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2.1 Classifying Matter

Reading StrategySummarizing Copy the diagram below. Asyou read, complete the classification of matter.

Key ConceptsWhy are elements andcompounds classified aspure substances?

How do mixtures differfrom pure substances?

What is the maindifference amongsolutions, suspensions,and colloids?

Vocabulary◆ pure substance◆ element◆ atom◆ compound◆ heterogeneous

mixture◆ homogeneous

mixture◆ solution◆ suspension◆ colloid

Each piece of clothing sold has a care label, which lists recommendedcleaning methods for the clothing. For a sweater, the instructionsmight say to machine wash the sweater using a gentle cycle, and thentumble dry at a low temperature. They might say to hand wash thesweater in cold water and lay the sweater flat to dry. The label mighteven say, “Dry clean only.”

Why is it necessary to put care instructions on a label? The samecleaning method will not work for all materials. For example, a shirtmade from 100 percent cotton may need to be ironed after washing.But a shirt made from a cotton and polyester blend may come out ofthe dryer wrinkle free. A wool jacket often needs to be dry cleanedbecause wool can shrink when washed in water.

The tendency to wrinkle when washed is a property of cotton.The tendency not to wrinkle when washed is a property of poly-ester. The tendency to shrink when washed is a property of wool.Cotton, wool, and polyester have different properties because

they have different compositions. The word compositioncomes from a Latin word meaning “a putting together,” orthe combining of parts into a whole. Based on their com-positions, materials can be divided into pure substancesand mixtures.

Matter

a. ? Mixture

Element b. ? c. ? d. ?

Figure 1 You can use the care labels on clothing tosort laundry into batches for cleaning. The carelabel shown is for a wool sweater that needs to bedry cleaned or washed by hand.

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FOCUS

Objectives2.1.1 Classify pure substances as

elements or compounds.2.1.2 Describe the characteristics of

an element and the symbolsused to identify elements.

2.1.3 Describe the characteristics ofa compound.

2.1.4 Distinguish pure substancesfrom mixtures.

2.1.5 Classify mixtures as hetero-geneous or homogeneous.

2.1.6 Classify mixtures as solutions,suspensions, or colloids.

Build VocabularyParaphrasing To help studentsunderstand the definitions of vocabu-lary terms, you may replace less familiarwords in a definition with a more familiarword or phrase. For example, you canreplace distributed in the definition of ahomogeneous mixture with “spreadout,” or shattering in the definition ofmalleability with “breaking into pieces.”

Reading Strategya. Substance b. Compoundc. and d. Homogeneous mixture orheterogeneous mixture

INSTRUCT

Some students may associate the termmaterial exclusively with solids becausethey can see and hold solid materials.They may have difficulty recognizingthat liquids and gases are also matter.Challenge this misconception by point-ing to the Materials list for the lab on p. 60 and asking students to identify the liquids. Verbal

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Reading Focus

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Section 2.1

Print• Reading and Study Workbook With

Math Support, Section 2.1• Math Skills and Problem Solving

Workbook, Section 2.1• Transparencies, Chapter Pretest and

Section 2.1

Technology• Interactive Textbook, Section 2.1• Presentation Pro CD-ROM, Chapter Pretest

and Section 2.1• Go Online, NSTA SciLinks, Mixtures

Section Resources

PPLS

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Pure SubstancesMatter that always has exactly the same composition is classified as apure substance, or simply a substance. Table salt and table sugar aretwo examples of pure substances. Every pinch of salt tastes equallysalty. Every spoonful of sugar tastes equally sweet. Every sample ofa given substance has the same properties because a substance hasa fixed, uniform composition. Substances can be classified into twocategories—elements and compounds.

ElementsAlthough there are millions of known substances, there are only about100 elements. An element is a substance that cannot be broken downinto simpler substances. Imagine cutting a copper wire into smallerand smaller pieces. Eventually you would end up with extremely tinyparticles called copper atoms. An atom is the smallest particle of anelement. An element has a fixed composition because it containsonly one type of atom.

No two elements contain the same type of atom. In Chapter 4, youwill find out more about atoms, including how the atoms of one ele-ment differ from the atoms of every other element.

Examples of Elements At room temperature (20°C, or 68°F),most elements are solids, including the elements aluminum andcarbon. You have seen aluminum foil used to wrap food. Most softdrink cans are made from aluminum. Carbon is the main element inthe marks you make with a pencil on a piece of paper. Some elementsare gases at room temperature. The elements oxygen and nitrogen arethe main gases in the air you breathe. Only two elements are liquids atroom temperature, bromine and mercury, both of which are extremelypoisonous. Figure 2 shows four elements and their symbols.

Figure 2 Aluminum, carbon, andgold are elements that you cansee in common objects, such ascans, pencils, and rings. Mixturescontaining iodine are used toprevent and treat infections.Analyzing Data Which of theseelements has a symbol that is notrelated to its name in English?

Properties of Matter 39

Aluminum (Al)

Carbon (C)

Iodine (I)

Gold (Au)

Pure SubstancesFYISamples of pure substances almostalways contain impurities, some that are unintentional, some intentional. For example, table salt is not puresodium chloride. It contains between0.006% and 0.01% potassium iodide.

ElementsFYISometimes an atom is defined as “thesmallest particle of an element that hasthe properties of the element.” However,an atom of copper has none of the prop-erties associated with bulk copper, suchas malleability, ductility, or density.

Build Science SkillsObserving Have students look at theelements in Figure 2. Ask, What aresome characteristics of the elements in Figure 2? (Gold is yellow and shiny.Aluminum is gray and shiny. Carbon is dulland black. Iodine is a dark purple solid.)Follow up on their observations by askingstudents to hypothesize about why theiodine sample is in a closed container. (Atroom temperature, iodine evaporates andforms a purple gas, which is visible in thecontainer. The gas is poisonous.)Visual

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Customize for English Language Learners

Sharing ExperiencesEncourage multilingual students to describewhat a particular element is called in theirnative language. This exercise will be mostuseful for those elements that have been

known for centuries or millennia. (Examplesinclude carbon, mercury, sulfur, iron, copper,tin, zinc, silver, gold, and lead.) Use thisinformation as a springboard for discussing the element names given in the text.

Answer to . . .

Figure 2 Gold


Symbols for Elements In 1813, Jöns Berzelius, a Swedishchemist, suggested that chemists use symbols to represent elements.Many of the symbols he assigned to elements are still used. Each symbolhas either one or two letters. The first letter is always capitalized. If thereis a second letter, it is not capitalized.

It is easy to see why C and Al are used to represent carbon and alu-minum. But why does gold have the symbol Au? The symbols thatBerzelius chose were based on the Latin names of the elements. TheLatin name for gold is aurum.

The symbols allow scientists who speak different languages to com-municate without confusion. For example, nitrogen is known as azotein France, as stickstoff in Germany, and as nitrógeno in Mexico. But sci-entists who speak English, French, German, and Spanish all agree thatthe symbol for the element nitrogen is N.

Sometimes an element’s name contains a clue to its properties. Forexample, the name hydrogen comes from the Greek words hydro andgenes, meaning “water” and “forming.”

CompoundsWater is composed of the elements hydrogen and oxygen. When elec-tricity passes through water, bubbles of oxygen and hydrogen gas formand rise to the surface of the water. If the gases are collected in a con-tainer and a flame is brought near the mixture, the hydrogen andoxygen react and form water. Water is classified as a compound. Acompound is a substance that is made from two or more simpler sub-stances and can be broken down into those simpler substances. Thesimpler substances are either elements or other compounds.

The properties of a compound differ from those of the substancesfrom which it is made. For example, oxygen and hydrogen are gases atroom temperature, but water is a liquid. Hydrogen can fuel a fire, andoxygen can keep a fire burning, but water does not burn or help othersubstances to burn. In fact, water is one of the substances commonlyused to put out fires.

Figure 3 shows another example of how properties change whenelements join and form compounds. Silicon dioxide is a compoundfound in most light-colored grains of sand. It is a colorless, transparentsolid. Yet, silicon dioxide is made from a colorless gas (oxygen) and agray solid (silicon). Silicon is used to make chips for computers.

A compound always contains two or more elements joined ina fixed proportion. For example, in silicon dioxide, there are always twooxygen atoms for each silicon atom. (Di- means “two.”) In water, thereare always two hydrogen atoms for each oxygen atom.

What happens if electricity passes through water?

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Silicon

Oxygen

Silicondioxide

Figure 3 Elements have differentproperties than their compounds.Silicon is a gray solid and oxygen isa colorless gas, which can bestored in a metal tank. Silicon andoxygen combine to form silicondioxide—a colorless, transparentsolid found in most grains of sand.

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Integrate Language ArtsHave students research and explain theorigin of element symbols (other thanAu) that are not abbreviations for ele-ment names in English. Examples includesilver (Ag), lead (Pb), tin (Sn), antimony(Sb), and mercury (Hg).Verbal, Portfolio

CompoundsFYIWhen the terms element, atom, com-pound, and molecule are defined in anintroductory section, the definition of a molecule is often both too broad and too narrow. Molecules are not thesmallest particle of all compounds, and many elements exist in nature asmolecules. For this reason, in this text,the formal definition of molecule doesnot appear until Chapter 6, the chapteron bonding, where it can be definedaccurately. (The term molecule isintroduced informally in Section 3.3when water molecules are described.)

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Section 2.1 (continued)

Electrolysis of Water Early chemists were able to use heat to break down manycompounds into their constituent elements.Because water cannot be decomposed by thismethod, water was classified as an elementuntil chemists were able to use electric current

to break down water into hydrogen andoxygen. (The decomposition of water intohydrogen and oxygen will not occur unlessions are added to the water because there arenot enough ions in pure water to conduct an electric current.)

Facts and Figures



MixturesSuppose you are making salsa using the ingredients shown in Figure 4.You have a choice. You can use exactly the amounts listed in the recipe,or you can adjust the ingredients according to your own taste. Youmight have to prepare the recipe a few times before deciding if youhave just the right amount of each ingredient.

Mixtures tend to retain some of the properties of their individualsubstances. But the properties of a mixture are less constant than theproperties of a substance. The properties of a mixture can varybecause the composition of a mixture is not fixed. The type of pepperand the quantity of pepper used in a salsa recipe determine the “hot-ness” of a batch of salsa. Chili peppers contain a compound calledcapsaicin (kap SAY uh sin) that can cause a burning sensation in yourmouth. The amount of capsaicin varies among types of peppers.Cayenne peppers, for example, contain more capsaicin than dojalapeño peppers.

No matter how well you stir a batch of salsa, the ingredientswill not be evenly distributed. There may, for example, bemore onion in one portion of the salsa than another.Mixtures can be classified by how well the parts of the mix-ture are distributed throughout the mixture.

Heterogeneous Mixtures If you look at a handfulof sand from a beach, the sand appears to be all the samematerial. However, if you use a hand lens, you will notice that thesample of sand is not the same throughout. Figure 5 shows that grainsof sand vary in size. Also, some grains are light in color and some aredark. Sand is an example of a heterogeneous mixture. Heterogeneous(het uh roh GEE nee us) comes from the Greek words hetero and genus,meaning “different” and “kind.” In a heterogeneous mixture, the partsof the mixture are noticeably different from one another.

Figure 4 The ingredients shownare used to make one kind ofsalsa, which is an example of aheterogeneous mixture.

Figure 5 Sand is a hetero-geneous mixture. The spoonis stainless steel, which is ahomogeneous mixture.Interpreting PhotographsExplain how viewing sandthrough a hand lens helps showthat sand is a heterogeneousmixture.

• 4 or 5 plum tomatoes

• 3–5 fresh Serrano

chili peppers

• 12 sprigs of cilantro

• large garlic clove

• small white onion

• 1 teaspoons

fresh lime juice

• teaspoon

salt

Salsa

12

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MixturesBuild Reading LiteracyCompare and Contrast Refer to page 226D in Chapter 8, which pro-vides the guidelines for comparing and contrasting.

Have students read pp. 41–44 and gatherinformation on different classifications ofmixtures. Then, have students create achart that compares and contrasts eachtype of mixture.Visual

Integrating Language ArtsHave students consider the differencebetween hot food (as in warm versuscold) and hot food (as in spicy versusmild). Encourage students to suggestexamples of dishes other than salsa that are hot in the second, spicy sense.Some students may want to record afamily recipe for a “hot” dish. Havethem identify the ingredient(s) that can be varied to control the “hotness”of the mixture. Logical

Build Science SkillsObserving Have students look atsamples of sand with a hand lens. Explainthat the composition of sand can varyfrom beach to beach because the rocksand shells from which sand forms havedifferent compositions. Ask, How canyou tell that sand is a heterogeneousmixture? (The grains of sand vary in colorand size.) (A sample of sand that containsonly ground-up shells would probably beclassified as homogeneous.) Visual

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Capsaicinoid Content Capsaicin (8-methyl-N-vanillyl-6-nonenamide) is the most abundantof a group of compounds called capsaicinoids.These compounds are found in the seeds andmembranes of chili peppers. When they areingested, they affect pain receptors in themouth and throat. Because capsaicin is an oil,and oil and water do not mix, drinking waterspreads the oil to more parts of the mouth and increases the burning sensation.

The capsaicinoid content of a pepper ismeasured in Scoville units. Wilbur Scovilledeveloped the method for measuring the“hotness” of chili peppers in 1912. He mixedground chilies with a sugar and water solutionand diluted the mixture until tasters no longerreported a burning sensation. The greater thedilution needed, the higher the assignednumber of Scoville units. Fifteen Scoville unitsis equivalent to one part per million.

Facts and Figures

Answer to . . .

Figure 5 More details are visible in the magnified sand, making it easier to observe the different parts of the mixture.

Bubbles of oxygen andhydrogen form and rise

to the surface of the water.

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Homogeneous Mixtures If you collect water from both theshallow end and the deep end of a swimming pool, the water sampleswill appear the same. The water in a swimming pool is a homogeneous(hoh moh GEE nee us) mixture of water and substances that dissolve inwater. In a homogeneous mixture, the substances are so evenly dis-tributed that it is difficult to distinguish one substance in the mixturefrom another. A homogeneous mixture appears to contain only onesubstance. The serving spoon in Figure 5 is made of stainless steel—ahomogeneous mixture of iron, chromium, and nickel.

Solutions, Suspensions, and ColloidsIt isn’t always easy to tell a homogeneous mixture from a heterogeneousmixture.You may need to observe the properties of a mixture before youdecide. The size of the particles in a mixture has an effect on the prop-erties of the mixture. Based on the size of its largest particles, amixture can be classified as a solution, a suspension, or a colloid.

Solutions If you place a spoonful of sugar in a glass of hot waterand stir, the sugar dissolves in the water. The result is a homogeneousmixture of sugar and water. When substances dissolve and form ahomogeneous mixture, the mixture that forms is called a solution.The windshield wiper fluid in Figure 6 is a solution. So is tap water.

Do the Contents of Two Cansof Mixed Nuts Meet FDARegulations?

The Food and Drug Administration (FDA) has twomain areas of concern about food. First, and mostimportant, the FDA ensures that food sold in theUnited States is safe to eat. Second, the FDAensures that the information on a food labelaccurately describes a food product.

What can you assume when you see the label“mixed nuts” on a can of nuts? According to theFDA regulations, a can labeled mixed nuts mustcontain at least four types of shelled nuts otherthan peanuts. The mass of each type of nut mustbe not less than 2 percent of the total mass andnot more than 80 percent of the total mass.

1. Comparing and Contrasting How arethe two brands of mixed nuts alike? Howare they different?

2. Calculating What is the percent by mass ofeach type of nut in each can?

3. Drawing Conclusions Do the contents ofeach can meet the FDA regulations? Explain.

4. Inferring On the Brand A label, the nutsare listed in this order: peanuts, Brazil nuts,almonds, cashews, pecans, and hazelnuts.What do you think determines the order?

Type of Nut

Peanut

Almond

Brazil nut

Cashew

Hazelnut

Pecan

152.39 g

47.02 g

57.88 g

46.20 g

19.90 g

21.40 g

Mass inBrand B

191.96 g

31.18 g

19.60 g

73.78 g

16.90 g

16.90 g

Contents of Two Cans of Mixed NutsMass inBrand A

For: Links on mixtures

Visit: www.SciLinks.org

Web Code: ccn-1021

Do the Contents of TwoCans of Mixed Nuts Meet FDA Regulations?Answers1. Both brands contain the same sixtypes of nuts, but the amount of eachtype of nut varies. 2. There are 344.79 g in Brand A and350.32 g in Brand B. The percents bymass in Brand A are: 44.2% peanuts,13.64% almonds, 16.79% Brazil nuts,13.4% cashews, 5.77% hazelnuts, and 6.21% pecans. The percents bymass in Brand B are: 54.8% peanuts,8.90% almonds, 5.59% Brazil nuts,21.06% cashews, 4.82% hazelnuts, and4.82% pecans.3. Yes. Both brands contain more thanfour types of nuts other than peanuts.The percent of each nut by mass is withinthe 2% to 80% range. 4. The ingredients are listed in order bytotal mass. The ingredient with thelargest total mass is listed first.

For Extra HelpHave students answer Question 3 by cal-culating 2% and 80% of each total massand checking to see if any of the massesfall outside that range. Logical

Solutions, Suspensions, and ColloidsFYIMany alloys are not true solutions.Sterling silver, a mixture of silver andcopper, is an example. Silver and copperare completely soluble in all proportionswhen molten. However, solid sterlingsilver is a two-phase alloy with pocketsof silver and pockets of a 71.9% silverand 28.1% copper mixture.

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Regulatory Agencies Responsibility forprotecting the food supply is shared amongthe FDA, the Department of Agriculture, andthe EPA, which regulates pesticides. Food that is adulterated or mislabeled may bevoluntarily destroyed or recalled, or seized by court order. The FDA is responsible forestablishing standards for identity, quality, and fill of container. If standards have been

set for a product, such as mixed nuts, theproduct must comply with those standards.Food labels must include a statement ofidentity (usual or common name), the netquantity of the contents, the name and place of business of the supplier, and a list of ingredients in descending order bymass. The goal is to have honest andinformative labels.

Facts and Figures

Download a worksheet onmixtures for students to complete,and find additional teacher supportfrom NSTA SciLinks.

42 Chapter 2


Liquid solutions are easy to recognize. They do not separate intodistinct layers over time. If you pour a liquid solution through a filter,none of the substances in the solution are trapped in the filter. You cansee through solutions that are liquids because light passes throughthem without being scattered in all directions. These three propertiesof liquid solutions can be traced to the size of the particles in a solu-tion. The particles in a solution are too small to settle out of thesolution, be trapped by a filter, or scatter light.

Suspensions Have you ever seen the instruction “Shake wellbefore using” on a bottle? This instruction is a clue that the material inthe bottle is a suspension. A suspension is a heterogeneous mixturethat separates into layers over time. For example, if you shake up a con-tainer of sand and water, the sand mixes with the water and forms asuspension. Over time, the suspended particles of sand settle to thebottom of the container.

You could use a filter to separate the sand from the water. The waterwould pass through the filter, but the sand would remain in the filterpaper. Suspended particles settle out of a mixture or are trapped by afilter because they are larger than the particles in a solution. Theworker in Figure 7 is using a mask to filter out particles of plastic foamthat are suspended in air. Because larger particles can scatter light in alldirections, suspensions are cloudy.

What happens to suspended particles over time?

Figure 6 The liquids shown representthree categories of mixtures. A Windshieldwiper fluid is a solution. B Muddy watercollected from a swamp is a suspension. C Milk is a colloid. Comparing and Contrasting Based onappearance, how are a solution and acolloid similar?

Figure 7 When a surfboard issanded, particles of plasticbecome suspended in air. Theworker wears a mask to keepfrom breathing in the particles.


B

A

C

Transmission VersusScatteringPurpose Students observe light passthrough different mixtures.

Materials 2 beakers, water, iodinesolution, table salt, stirring rods, milk,fish tank, flashlight, white paper

Procedure Demonstrate that asolution can have color and still transmitlight. Fill two beakers halfway withwater. Add a small amount of iodinesolution to one beaker and stir. Addtable salt to the second beaker. Askstudents to describe the color of eachmixture and state whether it is clear. Demonstrate transmission of light versusscattering. Fill the tank with water. Turnoff the lights. Shine a flashlight throughthe tank so that it lights up the whitepaper placed behind the tank. Addsome milk to the water and stir. Shinethe light through the tank again. (Thesebehaviors of light are discussed in detailin Section 18.3.)

Expected Outcome Although the salt solution is colorless and the iodinesolution is colored, both transmit light.Light passes through the water in thetank and makes a bright spot on thepaper. With milk added, the light isscattered so that the spot is much less distinct. Visual

Use VisualsFigure 7 To emphasize that suspendedparticles settle out over time, ask, Whatwill happen to the particles of plasticsuspended in the air? (The particles will eventually settle out.) Discuss withstudents the dangers of inhalingsuspended particles. Ask, How does amask prevent the worker from inhal-ing particles of plastic? (The particles insuspensions can be separated by filtration.The mask is a filter.)Visual

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Answer to . . .

Figure 6 The solution and the colloidboth appear homogeneous.

Suspended particlessettle out of a mixture.


Section 2.1 Assessment

Reviewing Concepts1. Why does every sample of a given

substance have the same properties?

2. Explain why the composition of anelement is fixed.

3. Describe the composition of a compound.

4. Why can the properties of a mixture vary?

5. On what basis can mixtures be classifiedas solutions, suspensions, or colloids?

Critical Thinking6. Predicting If you added salt instead of sugar

to a pitcher of lemonade, how would thischange the properties of the lemonade?

7. Interpreting Visuals Explain why silicondioxide cannot be the only compound in thesample of sand shown in Figure 5.

8. Inferring Fresh milk is a suspension. Afterfresh milk is homogenized, it is a colloid. Whathappens to the size of the drops of fat in milkwhen milk is homogenized?

Writing Instructions Pick a cereal that isan obvious mixture. Write rules that could beused to control the cereal’s composition. Usethe FDA rules for mixed nuts as a model.

44 Chapter 2

Low beam

High beam

Colloids Milk is a mixture of substances including water, sugar, pro-teins, and fats. When fresh cow’s milk is allowed to stand, a layer ofcream rises to the top. This layer contains much of the fat in the milk.In the milk you buy at the store, the cream does not form a separate layer.The milk has been processed so that the fat remains dispersed through-out the milk. The result is homogenized milk, which is a colloid.

A colloid contains some particles that are intermediate in sizebetween the small particles in a solution and the larger particles in asuspension. Like solutions, colloids do not separate into layers. Youcannot use a filter to separate the parts of a colloid.

Fog is a colloid of water droplets in air. Figure 8 shows how fogaffects which headlights a driver uses. Automobiles have headlightswith low beams for normal driving conditions and high beams forroads that are poorly lit. With the high beams, a driver can see a bendin the road or an obstacle sooner. But the high beams are not useful ona foggy night because the water droplets scatter light back toward thedriver and reduce visibility. With the low beams, much less light is scat-tered. The scattering of light is a property that can be used todistinguish colloids and suspensions from solutions.

Figure 8 The photograph showshow water droplets in fog scatterthe light from high beams. Thedrawing compares the areas lit byhigh beams and low beams.Interpreting Diagrams Whichbeams normally make a largerarea of a road visible?

44 Chapter 2

Build Science SkillsClassifying Have interested studentsresearch categories of colloids such asgels, foams, aerosols, and emulsions.Have students find out how scientistsdistinguish different types of colloids.Have them identify household examplesof each type. Visual, Portfolio

ASSESSEvaluate UnderstandingHave students make a game of concen-tration using the terms in the chapterand their definitions. Have groups ofstudents write each term on separateindex cards and the definition of eachterm on a second set of index cards. Toplay the game, students should shuffleall the cards together and then lay themface down in a grid. Each student takesturns flipping over two index cards. If the cards match, the student can removethe cards from the grid. If the cards donot match, the student places the cardsface down. After all of the cards aregone, the student who has removed the most cards wins the match.

ReteachUse Figure 6 as a visual aid to summarizethe key differences among differenttypes of mixtures.

Students should specify whichingredients are required for the cereal toqualify for a particular label and suggesta range for ingredients such as driedfruit or nuts. (Students could chooseanother edible mixture, such as cannedvegetable soup.)

If your class subscribes tothe Interactive Textbook, use it toreview key concepts in Section 2.1.

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6. The lemonade would taste salty instead of sweet.7. Silicon dioxide is colorless. There must be atleast one other compound in the sample toaccount for the dark-colored grains.8. Large drops are broken down into smallerdrops, which can remain dispersedthroughout the milk.


1. A pure substance has a fixed composition. 2. An element contains only one kind of atom. 3. Compounds contain two or more elementsjoined in a fixed proportion. 4. Because the composition of a mixture is not fixed5. Mixtures can be classified as solutions,suspensions, or colloids based on the size of their largest particles. Answer to . . .

Figure 8 High beams


If there are pitchers of ice water and lemonade on a picnic table, howdo you know which liquid is in each pitcher? It’s easy! The lemonadeis yellow. Lemonade also has a tart taste that is hard to miss. A yellowcolor and a tart taste are two properties of lemonade.

Examples of Physical PropertiesA physical property is any characteristic of a material that can beobserved or measured without changing the composition of the sub-stances in the material. Viscosity, conductivity, malleability,hardness, melting point, boiling point, and density are examplesof physical properties.

Viscosity Suppose you knock over an open bottle of vinegar andan open jar of honey at exactly the same time. In the time it takes forthe vinegar bottle to empty, the honey will scarcely start to flow. Thetendency of a liquid to keep from flowing—its resistance to flowing—is called its viscosity. The greater the viscosity, the slower the liquidmoves. Thick liquids, such as corn syrup and the honey in Figure 9, havea high viscosity. Thin liquids, such as vinegar, have a low viscosity.

The viscosity of a liquid usually decreases when it is heated. Forexample, a spoonful of cooking oil will spread more quickly across thebottom of a heated frying pan than across the bottom of a cold pan.

Why is the viscosity of a liquid important? Consider the motor oilused to keep the parts of an automobile engine from wearing away asthey move past one another. The motor oil must not be too thick in coldweather or too thin in hot weather.

2.2 Physical Properties

Reading StrategyBuilding Vocabulary Copy the table. Asyou read, write a definition for each property.

Key ConceptsWhat are some examplesof physical properties?

How can knowing thephysical properties ofmatter be useful?

What processes are usedto separate mixtures?

When does a physicalchange occur?

Vocabulary◆ physical property◆ viscosity◆ conductivity◆ malleability◆ melting point◆ boiling point◆ filtration◆ distillation◆ physical change

Viscosity

Malleability

Melting point

Physical Property Definition

a. ?

b. ?

c. ?

Figure 9 The object in thephotograph is called a honeydipper. Honey is an example of aliquid with a high viscosity.


FOCUS

Objectives2.2.1 Describe physical properties of

matter.2.2.2 Identify substances based on

their physical properties.2.2.3 Describe how properties are

used to choose materials.2.2.4 Describe methods used to

separate mixtures.2.2.5 Describe evidence that

indicates a physical change istaking place.

Build VocabularyLINCS Have students use the LINCSstrategy to learn and review the termsviscosity, conductivity, and malleability. InLINCS exercises, the students List whatthey know about each term, Imagine apicture that describes the word or phrase,Note a reminding “sound-alike” word(such as visible, train conductor, andmallet), Connect the terms to the sound-alike word by making up a short story,and then perform a brief Self-test.

Reading Strategya. The tendency of a liquid to resistflowing b. The ability of a solid to behammered without shattering c. Thetemperature at which a substancechanges from a solid to liquid

INSTRUCT

Examples of Physical PropertiesFYIThe text mentions that lemonade can be identified by its taste. Remind stu-dents that taste should never be used toidentify an unknown substance and thatno substances, known or unknown,should ever be ingested in a lab.

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Reading Focus

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

Math Support, Section 2.2 and Math Skill: Melting and Boiling Points

• Transparencies, Section 2.2

Technology• Interactive Textbook, Section 2.2• Presentation Pro CD-ROM, Section 2.2• Go Online, Science News, Properties of

matter

Section Resources

Section 2.2

PPLS


Conductivity Which spoon should you choose for stirring a potof soup heating on the stove—a metal spoon or a wooden spoon? Ifone end of a metal object is heated, the other end will soon feel hot. Amaterial’s ability to allow heat to flow is called conductivity.

Materials that have a high conductivity, such as metals, are calledconductors. If a material is a good conductor of heat, it is usually alsoa good conductor of electricity. Wood is not a good conductor of heat.You can stir hot soup with a wooden spoon without worrying aboutburning your hand because the wooden spoon stays cool to the touch.

Malleability The ancient gold objects in Figure 10 were found ina tomb in Greece. A goldsmith made the medallions by tapping goldwith a small hammer and punch. Gold can be shaped in this waybecause it is malleable. Malleability (mal ee uh BIL uh tee) is the abilityof a solid to be hammered without shattering. Most metals are mal-leable. By contrast, an ice cube breaks into small pieces when struckwith a hammer. So does ordinary glass when hit by a fast-moving objectsuch as a baseball. Solids that shatter when struck are brittle.

Why would a cook use a wooden spoon to stirhot soup?

Comparing Heat Conductors

Materials2 plastic foam cups, scissors, metric ruler, metalrod, wooden rod, 2 liquid crystal thermometers,hot water, clock or watch with second hand

Procedure 1. Make a data table with a column for time and a

column to record the temperature of each rod.

2. Cut 3 cm off the top of one cup. Turn the cupupside down. Use the metal rod to make twoholes about 3 cm apart in the bottom.

3. Attach a thermometer to each rod as shown.

4. Insert both rods so they will just touch thebottom of the second cup. Lay the cup withthe rods on the table. Do not touch the rodsand thermometers during the experiment.

5. Ask your teacher to add hot water to theintact cup until it is about three fourths full.

6. Hold the cup with the hot water firmly as youplace the cup with the rods on top of it.CAUTION Be careful not to overturn the cup.

7. Observe and record the temperatures everyminute for five minutes.

Analyze and Conclude1. Comparing and Contrasting Is metal or

wood a better conductor of heat? Explain.

2. Applying Concepts Is a metal cup a goodcontainer for keeping a drink hot for as long aspossible? Give a reason for your answer.

3. Evaluating Describe any variables, otherthan the type of material, that were notcontrolled in this lab.

Liquid crystalthermometer

Metal rod

Wooden rod

Foam cups

46 Chapter 2

Figure 10 Because gold is bothmalleable and beautiful, it isoften used to make jewelry. Theseancient gold medallions weremade to form a necklace.

46 Chapter 2

Comparing Heat Conductors

ObjectiveAfter completing this activity, studentswill be able to• distinguish a material that is a good

conductor of heat from a material thatis a poor conductor of heat.

Skills Focus Observing, Inferring

Prep Time 10 minutes

Advance Prep Acquire wood andmetal rods of similar length from themetal or woodworking classroom, a crafts shop, or a home repair shop. Liquid crystal thermometers may bepurchased at a pet or aquarium supplystore. You may want to cut the cups andattach the thermometers to the rods inadvance to save class time. Thermom-eters can be attached to the rods withtheir own adhesive backing or taped on.Heat enough water to fill each foam cupabout 3/4 full.

Class Time 20 minutes

Safety Remind students to handle thesetup carefully to prevent burns after thehot water is added.

Teaching Tips• You may need to provide tape to

attach thermometers to the rods.• Construct a blank data table on the

board, an overhead transparency, or a computer and have each groupenter its data in this table. All data can then be pooled and averaged for greater accuracy.

Expected Outcome Metal willconduct heat faster than wood.

Analyze and Conclude1. Metal is a better conductor. Studentsshould cite the differences in temperatureover time as evidence.2. Metal is not the best choice for thispurpose. Because metals are good con-ductors, the contents of the cup willcool quickly as heat is transferred to the surroundings.3. Depending on what rods are used,possible sources of error are differentlengths, different diameters, anddifferent densities of the materials.Kinesthetic, Logical

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Customize for Inclusion Students

Visually Impaired Provide students who have visual impairmentswith samples of two different nontoxic solidsthat have approximately the same mass. Byhandling the samples, students can infer theirrelative densities from their volumes.

Provide samples of materials with a range of hardness. Have students do scratch tests to determine an order of hardness. Studentsshould be able to feel any indentations on thesurfaces of the softer materials.


Hardness One way to compare the hardness of two materials is tosee which of the materials can scratch the other. The blade of a typicalkitchen knife, for example, can scratch a copper sheet because stainlesssteel is harder than copper. The stainless steel in a knife blade is a hardsolid that can be shaped into a sharp cutting edge. The material usedto sharpen the blade must be harder than stainless steel. Diamond isthe hardest known material. Some of the grinding wheels used tosharpen steel contain small grains of diamond. The man in Figure 11is carving a canoe from a soft wood—Western red cedar.

Melting and Boiling Points If you leave a tray of ice cubeson your kitchen counter, the ice cubes will melt. The temperature atwhich a substance changes from solid to liquid is its melting point.For water, this change normally occurs at 0°C. If you heat water to cookpasta, the water will normally start to boil at 100°C. The temperatureat which a substance boils is its boiling point. Figure 12 shows themelting point and the boiling point for some substances.

Density Density can be used to test the purity of a substance. Recallthat density is the ratio of the mass of a substance to its volume. Atroom temperature, silver has a density of 10.5 g/cm3. If a coin has adensity of 9.9 g/cm3 at room temperature, either the coin is not madefrom silver or the coin contains substances in addition to silver.

Density can be used to test the purity of methanol. Methanol is afuel burned in some racing motorcycles. The American MotorcycleAssociation (AMA) requires racers to use fuel that is at least 99.65 per-cent pure. Race officials may collect a sample of fuel and measure itstemperature and density. Then they compare the measured density tothe expected density of methanol at that temperature. These spotchecks keep racers from adding substances to the fuel that will givethem an unfair advantage in a race.

Melting Point Boiling Point

Hydrogen

Nitrogen

Ammonia

Octane (found in gasoline)

Water

Acetic acid (found in vinegar)

Table salt

Gold

�259.3�C

�210.0�C

�77.7�C

�56.8�C

0.0�C

16.6�C

800.7�C

1064.2�C

�252.9�C

�195.8�C

�33.3�C

125.6�C

100.0�C

117.9�C

1465�C

2856�C

Melting and Boiling Points of Some Substances

Substance

Figure 12 The table lists themelting points and boiling pointsfor several substances.Analyzing Data Which of thesesubstances are liquids at roomtemperature (20oC, or 68oF)?

Figure 11 This Tlingit carver isusing an adze to carve a canoefrom Western red cedar. Redcedar is a relatively soft wood.

Use VisualsFigure 10 Gold is the most malleablemetal. Ancient Greek jewelry was oftenhammered to a thickness of 0.1 to 0.15 mm. Ask, What properties of gold can you identify from Figure 10?(Gold is a shiny, yellow solid that can beshaped into small, detailed pieces.) Themedallions shown in Figure 10 werefound in the Lazlo region of central Italyand were likely made between the eighthand sixth centuries B.C. Ask, Given theage of this jewelry, what other prop-erty could you say gold exhibits? (Gold is durable.)Visual

Integrate Earth ScienceThere are different scales for describinghardness. The Brinell and Rockwell scalesare based on the indentation made by anobject. The Brinell hardness number isbased on indentation depth. The Rockwellhardness number is based on indentationarea. On a scale that measures resistanceto abrasion, only two substances are ratedabove 10: cubic boron nitride (cBN), 19,and diamond, 42.5. Have students findout about hardness scales and describehow they are used to characterize rocksand minerals.Logical, Portfolio

FYIBoiling points depend on bothtemperature and pressure. In Chapter 3,students will study the effect of pressureon boiling points.

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Answer to . . .

Figure 12 Octane, water, and acetic acid

A wooden spoon is apoor conductor of heat.

Specific Gravity The AMA race officialsdescribed in the text are using a hygrometerto test specific gravity. Specific gravity is theratio of the mass of a solid or liquid to themass of an equal volume of distilled water

at 4°C. Because specific gravity is a ratio ofvalues that have identical units, it does nothave units. (Density is discussed in greaterdetail in Chapter 1.)

Facts and Figures


Using Physical PropertiesPeople use data about physical properties to solve many different typesof problems. Physical properties are used to identify a material, tochoose a material for a specific purpose, or to separate the substancesin a mixture.

Using Properties to Identify Materials The steps used toidentify a material are similar to the steps used to test for purity. Thefirst step is to decide which properties to test. The second step is to dotests on a sample of the unknown. The final step is to compare theresults with the data reported for known materials.

The identification of a material can be a crucial step in solving acrime. A detective collects red paint chips from the scene of a hit-and-run accident. He asks a chemist at the crime lab to use the chips toidentify the model of the missing vehicle. Because paint is a mixture ofsubstances, the chemist can do tests that distinguish one type of redpaint from another. The technician compares the data she collects toan FBI database. The database contains information about the paintsused on different makes and models of cars. Once the detective knowsthe make and model, he uses a database of registered owners to createa list of possible suspects.

Using Properties to Choose Materials Properties deter-mine which materials are chosen for which uses. For example, youwouldn’t want shoelaces made from wood. Shoelaces must be flexible,that is, they must be able to bend without breaking. They must also bedurable, that is, they must be able to withstand repeated use. Laces inhiking boots like those in Figure 13 are usually nylon or leather.

People don’t consider just one property when choosing a material fora particular application. They look at a set of properties. For example,the How It Works box on page 49 explains how the properties of waxare used when clay molds are made for casting metal sculptures.

Figure 13 Shoelaces for hikingboots are sometimes made fromleather. So are some belts andshoes. Making GeneralizationsWhat properties of leather wouldmake it a suitable material for allthree types of objects?

48 Chapter 2

48 Chapter 2

Using PhysicalPropertiesBuild Reading LiteracyOutline Refer to page 156D inChapter 6, which provides theguidelines for an outline.

Have students read pp. 48–50 andgather information on identifying,choosing, and separating substancesbased on their physical properties. Then,have students use the headings as majordivisions in an outline. Have studentsrefer to their outlines when answeringthe questions in the Section 2.2Assessment. Visual

Comparing Melting PointsPurpose Students observe thedifferences in melting points of twosubstances.

Materials water, ethanol, foam cups,freezer, large beaker, thermometer

Procedure Tell students that you willdemonstrate that the melting points oftwo substances differ. The night before,place a foam cup filled with water and asecond foam cup filled with ethanol inthe freezer. During class, pull both cupsout of the freezer. Have students notethat the water is frozen, but the ethanolis not. Remove the ice from the cup andadd it to the large beaker. Add a smallamount of water and allow the ice tobegin melting. While the temperature of the ice water is leveling off, have avolunteer measure the temperature ofthe ethanol. Compare that temperatureto the temperature of the ice water.

Expected Outcome Because mostfreezers do not cool to temperaturesbelow the melting point of ethanol,–114.7°C, the ethanol will remain aliquid. After some of the ice has melted,the temperature of the solid-liquidsystem will stop decreasing and level off. This is the melting point of water,0°C. The temperature of the chilledethanol is below 0°C. Visual

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Identifying Substances Sometimes a singleproperty is not sufficient to positively identify asubstance. Iron, nickel, and cobalt, which areoften found together in nature, look quitesimilar. The density of iron is 7.87 g/cm3. Thedensity of nickel is 8.90 g/cm3. The density of

cobalt is 8.86 g/cm3. Measurements of massmade on the type of balance found in mostschool labs would be precise enough to distinguish iron from cobalt or nickel, butnot precise enough to distinguish cobalt from nickel.

Facts and Figures

Making a SculptureA process called lost-wax casting is used to make metalsculptures. Different stages of the process depend onphysical properties of wax, clay, and metal. Wax can becarved and molded. Clay becomes brittle when baked ata high temperature. When melted, most metals formhomogeneous mixtures, which can be poured into molds.Interpreting Diagrams Why is it important that waxhas a low melting point?

Preparing clay moldsThis artist from Nepal in southernAsia is preparing molds for lost-waxcasting. She is applying layers of clayto a wax model.

Wax model A wax model ofthe sculpture is built up around

a clay core. Wax is used because it issoft enough for carving fine details,but hard enough to retain its shapeat room temperature.

Clay moldClay is applied to

the wax model in layersof increasing coarseness,and left to dry. The inner,finer layers capture everydetail of the sculpture ina smooth mold. Theouter, coarser layers(clay mixed with sand)provide strength.

Molten metalMolten (liquid)

metal is poured into theclay mold and left tocool and harden. For thissculpture of an Africanhead, a mixture ofcopper, zinc, and leadwas used.

Metal sculptureWhen the metal

has cooled, the clayshell is broken open toreveal the finishedmetal sculpture.

Melting waxThe clay-covered

wax model is thenbaked in a kiln. Becausewax has a low meltingpoint, the wax modelmelts away inside theclay, leaving a hardenedshell mold. This claymold is then used tomake the final sculpture.

Molten metalpoured intomold

Layers of clayapplied to thewax model

Wax model

Clay core

Meltingwax

Wax model

Broken clay

Cast metalsculpture


Making a SculptureThere are a number of variations to the method described in the text. Forexample, sometimes there is no claycore. The lost-wax method of castingcan also be used to make replicas ofexisting sculptures.

Many other molding and castingtechniques also take advantage of the physical properties of differentsubstances to maximize the quality of the final product. For example, theproperties of some types of rubber makethem ideal materials for molds. Rubbermolds are flexible enough that the castcan be pulled out without breaking themold, yet strong enough to preservefine details in the final product. Becauseof their durability, rubber molds can bereused many times.

Interpreting Diagrams The waxmust melt away when the model isheated in a kiln.Logical

For EnrichmentBecause an imprint made in wet sandwill hold its shape, filling the imprintwith melted wax or wet plaster willproduce a cast of the object pressed intothe sand. This sculpting technique iscalled a sand mold. Have students findout about sand molds and use them tomake candles or plaster casts of theirfootprints or handprints.Visual, Portfolio

Use Community ResourcesArrange for a sculptor or jewelry makerto visit the class. Have the artist describethe molding and casting techniques he or she uses. Before the visit, havestudents prepare questions about howthe artist chooses materials based ontheir physical properties.Interpersonal

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Answer to . . .

Figure 13 Flexibility and durability


50 Chapter 2

Using Properties to Separate MixturesSome properties can be used to separate mixtures. Filtration anddistillation are two common separation methods.

Filtration One way to make a pot of tea is to pour hot water overloose tea leaves. Some compounds in the tea leaves, such as caffeine, dis-solve in the water. You can separate the hot tea from the loose leaves bypouring the mixture through a strainer. Using a strainer is a type of fil-tration. Filtration is a process that separates materials based on the sizeof their particles. Particles of the compounds that dissolve are smallenough to pass through the strainer, but the tea leaves themselves aretoo large to pass through the strainer. The drip method of brewing coffeealso uses a filter to separate the brewed coffee from the coffee grounds.

The students in Figure 14 are using a wire screen to locate smallobjects buried in the sand at an archaeological site. Particles of dirt aresmall enough to pass through the holes in the screen, but objects suchas broken bits of pottery are too large to pass through the screen.

Distillation How can you separate the parts of a solution when allthe particles in a solution are small enough to pass through a filter?Sometimes distillation can work. Distillation is a process that sepa-rates the substances in a solution based on their boiling points.

One practical use of distillation is to provide fresh water for sub-marines. Most submarines can store only enough fresh water to last aboutten days. Each submarine has equipment that can convert seawater intofresh water. The water is heated until it changes from a liquid to a gas. Thegas is cooled until it changes back to a liquid, which is collected in a sep-arate container. Boiling can separate fresh water from seawater becausewater has a much lower boiling point than the compounds dissolved inseawater. These compounds are left behind in the original container.

How can loose tea leaves be removed from a pot ofbrewed tea?

Figure 14 These Americorpsstudents are looking for artifactsat the San Diego Presidio—a fortthat was built in 1769. As thestudents sift dirt through ascreen, small objects buried in thedirt collect on the screen.Applying Concepts How couldchanging the size of the holes ina screen change the number ofobjects found?

For: Articles on propertiesof matter

Visit: PHSchool.com

Web Code: cce-1022

50 Chapter 2

Using Properties toSeparate MixturesBuild Science Skills

Applying Concepts

Purpose In this activity, students use properties to separate the components of a mixture.

Materials a mixture of table salt, sand,and iron filings; beaker; magnet; plasticbag; water; funnel; filter paper


Procedure Encourage students toconsider properties that can be used toseparate each material from the othertwo materials in the mixture. Then, havestudents design a procedure tocomplete the separation.

Expected Outcome Students cancollect the iron filings using a magnetcovered with a plastic bag. They can addwater to the mixture to dissolve the salt.They can use a filter to collect the sand.Students can collect the salt by lettingthe water evaporate for a few days. Visual, Portfolio

Students may think that all the particlesin mixtures are similar in size. This is truefor solutions and colloids, but not forsuspensions. Challenge this miscon-ception by displaying common devicesused to filter mixtures (such as a sieve, a colander, and a coffee filter) anddiscussing the mixtures these devicesare used to separate. The size of theholes in a filter determines whetherfiltration can be used to separate asuspension. Hole size also determineswhether all the particles in a colloid pass through the filter or all the particlesare trapped in the filter. Either way, theend result is that the colloid is notseparated by filtration.Verbal

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Science News provides studentswith current information onproperties of matter.

Tea Tea contains compounds calledpolyphenols, which are often referred to astannins. There are health claims made for bothblack and green tea. The health claims madefor green tea are related to the concentrationof polyphenols. During the processing of black

tea, most of the polyphenols are oxidized.Oxidation of polyphenols is limited during the processing of green tea. (The tea dumpedinto Boston harbor on December 16, 1773,was green tea, which outsold black tea in the Colonies.)

Facts and Figures


Reviewing Concepts1. List seven examples of physical properties.

2. Describe three uses of physical properties.

3. Name two processes that are used toseparate mixtures.

4. When you describe a liquid as thick, are yousaying that it has a high or a low viscosity?

5. Explain why sharpening a pencil is an exampleof a physical change.

6. What allows a mixture to be separatedby distillation?

Critical Thinking7. Designing Experiments How could you

find out whether copper is harder or softerthan the plastic used in a plastic cup?

8. Inferring Why would you expect thematerials used to make pot holders to bepoor conductors of heat?

9. Applying Concepts Silicon dioxide is asolid at room temperature and methanolis a liquid. Which substance has the highermelting point?

Organizing Data In what order are thesubstances arranged in Figure 12? Usewhat you studied about organizing data inSection 1.4 to explain why this order isuseful. Explain why you might choose adifferent order if you had similar data for500 substances.

Figure 15 Braiding hair andcutting hair are examples ofphysical changes. Braiding is areversible change. Cutting cannotbe reversed.

Recognizing Physical ChangesThe change of water from a liquid to a gas during boiling isa physical change. A physical change occurs when some ofthe properties of a material change, but the substances inthe material remain the same. For example, if you slowlyheat butter in a pan, it changes from a solid to a liquid, butthe substances in the butter do not change. Two other exam-ples of physical changes are crumpling a piece of paper andslicing a tomato. Crumpling and slicing are actions thatchange the size and shape of a material, butnot its composition.

Some physical changes can be reversed.You can freeze water, melt the ice that forms,and then freeze the water again. You can usean iron to remove the wrinkles from a cottonshirt. You can braid hair, unbraid the hair,and then braid it again. Some physicalchanges cannot be reversed. You would notexpect to reconstruct a whole tomato fromtomato slices or to replace the peel on apeeled orange. Figure 15 shows one physicalchange that can be reversed and one thatcannot be reversed.


Recognizing Physical ChangesFYIIf butter heated in a pan begins tobrown, this is a sign that a chemicalchange is starting to occur. Students areasked to compare the possible changesto heated butter in Question 5 of theSection 2.3 Assessment.

ASSESSEvaluate UnderstandingTell students that a layer of fat rises tothe top in a pot of chicken soup. Ask,How can the cook remove the fat?(Skim the fat from the surface or freeze the soup and remove the solid fat layer.)

ReteachUse the How It Works feature on p. 49to review with students the importanceof examining physical properties whenchoosing a material. Discuss why thephysical properties of wax, clay, andmetal determine what roles they play inthe making of a sculpture.

The substances are ordered from lowestto highest melting point, which groupsthem into gases, liquids, and solids atroom temperature. With a largerdatabase, alphabetical order would bemore useful for retrieving the data.

If your class subscribes to the InteractiveTextbook, use it to review key conceptsin Section 2.2.

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5. The shape and size of the pencil change,but the composition of the pencil remains the same.6. A difference in boiling points allowsmixtures to be separated by distillation.7. By scratching the cup with an object madeof copper8. Pot holders are designed to prevent thetransfer of heat from a cooking utensil.9. Silicon dioxide


1. Students may list hardness, viscosity,conductivity, density, malleability, meltingpoint, and boiling point. Students may also list color, taste, flexibility, and durability.2. Identify materials, choose a material for aspecific purpose, or separate the substances in a mixture3. Filtration and distillation4. A thick liquid has a high viscosity.

Answer to . . .

Figure 14 With smaller holes, moreobjects would collect on the screen.

Loose tea leaves can beremoved by filtration.


Getting a Fresh StartWater found in nature is never 100 percent pure. Thereare always substances dissolved in the water. Some ofthese substances, as well as bacteria and particles ofdirt, must be removed before the water is fit to drink.

Mixing tank

Lumpsdrainedoff

Reservoir

52 Chapter 2

■3 Forming a colloid Alumis added to the mixing tank.Alum causes small particles inthe water to form a type ofcolloid called a gel. Most ofthe bacteria in the water aretrapped in the gel.

■4 Sedimentation Inthe sedimentation tank,large lumps slowly settleto the bottom of thetank from where theycan be removed. Sedimentation tank

Cleaned waterspills over intoa trough.

Layer oflumps

Colloid forms.

■2 Aeration During aeration, airmay be bubbled into the water orwater may be sprayed into the air.Aeration removes substances fromwater, such as iron compounds, thatgive water an unpleasant taste.

Aeration tank

■1 Coarse filter A screenprevents large items suchas leaves from entering thewater treatment plant.Coarse

filterscreen

Alum(aluminumsulfate)added

Outlettower

52 Chapter 2

Getting a Fresh StartBackgroundAeration is of great importance duringwaste-water treatment because of theorganic compounds in sewage. Aerationis also important when the water sourceis groundwater because of the likelyconcentration of dissolved carbonates.Water that contains dissolved calciumand magnesium compounds is calledhard water because deposits of thesesalts can accumulate in water and lessenthe effectiveness of soap. Water softenersreplace calcium and magnesium com-pounds with sodium compounds, whichdo not cause the same problems.

Build Science SkillsObserving

Purpose Students observe that water can taste different depending on its source and on its purity.

Materials tap water, bottled water,distilled water


Procedure Have students tastesamples of tap water, bottled water, anddistilled water. Have them describe anydifferences or similarities they taste.

Safety Use only water that is intendedfor consumption.

Expected Outcome Students will beable to observe that the taste that theyassociate with water is due to substancesdissolved in the water, and not to thewater itself. Kinesthetic, Visual

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Water from the groundAbout half of the drinkingwater in the United Statescomes from natural under-ground sources. Although

this groundwater is filteredas it passes through rocks

and sand, it sometimescontains high levels of

dissolved minerals orchemical pollutants. So

groundwater is purifiedusing a modified version of

surface water filtration.


Carbonfilter

Waterstorage

tank

Sand layer

Gravel layerFiltered water

Fluorinecompound

added

Watersoftener

added

Chlorineadded

■6 Carbon filter The water issometimes passed through acarbon filter. This filter removestiny amounts of dissolvedimpurities, and improves thewater’s taste and color.

■5 Sand and gravel filterNext, the water tricklesthrough sand and gravel filter beds, which trap theremaining suspended particles.Because the filter beds canbecome clogged, they arewashed every 24 hours.

■ Some people use a system to purify water athome. Research and write about one of thesesystems. What is the systemdesigned to do? Compare themethod used to the processesthat occur at a waterpurification plant.

■ Take a Discovery Channel VideoField Trip by watching“Fresh-Squeezed Water.”

Going Further

Video Field Trip

■7 AdditivesThe fluorine compoundprevents tooth decay andthe chlorine kills bacteria.The water softenerremoves some magnesiumand calcium compounds.

■8 Storage Finally,the water is held instorage tanks beforepassing into thepublic water supply.

Going FurtherMost home water purification systemsuse activated charcoal (AC) filters orreverse osmosis (RO). Neither systemremoves all contaminants. Activatedcharcoal is an extremely porous solidwith a large surface area on whichsubstances are absorbed. Activatedcharcoal is effective for removing volatileorganic compounds, pesticides, somemetals, and chlorine. Sometimesbacteria grow on the filter. The surfaceof the activated charcoal may becomesaturated and ineffective before theconsumer replaces the filter.

During reverse osmosis, normalhousehold water pressure forces waterthrough a semipermeable membrane.Reverse osmosis is useful for removinglead and other transition metals,asbestos, and many dissolved organiccompounds. Reverse osmosis is a slow,wasteful process that uses three to ninegallons per gallon of purified water.(Some people use RO systems in theirfish tanks.)

In parts of the world withoutmunicipal treatment plants, UV light isused to kill waterborne pathogens.Verbal


After students have viewed the Video Field Trip,ask them the following questions: Why did thepeople on Catalina Island need to convertseawater to fresh water? (They were running outof drinkable water.) What is a semipermeablemembrane? (A semipermeable membrane is a

material that has microscopic holes to allow water topass through while blocking salts and viruses.) Whatis reverse osmosis? (During reverse osmosis, waterunder high pressure passes through a semipermeablemembrane, which removes dissolved salts and otherunwanted substances.) Why is NASA alsointerested in desalination methods? (Being ableto recycle the water on board a spacecraft reducesthe amount and the weight of water that mustaccompany the astronauts on their voyages.)

Video Field Trip

Fresh-Squeezed Water


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. Combustion of fossil fuels is discussed in Chapter 9.

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Reading Focus

1

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

PPLS



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



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

Materials 3 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?

56 Chapter 2

Recognizing ChemicalChanges

Identifying a Chemical Change

Objective After 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.


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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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.

A

B


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


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 a hard-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).

ASSESSEvaluate UnderstandingHave 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 review the clues that indicate that a chemicalchange is taking place.

Students might argue that observationand experimentation are the key steps in 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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3

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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 denim to 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.


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.



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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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.


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