KINETIC–MOLECULAR DESCRIPTION
OF LIQUIDS AND SOLIDSThe properties listed in Table 13-1 can be qualitatively explained in terms of the kinetic–
molecular theory of Chapter 12. We saw in Section 12-13 that the average kinetic energy
of a collection of gas molecules decreases as the temperature is lowered. As a sample of
gas is cooled and compressed, the rapid, random motion of gaseous molecules decreases.
The molecules approach one another, and the intermolecular attractions increase. Even-
tually, these increasing intermolecular attractions overcome the reduced kinetic energies.
At this point condensation (liquefaction) occurs. The temperatures and pressures required
for condensation vary from gas to gas, because different kinds of molecules have different
attractive forces.
In the liquid state the forces of attraction among particles are great enough that disor-
dered clustering occurs. The particles are so close together that very little of the volume
occupied by a liquid is empty space. As a result, it is very hard to compress a liquid. Parti-
cles in liquids have sufficient energy of motion to overcome partially the attractive forces
among them. They are able to slide past one another so that liquids assume the shapes of
their containers up to the volume of the liquid.
Liquids diffuse into other liquids with which they are miscible.For example, when a
drop of red food coloring is added to a glass of water, the water becomes red throughout
after diffusion is complete. The natural diffusion rate is slow at normal temperatures.
Because the average separations among particles in liquids are far less than those in gases,
the densities of liquids are much higher than the densities of gases (Table 12-1).
Cooling a liquid lowers its molecular kinetic energy and causes its molecules to slow
down even more. If the temperature is lowered sufficiently, at ordinary pressures, stronger
but shorter-range attractive interactions overcome the reduced kinetic energies of the
molecules to cause solidification.The temperature required for crystallizationat a given pres-
sure depends on the nature of short-range interactions among the particles and is
characteristic of each substance.
Most solids have ordered arrangements of particles with a very restricted range of
motion. Particles in the solid state cannot move freely past one another so they only vibrate
about fixed positions. Consequently, solids have definite shapes and volumes. Because the
particles are so close together, solids are nearly incompressible and are very dense rela-
tive to gases. Solid particles do not diffuse readily into other solids. However, analysis of13-1
Figure 13-2 A representation of
diffusion in solids. When blocks of
two different metals are clamped
together for a long time, a few
atoms of each metal diffuse into the
other metal.
486 CHAPTER 13: Liquids and Solids
Lead CopperIntermolecular attractions are those
between different molecules or ions.
Intramolecular attractions are those
between atoms within a single
molecule or ion.
The miscibilityof two liquids refers
to their ability to mix and produce a
homogeneous solution.
Solidificationand crystallizationrefer to
the process in which a liquid changes
to a solid.