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Figure 12
When mercury(II) oxide is
heated, the powdery red solid reacts
to form silvery liquid mercury and
colorless oxygen gas. The sum of the
masses of mercury and oxygen
produced during the reaction equals
the mass of mercury(II) oxide.
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Summary
• A physical change alters the
physical properties of a sub-
stance without changing its
composition.
• A chemical change, also known
as a chemical reaction, involves
a change in a substance’s
composition.
• In a chemical reaction, reactants
form products.
• The law of conservation of mass
states that mass is neither
created nor destroyed during a
chemical reaction; it is
conserved.
Check Your Progress
Demonstrate Understanding
10.
Classify
each change as physical or chemical.
a.
crushing an aluminum can
b.
recycling used aluminum cans to make new ones
c. aluminum combining with oxygen to form
aluminum oxide
11.
Describe
the results of a physical change and list three
examples of physical change.
12.
Describe
the results of a chemical change. List four
indicators of chemical change.
13.
Calculate
Solve each of the following.
a.
If 22.99 g of sodium and 35.45 g of chlorine fully
react, how much sodium chloride forms?
b. A 12.2-g sample of
X
reacts with a sample of
Y
to
form 78.9 g of
XY
. What mass of
Y
reacted?
14.
Evaluate
A friend tells you, “Because composition
does not change during a physical change, the
appearance of a substance does not change.”
Is your friend correct? Explain.
French scientist Antoine Lavoisier (1743–1794) was one of the first to use an analytical
balance to monitor chemical reactions. He studied the thermal decomposition of
mercury(II) oxide, shown in
Figure 12.
The color change and production of a gas are
indicators of a chemical reaction. In a closed container, the oxygen gas cannot escape
and the mass before and after the reaction can be measured. The masses are the same.
The law of conservation of mass is one of the most fundamental concepts in the study
of chemistry and chemical reactions.
Lesson 2 • Changes in Matter
59
Stephen Frisch/McGraw-Hill Education