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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
intermolecular forces; the particles are held close together by the intermolec-
ular 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 ofCHAPTER
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