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

Rates of vaporization and condensation of a liquid over time

As vaporization continues at a constant rate, the rate of condensation increases. Eventually, the two rates become equal. The partial pressure of vapor reaches a steady value at a given temperature, which is the vapor pressure of the liquid. The process is said tov reach equilibrium. The flasks, A, B, and C, present the molecular view of the process at positions A, B, and C on the curve.

11 States of Matter; Liquids and Solids

FIGURE 11.7

Variation of vapor pressure with temperature

Several common liquids are shown. Note that the vapor pressure increases rapidly with temperature.

When the rates of vaporization and condensation have become equal, the number of molecules in the vapor at the top of the column in Figure 11.4 stops increasing and remains unchanged. The partial pressure of the vapor (as measured by the change in height of the mercury column) reaches a steady value, which is its vapor pressure. The liquid and vapor are in equilibrium. Although the partial pressure of the vapor is unchanging, molecules are still leaving the liquid and coming back to the liquid. For this reason, you speak of this as a dynamic equilibrium—one in which the molecular processes (in this case, vaporization and condensation) are continuously occurring. You represent this dynamic equilibrium for the vaporization and condensation of water by the equation with a double arrow.
H ₂O_(l) H₂O_(g)
As Figure 11.7 demonstrates, the vapor pressure of a substance depends on the temperature. (Appendix B gives a table of the vapor pressures of water at various temperatures.) As the temperature increases, the kinetic energy of molecular motion becomes greater, and the vapor pressure increases. Liquids and solids with relatively high vapor pressure at normal temperatures are said to be volatile. Chloroform and carbon tetrachloride are volatile liquids. Naphthalene, C10H8, and para-dichlorobenzene, C6H4Cl2, are volatile solids: they have appreciable vapor pressures at room temperature. Both are used to make mothballs