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FIGURE 9.1 Water is the most abundant liquid on Earth and is
necessary for all life. Because of water’s great dissolving properties,
any sample is a solution containing solids, other liquids, and gases FIGURE 9.3 Magnesium is an alkaline earth metal that burns
from the environment. This stream also carries suspended, ground- brightly in air, releasing heat and light. As chemical energy is
up rocks, called rock flour, from a nearby glacier. released, new chemical substances are formed. One new chemi-
cal material produced here is magnesium oxide, a soft, powdery
material that forms an alkaline solution in water (called milk of
magnesia).
change, and the process is called a chemical reaction. A chemical
reaction is defined as
all involve chemical reactions. These reactions produce differ-
a change in matter in which different chemical substances ent chemical substances with greater or smaller amounts of in-
are created by breaking and/or forming chemical bonds. ternal potential energy (see chapter 4 for a discussion of internal
potential energy). Energy is absorbed to produce new chemi-
In general, chemical bonds are formed when atoms of elements cal substances with more internal potential energy. Energy is
are bound together to form compounds. Chemical bonds are released when new chemical substances are produced with less
broken when a compound is decomposed into simpler sub- internal potential energy (Figure 9.3). In general, changes in
stances. Chemical bonds are electrical in nature, formed by internal potential energy are called chemical energy. For ex-
electrical attractions, as discussed in chapter 6. ample, new chemical substances are produced in green plants
Chemical reactions happen all the time, all around you. A through the process called photosynthesis. A green plant uses
growing plant, burning fuels, and your body’s utilization of food radiant energy (sunlight), carbon dioxide, and water to produce
new chemical materials and oxygen. These new chemical mate-
rials, the stuff that leaves, roots, and wood are made of, contain
more chemical energy than the carbon dioxide and water they
Monatomic Diatomic were made from.
A chemical equation is a way of describing what happens
He O O H H
in a chemical reaction. Later, you will learn how to use formulas
A B in a chemical reaction. For now, the chemical reaction of photo-
synthesis will be described by using words in an equation:
Triatomic
energy carbon water plant oxygen
O O O (sunlight) + dioxide + molecules → material + molecules
molecules molecules
C
The substances that are changed are on the left side of the
FIGURE 9.2 (A) The noble gases are monatomic, occurring as
single atoms. (B) Many gases, such as hydrogen and oxygen, are di- word equation and are called reactants. The reactants are car-
atomic, with two atoms per molecule. (C) Ozone is a form of oxygen bon dioxide molecules and water molecules. The equation also
that is triatomic, occurring with three atoms per molecule. indicates that energy is absorbed, since the term energy appears

9-3 CHAPTER 9 Chemical Bonds 231