Introductory Chemistry: A Foundation - Zumdahl S.S., DeCoste D.J. 2019

Chemical Reactions: An Introduction
Evidence for a Chemical Reaction


Steven Baranek/

Lightning in Reno, Nevada.

Chemistry is about change. Grass grows. Steel rusts. Hair is bleached, dyed, “permed,” or straightened. Natural gas burns to heat houses. Nylon is produced for jackets, swimsuits, and pantyhose. Water is decomposed to hydrogen and oxygen gas by an electric current. Grape juice ferments in the production of wine. The bombardier beetle concocts a toxic spray to shoot at its enemies (see the “Chemistry in Focus” segment The Beetle That Shoots Straight).


© Ignacio Soto

Nylon is a strong material that makes sturdy parasails.

These are just a few examples of chemical changes that affect each of us. Chemical reactions are the heart and soul of chemistry, and in this chapter we will discuss the fundamental ideas about chemical reactions.


Kim Steele/Photodisc/Getty Images

© Cengage Learning

Production of plastic film for use in containers such as soft drink bottles (left). Nylon being drawn from the boundary between two solutions containing different reactants (right).

Evidence for a Chemical Reaction


· To learn the signals that show a chemical reaction has occurred.

How do we know when a chemical reaction has occurred? That is, what are the clues that a chemical change has taken place? A glance back at the processes in the introduction suggests that chemical reactions often give a visual signal. Steel changes from a smooth, shiny material to a reddish-brown, flaky substance when it rusts. Hair changes color when it is bleached. Solid nylon is formed when two particular liquid solutions are brought into contact. A blue flame appears when natural gas reacts with oxygen. Chemical reactions, then, often give visual clues: a color changes, a solid forms, bubbles are produced (Fig. 6.1), a flame occurs, and so on. However, reactions are not always visible. Sometimes the only signal that a reaction is occurring is a change in temperature as heat is produced or absorbed (Fig. 6.2).

Figure 6.1.A photo shows a glass beaker with a battery immersed in the water. An illustration zoomed from one terminal of the battery represents oxygen gas that shows diatomic molecules of oxygen. An illustration zoomed from other terminal of the battery represents hydrogen gas that shows diatomic molecules of hydrogen.

© Cengage Learning

Bubbles of hydrogen and oxygen gas form when an electric current is used to decompose water.

Figure 6.2.image

no limit pictures/

© Cengage Learning

Table 6.1 summarizes common clues to the occurrence of a chemical reaction, and Fig. 6.3 gives some examples of reactions that show these clues.

Table 6.1. Some Clues That a Chemical Reaction Has Occurred


The color changes.


A solid forms.


Bubbles form.


Heat and/or a flame is produced, or heat is absorbed.

Figure 6.3.

© Cengage Learning