The Foundations of Chemistry

We can understand the order of vapor pressures of the four liquids cited in Table

13-4 and Figure 13-13 by considering the strengths of their intermolecular attractions.

Water has the lowest vapor pressure (strongest cohesive forces) because each molecule has

two hydrogen atoms to act as hydrogen-bond donors and each molecule can accept

hydrogen bonds from two other molecules. Methyl alcohol has only one potential

hydrogen-bond donor, so its average cohesive forces are weaker than those in water and

its vapor pressure is higher. In benzene and diethyl ether, the hydrogen atoms are all

bonded to carbon, so strong hydrogen bonds are not possible. Electrons can move easily

throughout the delocalized -bonding orbitals of benzene, however, so benzene is quite

polarizable and exhibits significant dispersion forces. In addition, the hydrogen atoms of

benzene are more positive than most hydrogens that are bonded to carbon. The H atoms

of benzene are attracted to the electron-rich -bonding regions of nearby molecules. The

accumulation of these forces gives benzene rather strong cohesive forces, resulting in a

lower vapor pressure than we might expect for a hydrocarbon. The diethyl ether mole-

cule is only slightly polar, resulting in weak dipole–dipole forces and a high vapor pressure.

BOILING POINTS AND DISTILLATION

When heat energy is added to a liquid, it increases the kinetic energy of the molecules,

and the temperature of the liquid increases. Heating a liquid always increases its vapor

pressure. When a liquid is heated to a sufficiently high temperature under a given applied

(usually atmospheric) pressure, bubbles of vapor begin to form below the surface. If the

vapor pressure inside the bubbles is less than the applied pressure on the surface of the

liquid, the bubbles collapse as soon as they form. If the temperature is raised sufficiently,

the vapor pressure is high enough that the bubbles can persist, rise to the surface, and

burst, releasing the vapor into the air. This process is called boilingand is different from

evaporation. The boiling pointof a liquid is the temperature at which its vapor pressure

equals the external pressure. The normal boiling pointis the temperature at which the

vapor pressure of a liquid is equal to exactly one atmosphere (760 torr). The vapor pres-

sure of water is 760 torr at 100°C, its normal boiling point. As heat energy is added to a

pure liquid at its boiling point,the temperature remains constant, because the energy is

used to overcome the cohesive forces in the liquid to form vapor.

If the applied pressure is lower than 760 torr, say on the top of a mountain, water boils

below 100°C. The chemical reactions involved in cooking food occur more slowly at the

lower temperature, so it takes longer to cook food in boiling water at high altitudes than

at sea level. A pressure cooker cooks food rapidly because water boils at higher tempera-

tures under increased pressures. The higher temperature of the boiling water increases

the rate of cooking.

Different liquids have different cohesive forces, so they have different vapor pressures

and boil at different temperatures. A mixture of liquids with sufficiently different boiling

points can often be separated into its components by distillation.In this process the

mixture is heated slowly until the temperature reaches the point at which the most volatile

liquid boils off. If this component is a liquid under ordinary conditions, it is subsequently

recondensed in a water-cooled condensing column (Figure 13-14) and collected as a distil-

late. After enough heat has been added to vaporize all of the most volatile liquid, the

temperature again rises slowly until the boiling point of the next substance is reached, and

the process continues. Any nonvolatile substances dissolved in the liquid do not boil, but

remain in the distilling flask. Impure water can be purified and separated from its dissolved

salts by distillation. Compounds with similar boiling points, especially those that interact

13-8

As water is being heated, but before it
boils, small bubbles may appear in the
container. This is not boiling, but
rather the formation of bubbles of
dissolved gases such as CO 2 and O 2
whose solubilities in water decrease
with increasing temperature.

498 CHAPTER 13: Liquids and Solids

stronger lower

attractive mn vapor

forces pressure

increasing higher

temperature mn vapor

pressure





See the Saunders Interactive
General Chemistry CD-ROM,
Screen 13.10, Properties of Liquids (3):
Boiling Point.

Figure 13-14 A laboratory setup
for distillation. During distillation
of an impure liquid, nonvolatile
substances remain in the distilling
flask. The liquid is vaporized and
condensed before being collected in
the receiving flask. If any of the
substances are flammable, using an
open flame would be dangerous; in
such a case, another source of heat,
such as an electric heater, should be
used.