SAT Subject Test Chemistry




Liquids, Solids, and Phase Changes


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 Equilibrium

In 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.


More about equilibrium is discussed in Chapter 9.

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.

In the discussion above, if the 20°C system is heated to 30°C, the number of gas molecules will be increased while the number of liquid molecules will be decreased:


Le Châtelier’s Principle. It will occur again in Chapter 10.

The equation shifts to the right (any similar system that is endothermic shifts to the right when temperature is increased) until equilibrium is reestablished at the new temperature.

The molecules in the vapor that are in equilibrium with the liquid at a given temperature exert a constant pressure. This is called the equilibrium vapor pressure at that temperature.