043-Rogers September 13, 2010 11:26 Printer Name: Yet to Come
11 Liquids and Solids
Surface tension causes raindrops to be spherical, enables you to float a steel needle on
the surface of water (if you’re careful), and enables trees to draw liquid nourishment
from the earth by capillary attraction. These phenomena and the beautiful hexagonal
form of snowflakes are properties of liquids and solids, the condensed states of matter.11.1 SURFACE TENSION
The surface of any liquid is enveloped by molecules that are not like other molecules
because of an imbalance of forces acting on them that is different from the balanced
forces in the interior (Fig. 11.1). An imbalance of forces on surface molecules causes
them to be drawn in so as to form an elastic film of surface molecules enclosing
the bulk molecules. The existence of such a film completely surrounding a falling
rain droplet causes it to arrive at a stable structure with the minimum surface area
surrounding the volume of the drop. The minimum surface area for a specified volume
is that of a sphere. If there were no distortions due to gravity, air resistance, and so
on, raindrops would be perfect spheres.
A model of the liquid surface film is not difficult to set up and analyze. Consider
a small rectangular frame with one movable edge enclosing an elastic film such as a
soap film, as in Fig. 11.2. The model is analogous to the three-dimensional expansion
of a gas against a movable piston of areaAexcept that areaAis replaced by lengthl.
The work done in the case of stretching a membrane is the product of the intensiveConcise Physical Chemistry,by Donald W. Rogers
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