What are chemical changes?
Some changes in matter change the identity of the
substance. A chemical change is a change in matter in which the
substances that make up that matter change into substances with
different chemical and physical properties. Another name for a
chemical change is a chemical reaction. In a chemical
reaction, the particles that make up two or more substances
react, or combine, with each other and form a new
substance.
Signs of a Chemical Change
Key Concept Check
A chemical change causes changes in physical properties.
For many chemical reactions, changes in color, density, or
state of matter are signs that a chemical change has taken
place. The only sure sign of a chemical reaction is the
formation of a new substance.
3. Describe how chemical
changes are different from
physical changes.
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
Formation of Gas When a liquid boils, the bubbles show
that it is changing into a gas. This is a physical change.
When you add a medicine tablet to water, gas bubbles form.
The water does not boil, so the gas must be a new substance
formed by a chemical reaction.
Reading Check
Formation of a Precipitate Some chemical reactions result in
4. Describe how you can
a solid forming when two liquids combine. This solid is
called a precipitate (prih SIH puh tut). A precipitate is not
the result of a change in state from a liquid to a solid. It is a
chemical change. A precipitate forms when the particles that
were dissolved in the two liquids react to form the particles
of the solid, a new substance.
tell whether bubbles are a
result of a chemical change
or a physical change.
Color Change A change in color is another sign of a chemical
change. If you painted the walls of your room a different
color, this would not be a chemical change. But if a substance
changes color, it is a sign that a chemical change has taken
place. When you roast marshmallows or leave an apple slice
on a plate, the change in color is a sign that there has been a
chemical change.
lm
0
06 81
0 1
0 41
00121
0
0 8
04 6
02
0
Visual Check
5. Explain In each of the
images to the left, what new
substance can you see that
has formed?
150 ml
135
120
105
90
75
60
45
30
15
0
Reading Essentials
Matter: Properties and Changes
201