General Chemistry Principles, Patterns, and Applications, 2011

Chapter 11
Liquids
In Chapter 10 "Gases", you learned that attractive intermolecular forces cause most gases to condense to liquids at
high pressure, low temperature, or both. Substances that normally are liquids are held together by exactly the same
forces that are responsible for the liquefaction of gases. One such substance is water, the solvent in which all
biochemical reactions take place. Because of its thermal properties, water also modulates Earth’s temperature,
maintaining a temperature range suitable for life. Other liquids are used to manufacture objects that we use every
day—for example, a solid material is converted to a liquid, the liquid is injected into a mold, and it is then solidified
into complex shapes under conditions that are carefully controlled. To understand such processes, our study of the
macroscopic properties of matter must include an understanding of the properties of liquids and the interconversion
of the three states of matter: gases, liquids, and solids.
In this chapter, we look more closely at the intermolecular forces that are responsible for the properties of liquids,
describe some of the unique properties of liquids compared with the other states of matter, and then consider changes
in state between liquids and gases or solids. By the end of the chapter, you will understand what is happening at the
molecular level when you dry yourself with a towel, why you feel cold when you come out of the water, why ice is
slippery, and how it is possible to decaffeinate coffee without removing important flavor components. You will also
learn how liquid crystal display (LCD) devices in electronic devices function, and how adhesive strips used to measure
body temperature change color to indicate a fever.
11.1 The Kinetic Molecular Description of Liquids
L E A R N I N G O B JE C T I V E
1. To be familiar with the kinetic molecular description of liquids.
The kinetic molecular theory of gases described in Chapter 10 "Gases" gives a reasonably accurate description of the
behavior of gases. A similar model can be applied to liquids, but it must take into account the nonzero volumes of
particles and the presence of strong intermolecular attractive forces.
In a gas, the distance between molecules, whether monatomic or polyatomic, is very large compared with the size of
the molecules; thus gases have a low density and are highly compressible. In contrast, the molecules in liquids are
very close together, with essentially no empty space between them. As in gases, however, the molecules in liquids are
in constant motion, and their kinetic energy (and hence their speed) depends on their temperature. We begin our
discussion by examining some of the characteristic properties of liquids to see how each is consistent with a modified
kinetic molecular description.
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