Of state. Contents and Concepts 1. Comparison of Gases, Liquids, and Solids Changes of State

Surface tension is the energy required to increase the surface area of a liquid

by a unit amount. Column 4 in Table 11.2 lists values of the surface tension of some liquids at 20_Cction, depend on intermolecular forces, which are related to molecular structure.

11.4 Properties of Liquids; Surface Tension and Viscosity

FIGURE 11.17

Water strider on a water surface

The water strider seems to skate on the surface “skin” of the water, which is actually a result of surface tension.

The surface tension of a liquid can be affected by dissolved substances. Soaps and

detergents, in particular, drastically decrease the surface tension of water. An interesting but simple experiment shows the effect of soap on surface tension. Because of surface tension, a liquid behaves as though it had a skin. Water bugs (striders) seem to skitter across this skin as if ice-skating (Figure 11.17). You can actually float a pin on water, if you carefully lay it across the surface (Figure 11.18). If you then put a drop of soap solution onto the water, the soap spreads across the surface and the pin sinks. Water bugs also sink in soapy water. Capillary rise is a phenomenon related to surface tension. When a small-diameter glass tube, or capillary, is placed upright in water, a column of the liquid rises in the tube (Figure 11.19A). This capillary rise can be explained in the following way: water molecules happen to be attracted to glass. Because of this attraction, a thin film of

water starts to move up the inside of the glass capillary. But in order to reduce the