chromatography experiment in a later chapter. Capillary action is at least partly
responsible for the transportation of water from the roots of a plant to its leaves.
The same process is responsible for the concave liquid surface, called a
meniscus, that forms in a test tube or graduated cylinder.
PHASE EQUILIBRIUM
Figure 23 shows water in a container enclosed by a bell jar. Observation of this
closed system would show an initial small drop in the water level, but after some
time the level would become constant. The explanation is that, at first, more
energetic molecules near the surface are escaping into the gaseous phase faster
than some of the gaseous water molecules are returning to the surface and
possibly being caught by the attractive forces that will retain them in the liquid
phase. After some time the rates of evaporation and condensation equalize. This
is known as phase equilibrium.
Figure 23. Closed System in Dynamic EquilibriumIn a closed system like this, when opposing changes are taking place at equal
rates, the system is said to have dynamic equilibrium. At higher temperatures,
since the number of molecules at higher energies increases, the number of
molecules in the liquid phase will be reduced and the number of molecules in the
gaseous phase will be increased. The rates of evaporation and condensation,
however, will again become equal.
The behavior of the system described above illustrates what is known as Le
Châtelier’s Principle. It is stated as follows: When a system at equilibrium is
disturbed by the application of a stress (a change in temperature, pressure, or
concentration), it reacts so as to minimize the stress and attain a new equilibrium
position.