Solids, Liquids, and
Intermolecular Forces
IN THIS CHAPTER
Summary: In the chapter on Gases, we discussed the gaseous state. In this chapter,
we will discuss the liquid and solid states and the forces that exist between the
particles—the intermolecular forces. A substance’s state of matter depends on two
factors: the average kinetic energy of the particles, and the intermolecular forces
between the particles. The kinetic energy tends to move the particles away from
each other. The temperature of the substance is a measure of the average kinetic
energy of the molecules. As the temperature increases, the average kinetic energy
increases and the particles tend to move farther apart. This is consistent with our
experience of heating ice, for example, and watching it move from the solid state
to the liquid state and finally to the gaseous state. For this to happen, the kinetic
energy overcomes the forces between the particles, the intermolecular forces.
In the solid state, the kinetic energy of the particles cannot overcome the inter-
molecular forces; the particles are held close together by the intermolecular forces.
As the temperature increases, the kinetic energy increases and begins to overcome
the attractive intermolecular forces. The substance will eventually melt, going from
the solid to the liquid state. As this melting takes place, the temperature remains
constant even though energy is being added. The temperature at which the solid
converts into the liquid state is called the melting point (m.p.) of the solid.
After all the solid has been converted into a liquid, the temperature again
starts to rise as energy is added. The particles are still relatively close together,
but possess enough kinetic energy to move with respect to each other. Finally,
if enough energy is added, the particles start to break free of the intermolecu-
lar forces keeping them relatively close together and they escape the liquid as
essentially independent gas particles. This process of going from the liquid state
to the gaseous state is called boiling, and the temperature at which this occurs
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