This section will investigate a fundamental property of liquids: surface tension. Liquids and solids share a common attribute: a clear and discernible phase boundary that gives the sample a simple but definite shape. Liquids and solids share something else, too: most of their molecular units are in some degree of direct contact. At the same time, however, liquids, like gases, are fluids, meaning that their molecular units can move more or less independently of each other. But whereas the volume of a gas depends entirely on the pressure (and thus generally on the volume within which it is confined), the volume of a liquid is largely independent of the pressure.
The molecules of a liquid that find themselves fully in the interior volume are at a lower energy than those that are at the interface, as shown in .
Surface tension is responsible for the shape of a liquid droplet. Although easily deformed, droplets of water tend to be pulled into a spherical shape by the cohesive forces of the surface layer. In the absence of other forces, including gravity, drops of virtually all liquids would be perfectly spherical. If no force acts normal (perpendicular) to a tensioned surface, the surface must remain flat. But if the pressure on one side of the surface differs from pressure on the other side, the pressure difference times the surface area results in a normal force. In order for the surface tension forces to cancel out this force due to pressure, the surface must be curved. When all the forces are balanced, the curvature of the surface is a good measure of the surface tension, which is descibed by the Young-Laplace equation:
where P is the pressure differential across the interface,
This equation describes the shape and curvature of water bubbles and puddles, the "footprints" of water-walking insects, and the phenomenon of a needle floating on the surface of water, as shown in . Even though the needle is denser than water, it floats because surface tension is a contractive tendency of the surface of a liquid that allows it to resist an external force. This property is caused by cohesion of similar molecules and is responsible for many of the behaviors of liquids.
In imagining the shape of a liquid droplet or the curvature of the surface of a liquid, one must keep in mind that the molecules at the surface are at a different level of potential energy than are those of the interior. That is to say, there is an energy difference between the interior and the surface: to move a molecule from the interior to the surface requires energy. The object of the "game" is for a droplet or surface to form that minimizes the energy at the surface. Again, since the energy at the surface is due in large part to the attractive forces between particles in the bulk and those in the interior, the surface tension is an indicator of the extent of those forces. Different liquids and solutions have different surface tensions, as seen in .
Surface tension is expressed in units of force per unit length, or of energy per unit area. The two are equivalent, but when referring to energy per unit area, people use the term "surface energy," which is a more general term in the sense that it applies to solids as well as to liquids.