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OVERVIEW

Heat has been closely associated with the comfort and support of people throughout history. You can imagine the
appreciation when your earliest ancestors first discovered fire and learned to keep themselves warm and cook their
food. You can also imagine the wonder and excitement about 3000 B.c., when people put certain earthlike substances
on the hot, glowing coals of a fire and later found metallic copper, lead, or iron. The use of these metals for simple
tools followed soon afterward. Today, metals are used to produce complicated engines that use heat for transportation
and that do the work of moving soil and rock, construction, and agriculture. Devices made of heat-extracted metals
are also used to control the temperature of structures, heating or cooling the air as necessary. Thus, the production
and control of heat gradually built the basis of civilization today (Figure 4.1).
The sources of heat are the energy forms that you learned about in chapter 3. The fossil fuels are chemical
sources of heat. Heat is released when oxygen is combined with these fuels. Heat also results when mechanical
energy does work against friction, such as in the brakes of a car coming to a stop. Heat also appears when radiant
energy is absorbed. This is apparent when solar energy heats water in a solar collector or when sunlight melts snow.
The transformation of electrical energy to heat is apparent in toasters, heaters, and ranges. Nuclear energy provides
the heat to make steam in a nuclear power plant. Thus, all energy forms can be converted to heat.
The relationship between energy forms and heat appears to give an order to nature, revealing patterns that you
will want to understand. All that you need is some kind of explanation for the relationships—a model or theory that
helps make sense of it all. This chapter is concerned with heat and temperature and their relationship to energy. It
begins with a simple theory about the structure of matter and then uses the theory to explain the concepts of heat,
energy, and temperature changes.

Atoms are not divided, created, or destroyed during any type of
4.1 THE KINETIC MOLECULAR THEORY

chemical or physical change. There are similar groups of atoms

The idea that substances are composed of very small particles that make up the pure substances known as chemical elements.
can be traced back to certain early Greek philosophers. Th e Each element has its own kind of atom, which is diff erent from
earliest record of this idea was written by Democritus during the atoms of other elements. For example, hydrogen, oxygen,
the fi fth century b.c. He wrote that matter was empty space carbon, iron, and gold are chemical elements, and each has its

filled with tremendous numbers of tiny, indivisible particles own kind of atom.

called atoms. This idea, however, was not acceptable to most In addition to the chemical elements, there are pure sub-
of the ancient Greeks, because matter seemed continuous, and stances called compounds that have more complex units of

empty space was simply not believable. The idea of atoms was structure (Figure 4.2). Pure substances, such as water, sugar,
rejected by Aristotle as he formalized his belief in continuous and alcohol, are composed of atoms of two or more elements

matter composed of Earth, air, fire, and water elements. Aris- that join together in definite proportions. Water, for example,
totle’s belief about matter, like his beliefs about motion, pre- has structural units that are made up of two atoms of hydrogen

dominated through the 1600s. Some people, such as Galileo tightly bound to one atom of oxygen (H 2 O). These units are not
and Newton, believed the ideas about matter being composed of easily broken apart and stay together as small physical particles
tiny particles, or atoms, since this theory seemed to explain the of which water is composed. Each is the smallest particle of
behavior of matter. Widespread acceptance of the particle model water that can exist, a molecule of water. A molecule is generally

did not occur, however, until strong evidence was developed defined as a tightly bound group of atoms in which the atoms
through chemistry in the late 1700s and early 1800s. Th e experi- maintain their identity. How atoms become bound together to
ments finally led to a collection of assumptions about the small form molecules is discussed in chapters 8–10.

particles of matter and the space around them. Collectively, the Some elements exist as gases at ordinary temperatures,
assumptions could be called the kinetic molecular theory. Th e and all elements are gases at suffi ciently high temperatures.
following is a general description of some of these assumptions. At ordinary temperatures, the atoms of oxygen, nitrogen, and
other gases are paired in groups of two to form diatomic mol-
ecules. Other gases, such as helium, exist as single, unpaired
MOLECULES atoms at ordinary temperatures. At suffi ciently high tempera-

The basic assumption of the kinetic molecular theory is that all tures, iron, gold, and other metals vaporize to form gaseous,
matter is made up of tiny, basic units of structure called atoms. single, unpaired atoms. In the kinetic molecular theory, the

86 CHAPTER 4 Heat and Temperature 4-2