2.3 Chemical Properties

Reading StrategyRelating Text and Visuals Copy the table.Find examples of the clues for recognizingchemical changes in Figures 19 and 20.

Key ConceptsWhen can chemicalproperties be observed?

What observations mightindicate that a chemicalchange has occurred?

What is the differencebetween chemical andphysical changes?

Vocabulary◆ chemical property◆ flammability◆ reactivity◆ chemical change◆ precipitate

How would you describe candles like the ones in Figure 16? Youmight describe their color or their hardness. Or, you might observethat the candles float on water, and conclude that the density of candlewax is less than that of water. Color, hardness, and density are physi-cal properties. But there is something else about the candles that mayseem even more obvious to you: the fact that the candles are burning.The ability to burn is not a physical property because you cannotobserve burning without changing the composition of the materialthat is burning. As a candle burns, new substances form.

Observing Chemical PropertiesMost candles are made from paraffin, which is a mixture of com-pounds containing carbon and hydrogen. As a candle burns, thecompounds combine with oxygen in the air to form water and carbondioxide. (Carbon dioxide is the gas that gives a carbonated beverage itsfizz.) The ability to burn is a chemical property. A chemical propertyis any ability to produce a change in the composition of matter.

Chemical properties can be observed only when the substancesin a sample of matter are changing into different substances.Flammability and reactivity are two examples of chemical properties.

Flammability Materials that burn can be used as fuel. Sometimes,people use burning newspapers to start a fire in a fireplace. Gasoline isthe fuel burned in most automobiles. The chemical property that paperand gasoline share is their flammability. Flammability is a material’sability to burn in the presence of oxygen.

Change in color

Production of gas

Formation of precipitate

Clue Example

a. ?

b. ?

c. ?

54 Chapter 2

Figure 16 When candles burn,new substances form. The abilityto burn is a chemical property.

FOCUS

Objectives2.3.1 Describe chemical properties

of matter.2.3.2 Describe clues that indicate

that a chemical change istaking place.

2.3.3 Distinguish chemical changesfrom physical changes.

Build VocabularyWord-Part Analysis Have studentslook up the words flammable, inflam-mable, and nonflammable. Have them findtwo meanings of the prefix in- to explainwhy the terms are confusing. Ask them toconsider why it is very important to knowwhat each term means. (Inflammable andflammable are synonyms. The prefix in- canmean “not,” or “into” as in “burst intoflames.” It is important to use the correctmeanings when writing or interpreting firesafety warnings.)

Reading Strategya. Copper roof changing color from redto green when exposed to moist airb. Formation of carbon dioxide gaswhen vinegar is added to baking sodac. Formation of cottage cheese curdswhen acid is added to milk

INSTRUCT

Observing ChemicalPropertiesBuild Reading LiteracyPreview Refer to page 36D in thischapter, which provides the guidelinesfor previewing.

Before they read the section, havestudents skim the headings, visuals, andboldfaced material to preview how thetext is organized.Visual

FYIIf the combustion of paraffin is notcomplete, carbon monoxide will form.Combustion as a type of reaction isaddressed in Chapter 7. Combustionof fossil fuels is discussed in Chapter 9.

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Reading Focus

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Section 2.3

Print• Laboratory Manual, Investigation 2A• Reading and Study Workbook With

Math Support, Section 2.3• Transparencies, Section 2.3

Technology• Interactive Textbook, Section 2.3• Presentation Pro CD-ROM, Section 2.3• Go Online, NSTA SciLinks, Chemical and

physical changes

Section Resources

54 Chapter 2

Properties of Matter 55

Sometimes flammability is not a desirable property. For example,there are laws that regulate the flammability of fabrics. The fabrics usedin children’s sleepwear must have a low flammability. These fabrics aredescribed as flame-resistant because they are difficult to ignite. If theydo ignite, they burn slowly.

Reactivity The property that describes how readily a substancecombines chemically with other substances is reactivity. Nitrogen andoxygen 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 showsthe rust that forms when oxygen reacts with iron and water. Rust is abrittle, reddish-brown compound. A rusty chain or bolt is more likelyto break than a new chain or bolt because rust is weaker than iron.Because iron is highly reactive, you would not choose iron to makedecorative 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 ofa ship. The seawater helps to keep the ship stable in the water. Overtime, rust forms in the tanks because iron in the steel reacts withoxygen dissolved in the water. Researchers in Japan have developed away to reduce the amount of rust produced. They pump nitrogen gasinto the tanks, and the nitrogen displaces the dissolved oxygen.

Which element is more reactive—oxygen or nitrogen?

Figure 17 This automobile musthave been exposed to air andwater for many years.Drawing Conclusions Whatevidence is there that parts of theautomobile contained iron?

For: Links on chemical andphysical changes

Visit: www.SciLinks.org

Web Code: ccn-1023

Oxygen Is NeededPurpose Students observe that air isneeded for a candle to burn.

Materials 2 small candles, matches,large beaker

Procedure Tell students that theflammability of a material is tested in air.Light two candles with a match. Care-fully place a large beaker over one of thecandles and have students observe whathappens. Remove the beaker and relightthe candle to show that it can still burn.Discuss how a scientist might test thehypothesis that oxygen is the gas in airthat supports burning.

Expected Outcome After the beakeris placed over the candle, the flame diesout. The other candle continues to burn.To demonstrate that oxygen is the gasthat supports burning, the scientistcould try burning the candle in puresamples of the gases in air. Visual, Logical

Build Science SkillsDesigning Experiments Challengestudents to describe a material that they could test for flammability withoutchanging its composition. (The materialwould have to be nonflammable.) Helpstudents by asking them how theywould test the material for flammabilityand what the results of the test wouldbe. Explain that the absence of anobserved event is useful data. Verbal

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Customize for English Language Learners

Paired Brainstorming Have students work in pairs to think of otheritems—besides cars, chains, or bolts—that rust.Examples include bicycle frames, door andgate hinges, and old nails. Strengthen dis-cussion skills by having students share theirexamples with the class. Encourage them to

comment on what these items have incommon. Although many items made fromiron 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 onchemical and physical changes forstudents to complete, and findadditional teacher support fromNSTA SciLinks.

Answer to . . .

Figure 17 The amount of rust isevidence that much of the automobilewas made of iron.

Oxygen

Properties of Matter 55

56 Chapter 2

Recognizing Chemical ChangesFigure 18 shows what happens to banana peels as bananas ripen. Thecolor change in a banana peel is caused by chemical changes that aretaking place in the cells of the banana. A chemical change occurs whena substance reacts and forms one or more new substances. Chemicalchanges occur when a cake bakes in an oven, leaves on trees changecolor, and food is digested in your stomach.

How can you recognize a chemical change? You have to look forclues. For example, when food spoils, it often gives off an unpleasantodor. Three common types of evidence for a chemical changeare a change in color, the production of a gas, and the formation ofa precipitate.

Identifying a Chemical Change

Materials3 test tubes; test-tube rack; glass-marking pencil;3 10-mL graduated cylinders; solutions of coppersulfate, calcium chloride, and sodium chloride

Procedure1. Construct a data table with columns labeled

Test Tube, Contents, and Observations.

2. Label the test tubes A, B, and C.

3. Pour 5 mL of copper sulfate solution into testtube A. Pour 5 mL of calcium chloride solutioninto test tube B. Pour 5 mL of sodium chloridesolution into test tube C. CAUTION Do notallow the solutions to touch your skin. They maycause irritation.

4. Add 5 mL of calcium chloride solution to testtube A. Add 5 mL of sodium chloride solutionto test tube B. Add 5 mL of copper sulfatesolution to test tube C.

5. Examine the test tubes for evidence of achemical change. Record your observations.

6. Pour the contents of the test tubes into thesink. Rinse out the test tubes and flush thecontents down the drain. CAUTION Washyour hands thoroughly with soap or detergentbefore leaving the laboratory.

Analyze and Conclude1. Inferring In which test tube(s) did a

chemical change occur? Explain your answer.

2. Evaluating Can you be sure that a chemicalchange occurred? Explain your answer.

Figure 18 As a banana ripens,chemical changes cause the peelto change color from green toyellow. In a banana that is overlyripe, different chemical changescause the peel to turn brown.Observing Based on yourexperience, what otherproperties of a banana changeas it ripens?

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Recognizing ChemicalChanges

Identifying aChemical Change

ObjectiveAfter completing this activity, studentswill be able to• recognize evidence of a chemical

change.

Skills Focus Observing, DrawingConclusions

Prep Time 20 minutes

Advance Prep To prepare 1-Msolutions of calcium chloride, sodiumchloride, 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 ordeionized water in a 1-L volumetricflask. Once a salt has dissolved, addenough water to make a 1000-mLsolution. Stir the solution. Provide eachgroup with 25 mL of each solution in awash bottle, which is the easiest con-tainer for students to handle. Be sure tolabel each container with its contents.

Class Time 20 minutes

Safety Remind students to wash theirhands thoroughly at the end of the lab, tobe careful when handling glassware, andto dispose properly of chemical wastes.

Teaching Tips• Review how to pour a liquid from a

graduated cylinder and accuratelyread the volume.

Expected Outcome A precipitate ofcalcium sulfate will form in test tube A.The other two combinations will showno evidence of a chemical change.

Analyze and Conclude1. Formation of a precipitate indicatedthat a chemical change occurred in testtube A.2. No, formation of a precipitate isevidence for a chemical change, but it is not conclusive. Visual, Logical

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Section 2.3 (continued)

Banana Science There are four stages ofbanana development: growth, maturation,ripening, and senescence. During ripening,bananas produce increased amounts of ethylenegas, which triggers chemical changes. The peelchanges color and becomes more permeable,

the pulp softens, starches break down intosugar, and compounds responsible for flavorand aroma are produced. Once ripening begins,the process cannot be stopped. Ripening occurswhether the banana is attached or detachedfrom the parent plant.

Facts and Figures

A Change in Color Over time, a shiny silver bracelet that isexposed to air will darken. As a match burns, it shrivels up and turnsblack. The new copper roof and the old copper roof in Figure 19 havedifferent colors. In each of these examples, a change in color is a cluethat a chemical change has produced at least one new substance.

Production of a Gas Figure 20A shows what happens whenyou mix vinegar with baking soda. Bubbles of carbon dioxide formimmediately. A similar chemical change happens when you use bakingpowder as an ingredient in a cake recipe. Baking powder is a mixtureof baking soda and one or more acids that react when wet. As the cakebakes, the bubbles of carbon dioxide expand and cause the cake to rise.

Formation of a Precipitate Another chemical change youcan observe in the kitchen is the curdling of milk. If you add lemon juiceor 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 aprecipitate. When an acid is added to milk, proteins in the milk undergoa chemical change that alters their structure, causing them to sticktogether in clumps. They form the precipitate shown in Figure 20B.

What happens when you add vinegar tobaking soda?

Figure 19 When copper isexposed to moist air, it forms athin coating called a patina. A new copper roof has a reddishcolor. The green patina on an oldcopper roof is a mixture ofcopper compounds. Predicting Would a patina formfaster in a rainy climate or in adry climate?

Figure 20 The formation of a gas ora precipitate can be a clue to chemicalchange. A Carbon dioxide gas formswhen vinegar is mixed with bakingsoda. B The curds in cottage cheeseform when an acid is added to milk.

AA

BB

Properties of Matter 57

Use VisualsFigure 20 Emphasize that the best way to determine if a chemical change is taking place is to compare materialsbefore and after the reaction. Ask,Describe baking soda and vinegarbefore they are mixed. (Baking soda is awhite solid, and vinegar is a clear solution.)What clue indicates that a chemicalchange is taking place? (There isproduction of a gas.) When you see noobvious clues to a chemical change,can you be certain that a chemicalchange has not occurred? (No)Visual

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Acids in Action When acid is added to milk,one of the proteins in milk (casein) coagulates.The milk may thicken (as in buttermilk oryogurt) or separate into solid curds and liquidwhey (as in cottage cheese). This process iscalled curdling. Acids may be added directly tothe 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 powderscontain two acids, one that reacts faster than the other. Recipes that use baking soda haveanother ingredient that provides the acid.

Facts and Figures

Answer to . . .

Figure 18 Students may say that thebanana becomes softer and that itsodor increases.

Figure 19 In a rainy climate

Bubbles of carbondioxide form.

58 Chapter 2

Section 2.3 Assessment

Reviewing Concepts1. Under what conditions can chemical

properties be observed?

2. List three common types of evidence for achemical change.

3. How do chemical changes differ fromphysical changes?

4. Explain why the rusting of an iron bardecreases the strength of the bar.

5. A pat of butter melts and then burns in a hotfrying pan. Which of these changes is physicaland which is chemical?

Critical Thinking6. Comparing and Contrasting Compare

the properties of a raw egg to those of ahard-boiled egg.

7. Classifying If you spill household bleach ondenim jeans, you will observe that the area ofthe spill no longer has a blue color. Is thischange chemical or physical? Give a reason foryour answer.

8. Inferring Gold and platinum are often usedto make jewelry. What can you infer about thereactivity of these elements?

Is a Change Chemical or Physical?It is not always easy to distinguish a chemical change from a physicalchange. Even if you observe a color change, a gas, or a precipitate, youcannot be sure that a chemical change has taken place. When the ironhorseshoe 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 ona 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 thechange takes place? If not, then the change is physical, not chemical.

When matter undergoes a chemical change, the composition ofthe matter changes. When matter undergoes a physical change, thecomposition of the matter remains the same.

Figure 21 A blacksmith uses ahammer to shape a horseshoethat has been heated. Althoughthe color of the iron horseshoechanges, no chemical change isoccurring. Inferring Other thancolor, what physical property ofiron is affected by heating?

Scientific Methods Section 1.2 listedtypical steps scientists use. Which stepsmight you use to decide whether a changeis physical or chemical? Explain.

58 Chapter 2

Is a Change Chemicalor Physical?

Students often have trouble distinguish-ing chemical from physical changes.Explain that the composition of somesubstances must change during a chem-ical change, but the composition of allsubstances must remain the sameduring a physical change.Verbal

FYIThe release of heat or light is sometimeslisted as a clue for a chemical change, butenergy changes occur with both physicaland chemical changes. Exothermic andendothermic changes are discussed inSections 3.3 (Phase Changes) and 7.3(Energy Changes in Reactions).

ASSESSEvaluateUnderstandingHave students list three clues that indi-cate that a chemical change is takingplace and give an example of each. Then,have them exchange their work with apartner and discuss examples of physicalchanges that also exhibit these clues.

ReteachUse Figures 18, 19, and 20 to reviewthe clues that indicate that a chemicalchange is taking place.

Students might argue that observationand experimentation are the key stepsin a scientific method for determiningwhether a change is physical or chemical.

If your class subscribes tothe Interactive Textbook, use it toreview key concepts in Section 2.3.

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Section 2.3 (continued)

5. Melting is a physical change. Burning is achemical change.6. The raw egg is a viscous liquid. The eggwhite is cloudy and the yolk is orange. Thehard-boiled egg is a soft solid. The egg whiteis opaque and the yolk is yellow.7. Because the bleach has caused the denimto change color, the change is likely to be achemical change.8. Because jewelry is designed to last a longtime, the reactivity of gold and platinum mustbe relatively low.

Section 2.3 Assessment

1. When the substances in a sample of matterare changing into different substances2. A change in color, the production of a gas,or the formation of a precipitate3. A chemical change produces newsubstances. A physical change produces nonew substances.4. When iron rusts, the compound that formsis brittle.Answer to . . .

Figure 21 Malleability increases.

Properties of Matter 59

What Should Be Done WithArsenic-Treated Wood?

Old CCA-Treated Wood Does Not Need to Be RemovedThe EPA did not recommend tearing down existingstructures made from CCA-treated wood. A panelof Florida doctors reported that they found nostudies linking cancer in children and exposure toCCA-treated wood. They concluded that theamount of arsenic that children could absorb fromtreated wood is small compared to the amount ofarsenic that occurs naturally in soil.

There are risks associated with disposing oftreated wood. Burning arsenic-treated woodproduces ash with high levels of arsenic. The ashwould poison a person who inhaled, ingested, ortouched it. The only acceptable method of disposalis in landfills, which are rapidly filling. Also, arsenicfrom landfills can end up in groundwater.

Old CCA-Treated Wood Needs to Be RemovedA report produced by one state’s Department ofEnvironmental Protection concluded that there is aserious risk for children exposed to arsenic fromtreated wood. The level of arsenic remains high for20 years in CCA-treated wood. The useful life ofmost 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 theamount of arsenic allowed in a 6-ounce glass ofwater. (The EPA limit for arsenic in drinking water is10 parts per billion.) Rainwater penetrates woodand dissolves arsenic. The arsenic ends up on thesurface of the wood or in the soil near the wood.

1. Defining the Issue In your own words,describe the issue that needs to be resolved aboutexisting structures made from CCA-treated wood.

2. Analyzing the Viewpoints List threearguments of those who don’t think that existingstructures made from CCA-treated wood need tobe removed. List three arguments of those whowant to remove existing structures made fromCCA-treated wood.

3. Forming Your Opinion Should existing struc-tures built from CCA-treated wood be removed?Which argument did you find most convincing?

Termites are among the organisms that attack untreated wood. In 1950,United States suppliers of lumber began to treat wood with a mixture ofcopper, chromium, and arsenic (CCA). The mixture slows the damage towood by poisoning the attacking organisms. Outdoor structures such asdecks and porches were made from CCA-treated wood. By 2002, more than95 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-treatedwood and then placing their unwashed fingers in their mouths. Someconsumers asked the Environmental Protection Agency (EPA) to ban the useof CCA. In 2002, the chemical and home-improvement industries agreed tostop producing CCA-treated wood for home use.

The Viewpoints

Research and Decide

For: More on this issueVisit: PHSchool.comWeb Code: cch-1020

What Should Be Done WithArsenic-Treated Wood?BackgroundWoods that are naturally resistant toattack, such as cedar and redwood, tend to be more expensive than timbertreated with CCA. Alternatives towaterborne preservatives such as CCAincluded creosote and pentachloro-phenol, or penta. Creosote is a smellymixture of chemicals distilled from coaltar. Railroads began to treat railroad tieswith creosote in 1889. Penta wasgenerally applied in a 5% solution ofpetroleum solvents. Its use becamelimited after the price of oil rose in the1970s. Because there are health risksassociated with both penta andcreosote, their use has been restrictedsince 1986.

After students answer Question 3, ask,What evidence did the opponents andsupporters of CCA removal have thatyou do not have? Explain how thatevidence might have influenced youropinion. (The viewpoints as presented were based on opposing interpretations ofscientific studies. Being able to see the actualdata might have influenced their opinions.)

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Properties of Matter 59

Answers1. Are structures built from CCA-treated woodenough of a health hazard for children that thestructures should be removed? 2. Old CCA-Treated Wood Does Not Need toBe Removed: The EPA has not recommendedremoval of the structures. No studies link cancerin children and exposure to CCA-treated wood.There are serious risks associated with disposal ofthe wood.

Old CCA-Treated Wood Needs to Be Removed:The level of arsenic remains high over thelifetime of the wood. The level of arsenic in thewood is much higher than the level allowed inwater. Rain causes arsenic to collect on thesurface of the wood or in soil. 3. Students should provide a reason for theirdecision.

Have students further research theissues related to this topic.