Section 2.3
2.3 Chemical Properties
1 FOCUS
Objectives
2.3.1
2.3.2
2.3.3
Describe chemical properties
of matter.
Describe clues that indicate
that a chemical change is
taking place.
Distinguish chemical changes
from physical changes.
Key Concepts
Vocabulary
When can chemical
properties be observed?
◆
What observations might
indicate that a chemical
change has occurred?
◆
◆
◆
◆
chemical property
flammability
reactivity
chemical change
precipitate
What is the difference
between chemical and
physical changes?
Reading Strategy
Relating Text and Visuals Copy the table.
Find examples of the clues for recognizing
chemical changes in Figures 19 and 20.
Clue
Example
Change in color
a.
?
Production of gas
b.
?
Formation of precipitate
c.
?
Reading Focus
Build Vocabulary
L2
H
Word-Part Analysis Have students
look up the words flammable, inflammable, and nonflammable. Have them find
two meanings of the prefix in- to explain
why the terms are confusing. Ask them to
consider why it is very important to know
what each term means. (Inflammable and
flammable are synonyms. The prefix in- can
mean “not,” or “into” as in “burst into
flames.” It is important to use the correct
meanings when writing or interpreting fire
safety warnings.)
Reading Strategy
ow would you describe candles like the ones in Figure 16? You
might describe their color or their hardness. Or, you might observe
that the candles float on water, and conclude that the density of candle
wax is less than that of water. Color, hardness, and density are physical properties. But there is something else about the candles that may
seem even more obvious to you: the fact that the candles are burning.
The ability to burn is not a physical property because you cannot
observe burning without changing the composition of the material
that is burning. As a candle burns, new substances form.
Observing Chemical Properties
L2
a. Copper roof changing color from red
to green when exposed to moist air
b. Formation of carbon dioxide gas
when vinegar is added to baking soda
c. Formation of cottage cheese curds
when acid is added to milk
Figure 16 When candles burn,
new substances form. The ability
to burn is a chemical property.
2 INSTRUCT
Flammability Materials that burn can be used as fuel. Sometimes,
people use burning newspapers to start a fire in a fireplace. Gasoline is
the fuel burned in most automobiles. The chemical property that paper
and gasoline share is their flammability. Flammability is a material’s
ability to burn in the presence of oxygen.
Observing Chemical
Properties
Build Reading Literacy
L1
Preview Refer to page 36D in this
chapter, which provides the guidelines
for previewing.
Before they read the section, have
students skim the headings, visuals, and
boldfaced material to preview how the
text is organized.
Visual
FYI
If the combustion of paraffin is not
complete, carbon monoxide will form.
Combustion as a type of reaction is
addressed in Chapter 7. Combustion
of fossil fuels is discussed in Chapter 9.
54 Chapter 2
Most candles are made from paraffin, which is a mixture of compounds containing carbon and hydrogen. As a candle burns, the
compounds combine with oxygen in the air to form water and carbon
dioxide. (Carbon dioxide is the gas that gives a carbonated beverage its
fizz.) The ability to burn is a chemical property. A chemical property
is any ability to produce a change in the composition of matter.
Chemical properties can be observed only when the substances
in a sample of matter are changing into different substances.
Flammability and reactivity are two examples of chemical properties.
54
Chapter 2
Section Resources
Print
• Laboratory Manual, Investigation 2A
• Guided Reading and Study Workbook
With Math Support, Section 2.3
• Transparencies, Section 2.3
Technology
• iText, Section 2.3
• Presentation Pro CD-ROM, Section 2.3
• Go Online, NSTA SciLinks, Chemical and
physical changes
Sometimes flammability is not a desirable property. For example,
there are laws that regulate the flammability of fabrics. The fabrics used
in children’s sleepwear must have a low flammability. These fabrics are
described as flame-resistant because they are difficult to ignite. If they
do ignite, they burn slowly.
Reactivity The property that describes how readily a substance
combines chemically with other substances is reactivity. Nitrogen and
oxygen are the main gases in air. Oxygen is a highly reactive element.
Nitrogen has an extremely low reactivity.
Oxygen reacts easily with most other elements. Figure 17 shows
the rust that forms when oxygen reacts with iron and water. Rust is a
brittle, reddish-brown compound. A rusty chain or bolt is more likely
to break than a new chain or bolt because rust is weaker than iron.
Because iron is highly reactive, you would not choose iron to make
decorative objects, such as jewelry or coins.
Many uses of nitrogen depend on its low reactivity. For example,
seawater is often stored in steel tanks located below the lowest deck of
a ship. The seawater helps to keep the ship stable in the water. Over
time, rust forms in the tanks because iron in the steel reacts with
oxygen dissolved in the water. Researchers in Japan have developed a
way to reduce the amount of rust produced. They pump nitrogen gas
into the tanks, and the nitrogen displaces the dissolved oxygen.
Which element is more reactive—oxygen or nitrogen?
Oxygen Is Needed
For: Links on chemical and
physical changes
Visit: www.SciLinks.org
Web Code: ccn-1023
L2
Purpose Students observe that air is
needed for a candle to burn.
Materials 2 small candles, matches,
large beaker
Procedure Tell students that the
flammability of a material is tested in air.
Light two candles with a match. Carefully place a large beaker over one of the
candles and have students observe what
happens. Remove the beaker and relight
the candle to show that it can still burn.
Discuss how a scientist might test the
hypothesis that oxygen is the gas in air
that supports burning.
Figure 17 This automobile must
have been exposed to air and
water for many years.
Drawing Conclusions What
evidence is there that parts of the
automobile contained iron?
Expected Outcome After the beaker
is placed over the candle, the flame dies
out. The other candle continues to burn.
To demonstrate that oxygen is the gas
that supports burning, the scientist
could try burning the candle in pure
samples of the gases in air.
Visual, Logical
Build Science Skills
L2
Designing Experiments Challenge
students to describe a material that
they could test for flammability without
changing its composition. (The material
would have to be nonflammable.) Help
students by asking them how they
would test the material for flammability
and what the results of the test would
be. Explain that the absence of an
observed event is useful data.
Verbal
Properties of Matter 55
Customize for English Language Learners
Paired Brainstorming
Have students work in pairs to think of other
items—besides cars, chains, or bolts—that rust.
Examples include bicycle frames, door and
gate hinges, and old nails. Strengthen discussion skills by having students share their
examples with the class. Encourage them to
comment on what these items have in
common. Although many items made from
iron are coated with paint to prevent rusting,
paint does chip and items containing iron
will rust if left outdoors unprotected from
the elements.
Download a worksheet on
chemical and physical changes for
students to complete, and find
additional teacher support from
NSTA SciLinks.
Answer to . . .
Figure 17 The amount of rust is
evidence that much of the automobile
was made of iron.
Oxygen
Properties of Matter 55
Section 2.3 (continued)
Recognizing Chemical
Changes
Identifying a Chemical Change
Materials
4. Add 5 mL of calcium chloride solution to test
tube A. Add 5 mL of sodium chloride solution
to test tube B. Add 5 mL of copper sulfate
solution to test tube C.
3 test tubes; test-tube rack; glass-marking pencil;
3 10-mL graduated cylinders; solutions of copper
sulfate, calcium chloride, and sodium chloride
Identifying a
Chemical Change
5. Examine the test tubes for evidence of a
chemical change. Record your observations.
Procedure
L2
1. Construct a data table with columns labeled
Test Tube, Contents, and Observations.
Objective
After completing this activity, students
will be able to
• recognize evidence of a chemical
change.
2. Label the test tubes A, B, and C.
3. Pour 5 mL of copper sulfate solution into test
tube A. Pour 5 mL of calcium chloride solution
into test tube B. Pour 5 mL of sodium chloride
solution into test tube C. CAUTION Do not
allow the solutions to touch your skin. They may
cause irritation.
Skills Focus Observing, Drawing
Conclusions
Analyze and Conclude
1. Inferring In which test tube(s) did a
chemical change occur? Explain your answer.
2. Evaluating Can you be sure that a chemical
change occurred? Explain your answer.
Prep Time 20 minutes
Advance Prep To prepare 1-M
solutions of calcium chloride, sodium
chloride, and copper sulfate, dissolve
1 mole of each salt (111.0 g CaCl2;
58.5 g NaCl; or 249.69 g CuSO4•5H2O)
in approximately 800 mL of distilled or
deionized water in a 1-L volumetric
flask. Once a salt has dissolved, add
enough water to make a 1000-mL
solution. Stir the solution. Provide each
group with 25 mL of each solution in a
wash bottle, which is the easiest container for students to handle. Be sure to
label each container with its contents.
6. Pour the contents of the test tubes into the
sink. Rinse out the test tubes and flush the
contents down the drain. CAUTION Wash
your hands thoroughly with soap or detergent
before leaving the laboratory.
Recognizing Chemical Changes
Figure 18 As a banana ripens,
chemical changes cause the peel
to change color from green to
yellow. In a banana that is overly
ripe, different chemical changes
cause the peel to turn brown.
Observing Based on your
experience, what other
properties of a banana change
as it ripens?
Figure 18 shows what happens to banana peels as bananas ripen. The
color change in a banana peel is caused by chemical changes that are
taking place in the cells of the banana. A chemical change occurs when
a substance reacts and forms one or more new substances. Chemical
changes occur when a cake bakes in an oven, leaves on trees change
color, and food is digested in your stomach.
How can you recognize a chemical change? You have to look for
clues. For example, when food spoils, it often gives off an unpleasant
odor.
Three common types of evidence for a chemical change
are a change in color, the production of a gas, and the formation of
a precipitate.
Class Time 20 minutes
Safety Remind students to wash their
hands thoroughly at the end of the lab, to
be careful when handling glassware, and
to dispose properly of chemical wastes.
Teaching Tips
• Review how to pour a liquid from a
graduated cylinder and accurately
read the volume.
Expected Outcome A precipitate of
calcium sulfate will form in test tube A.
The other two combinations will show
no evidence of a chemical change.
Analyze and Conclude
1. Formation of a precipitate indicated
that a chemical change occurred in test
tube A.
2. No, formation of a precipitate is
evidence for a chemical change, but
it is not conclusive.
Visual, Logical
56 Chapter 2
56
Chapter 2
Facts and Figures
Banana Science There are four stages of
banana development: growth, maturation,
ripening, and senescence. During ripening,
bananas produce increased amounts of ethylene
gas, which triggers chemical changes. The peel
changes color and becomes more permeable,
the pulp softens, starches break down into
sugar, and compounds responsible for flavor
and aroma are produced. Once ripening begins,
the process cannot be stopped. Ripening occurs
whether the banana is attached or detached
from the parent plant.
L1
Use Visuals
A Change in Color Over time, a shiny silver bracelet that is
exposed to air will darken. As a match burns, it shrivels up and turns
black. The new copper roof and the old copper roof in Figure 19 have
different colors. In each of these examples, a change in color is a clue
that a chemical change has produced at least one new substance.
Production of a Gas Figure 20A shows what happens when
you mix vinegar with baking soda. Bubbles of carbon dioxide form
immediately. A similar chemical change happens when you use baking
powder as an ingredient in a cake recipe. Baking powder is a mixture
of baking soda and one or more acids that react when wet. As the cake
bakes, the bubbles of carbon dioxide expand and cause the cake to rise.
Figure 19 When copper is
exposed to moist air, it forms a
thin coating called a patina.
A new copper roof has a reddish
color. The green patina on an old
copper roof is a mixture of
copper compounds.
Predicting Would a patina form
faster in a rainy climate or in a
dry climate?
Figure 20 Emphasize that the best
way to determine if a chemical change
is taking place is to compare materials
before and after the reaction. Ask,
Describe baking soda and vinegar
before they are mixed. (Baking soda is a
white solid, and vinegar is a clear solution.)
What clue indicates that a chemical
change is taking place? (There is
production of a gas.) When you see no
obvious clues to a chemical change,
can you be certain that a chemical
change has not occurred? (No)
Visual
Formation of a Precipitate Another chemical change you
can observe in the kitchen is the curdling of milk. If you add lemon juice
or vinegar to milk, small bits of white solid will separate from the liquid.
Any solid that forms and separates from a liquid mixture is called a
precipitate. When an acid is added to milk, proteins in the milk undergo
a chemical change that alters their structure, causing them to stick
together in clumps. They form the precipitate shown in Figure 20B.
What happens when you add vinegar to
baking soda?
A
Figure 20 The formation of a gas or
a precipitate can be a clue to chemical
change. A Carbon dioxide gas forms
when vinegar is mixed with baking
soda. B The curds in cottage cheese
form when an acid is added to milk.
B
Properties of Matter 57
Facts and Figures
Acids in Action When acid is added to milk,
one of the proteins in milk (casein) coagulates.
The milk may thicken (as in buttermilk or
yogurt) or separate into solid curds and liquid
whey (as in cottage cheese). This process is
called curdling. Acids may be added directly to
the milk or produced by bacteria. Adding rennin
(a coagulating enzyme) produces larger curds.
Single-action baking powder contains three
dry ingredients: an acid (e.g., cream of tartar),
a base (e.g., baking soda), and filler (e.g.,
cornstarch). When water is added, the acid
and base react. Double-action baking powders
contain two acids, one that reacts faster than
the other. Recipes that use baking soda have
another ingredient that provides the acid.
Answer to . . .
Figure 18 Students may say that the
banana becomes softer and that its
odor increases.
Figure 19 In a rainy climate
Bubbles of carbon
dioxide form.
Properties of Matter 57
Section 2.3 (continued)
Figure 21 A blacksmith uses a
hammer to shape a horseshoe
that has been heated. Although
the color of the iron horseshoe
changes, no chemical change is
occurring. Inferring Other than
color, what physical property of
iron is affected by heating?
Is a Change Chemical
or Physical?
L2
Students often have trouble distinguishing chemical from physical changes.
Explain that the composition of some
substances must change during a chemical change, but the composition of all
substances must remain the same
during a physical change.
Verbal
Is a Change Chemical or Physical?
It is not always easy to distinguish a chemical change from a physical
change. Even if you observe a color change, a gas, or a precipitate, you
cannot be sure that a chemical change has taken place. When the iron
horseshoe in Figure 21 is heated, its color changes from gray to red.
Despite this change in color, the iron is still iron. When water boils on
a stove, the bubbles of gas that rise to the surface are still water.
Before you decide whether or not a chemical change has occurred,
ask yourself this question: Are different substances present after the
change takes place? If not, then the change is physical, not chemical.
When matter undergoes a chemical change, the composition of
the matter changes. When matter undergoes a physical change, the
composition of the matter remains the same.
FYI
The release of heat or light is sometimes
listed as a clue for a chemical change, but
energy changes occur with both physical
and chemical changes. Exothermic and
endothermic changes are discussed in
Sections 3.3 (Phase Changes) and 7.3
(Energy Changes in Reactions).
3 ASSESS
Evaluate
Understanding
Section 2.3 Assessment
L2
Reviewing Concepts
Have students list three clues that indicate that a chemical change is taking
place and give an example of each. Then,
have them exchange their work with a
partner and discuss examples of physical
changes that also exhibit these clues.
Reteach
1.
2.
3.
4.
L1
5.
Use Figures 18, 19, and 20 to review
the clues that indicate that a chemical
change is taking place.
Under what conditions can chemical
properties be observed?
List three common types of evidence for a
chemical change.
How do chemical changes differ from
physical changes?
Explain why the rusting of an iron bar
decreases the strength of the bar.
A pat of butter melts and then burns in a hot
frying pan. Which of these changes is physical
and which is chemical?
Critical Thinking
6. Comparing and Contrasting Compare
the properties of a raw egg to those of a
hard-boiled egg.
Students might argue that observation
and experimentation are the key steps
in a scientific method for determining
whether a change is physical or chemical.
If your class subscribes to
iText, use it to review key concepts in
Section 2.3.
Answer to . . .
Figure 21 Malleability increases.
58 Chapter 2
58
7. Classifying If you spill household bleach on
denim jeans, you will observe that the area of
the spill no longer has a blue color. Is this
change chemical or physical? Give a reason for
your answer.
8. Inferring Gold and platinum are often used
to make jewelry. What can you infer about the
reactivity of these elements?
Scientific Methods Section 1.2 listed
typical steps scientists use. Which steps
might you use to decide whether a change
is physical or chemical? Explain.
Chapter 2
Section 2.3
Assessment
1. When the substances in a sample of matter
are changing into different substances
2. A change in color, the production of a gas,
or the formation of a precipitate
3. A chemical change produces new
substances. A physical change produces no
new substances.
4. When iron rusts, the compound that forms
is brittle.
5. Melting is a physical change. Burning is a
chemical change.
6. The raw egg is a viscous liquid. The egg
white is cloudy and the yolk is orange. The
hard-boiled egg is a soft solid. The egg white
is opaque and the yolk is yellow.
7. Because the bleach has caused the denim
to change color, the change is likely to be a
chemical change.
8. Because jewelry is designed to last a long
time, the reactivity of gold and platinum must
be relatively low.
What Should Be Done With
Arsenic-Treated Wood?
Termites are among the organisms that attack untreated wood. In 1950,
United States suppliers of lumber began to treat wood with a mixture of
copper, chromium, and arsenic (CCA). The mixture slows the damage to
wood by poisoning the attacking organisms. Outdoor structures such as
decks and porches were made from CCA-treated wood. By 2002, more than
95 percent of treated lumber sold for use outdoors contained CCA.
Arsenic is a poison that has been linked with certain types of cancer.
Arsenic can be absorbed through the skin or ingested in water or food.
Consumers were especially concerned about children touching CCA-treated
wood and then placing their unwashed fingers in their mouths. Some
consumers asked the Environmental Protection Agency (EPA) to ban the use
of CCA. In 2002, the chemical and home-improvement industries agreed to
stop producing CCA-treated wood for home use.
The Viewpoints
Old CCA-Treated Wood Does
Not Need to Be Removed
Old CCA-Treated Wood
Needs to Be Removed
The EPA did not recommend tearing down existing
structures made from CCA-treated wood. A panel
of Florida doctors reported that they found no
studies linking cancer in children and exposure to
CCA-treated wood. They concluded that the
amount of arsenic that children could absorb from
treated wood is small compared to the amount of
arsenic that occurs naturally in soil.
A report produced by one state’s Department of
Environmental Protection concluded that there is a
serious risk for children exposed to arsenic from
treated wood. The level of arsenic remains high for
20 years in CCA-treated wood. The useful life of
most treated wood products is about 20 years.
An area of CCA-treated wood the size of a
four-year-old’s hand contains about 120 times the
amount of arsenic allowed in a 6-ounce glass of
water. (The EPA limit for arsenic in drinking water is
10 parts per billion.) Rainwater penetrates wood
and dissolves arsenic. The arsenic ends up on the
surface of the wood or in the soil near the wood.
There are risks associated with disposing of
treated wood. Burning arsenic-treated wood
produces ash with high levels of arsenic. The ash
would poison a person who inhaled, ingested, or
touched it. The only acceptable method of disposal
is in landfills, which are rapidly filling. Also, arsenic
from landfills can end up in groundwater.
What Should Be Done With
L2
Arsenic-Treated Wood?
Background
Woods that are naturally resistant to
attack, such as cedar and redwood,
tend to be more expensive than timber
treated with CCA. Alternatives to
waterborne preservatives such as CCA
included creosote and pentachlorophenol, or penta. Creosote is a smelly
mixture of chemicals distilled from coal
tar. Railroads began to treat railroad ties
with creosote in 1889. Penta was
generally applied in a 5% solution of
petroleum solvents. Its use became
limited after the price of oil rose in the
1970s. Because there are health risks
associated with both penta and
creosote, their use has been restricted
since 1986.
After students answer Question 3, ask,
What evidence did the opponents and
supporters of CCA removal have that
you do not have? Explain how that
evidence might have influenced your
opinion. (The viewpoints as presented
were based on opposing interpretations of
scientific studies. Being able to see the actual
data might have influenced their opinions.)
Research and Decide
1. Defining the Issue In your own words,
describe the issue that needs to be resolved about
existing structures made from CCA-treated wood.
3. Forming Your Opinion Should existing structures built from CCA-treated wood be removed?
Which argument did you find most convincing?
2. Analyzing the Viewpoints List three
arguments of those who don’t think that existing
structures made from CCA-treated wood need to
be removed. List three arguments of those who
want to remove existing structures made from
CCA-treated wood.
Have students further research the
issues related to this topic.
For: More on this issue
Visit: PHSchool.com
Web Code: cch-1020
Properties of Matter 59
Answers
1. Are structures built from CCA-treated wood
enough of a health hazard for children that the
structures should be removed?
2. Old CCA-Treated Wood Does Not Need to
Be Removed: The EPA has not recommended
removal of the structures. No studies link cancer
in children and exposure to CCA-treated wood.
There are serious risks associated with disposal of
the wood.
Old CCA-Treated Wood Needs to Be Removed:
The level of arsenic remains high over the
lifetime of the wood. The level of arsenic in the
wood is much higher than the level allowed in
water. Rain causes arsenic to collect on the
surface of the wood or in soil.
3. Students should provide a reason for their
decision.
Properties of Matter 59