2.1 Properties of Matter 2 - Ms. Lara La Cueva...

Matter and Change 39

2.1

FOCUSObjectives2.1.1 Identify properties of matter

as extensive or intensive.2.1.2 Define physical property and

list several common physical properties of substances.

2.1.3 Differentiate among three states of matter.

2.1.4 Describe a physical change.

Guide for Reading

Build VocabularyImagine a Picture Before students read this section, have them visualize the structure of solids, liquids, and gases at the microscopic level. Then have them compare their mental pic-tures with the drawings in Figure 2.3.

Reading StrategyActive Comprehension Read the first paragraph of Describing Matter. Ask students what they would like to know about extensive and intensive proper-ties. Students can look for answers as they read the rest of Describing Matter.

INSTRUCT

Tell students that the characters on the chopsticks translate as good fortune, prosperity, longevity, and good health. Why are bamboo chopsticks usually disposable? (Bamboo is inexpensive and abundant.) What characteristic makes bamboo useful for cooking tools? (It is heat resistant.)

Describing MatterDiscussTo help students distinguish extensive from intensive, you may want to note that the prefix in- can mean “within.” Also, the related word extend often refers to an increase in a dimension, such as length, area, or time.

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Section ResourcesPrint• Guided Reading and Study Workbook,

Section 2.1• Core Teaching Resources,

Section 2.1 Review• Transparencies, T10–T11

Technology• Interactive Textbook with ChemASAP,

Animation 1, Assessment 2.1• Go Online, Section 2.1

Connecting to Your World

Section 2.1 Properties of Matter 39

2.1 Properties of Matter

The more than 1200 species of bamboo belong to a family of grasses that includes wheat and corn. In tropical regions, bamboo plants grow rapidly to great heights.

The tender shoots of some bamboo plants are a favorite food of pandas. People use the

woody stems of mature plants to make furniture, fishing rods, and flooring. Because bamboo is inexpensive and abundant, disposable chopsticks are

usually made from bamboo. Bamboo has properties that make it a good choice for

use in chopsticks. It has no noticeable odor or taste. It is hard, yet easy to split, and it is

heat resistant. In this section, you will learn how properties can be used to classify and identify matter.

Guide for Reading

Key Concepts• How can properties used to

describe matter be classified?• Why do all samples of a

substance have the same intensive properties?

• What are three states of matter?• How can physical changes be

classified?

Vocabularymass

volume

extensive property

intensive property

substance

physical property

solid

liquid

gas

vapor

physical change

Reading StrategyUsing Prior Knowledge Before you read, write a definition for the term liquid. After you read this section, compare and contrast the definition of liquid in the text with your original definition.

Describing MatterUnderstanding matter begins with observation and what you observewhen you look at a particular sample of matter is its properties. Is a solidshiny or dull? Does a liquid flow quickly or slowly? Is a gas odorless or doesit have a smell? Properties used to describe matter can be classified asextensive or intensive.

Extensive Properties Recall that matter is anything that has mass andtakes up space. The mass of an object is a measure of the amount of matterthe object contains. The mass of a bowling ball with finger holes is five orsix times greater than the mass of the bowling ball shown in Figure 2.1,which is used to play a game called candlepins. There is also a difference inthe volume of the balls. The volume of an object is a measure of the spaceoccupied by the object. Mass and volume are examples of extensive prop-erties. An extensive property is a property that depends on the amount ofmatter in a sample.

Intensive Properties There are properties to consider when selecting abowling ball other than mass. Beginning bowlers want a bowling ball that islikely to maintain a straight path. They use bowling balls with a hard sur-face made from polyester. Experienced bowlers want a bowling ball theycan curve, or hook, toward the pins. Often, they use a polyurethane ball,which has a softer surface. Hardness is an example of an intensive property.An intensive property is a property that depends on the type of matter in asample, not the amount of matter.

Figure 2.1 This bowling ball and candlepin are used in a game played mainly in New England.

40 Chapter 2

Section 2.1 (continued)

TEACHER DemoTEACHER Demo

Volume and MassPurpose Students observe that air takes up space and has mass.

Materials plastic syringe (50 mL or larger), balance that reads to 0.01 g, Erlenmeyer flask with tightly fitting 1-hole rubber stopper, funnel, water

Procedure Ask students whether or not air takes up space and has mass. Determine the mass of the empty syringe. Then determine the mass when the syringe is full of air. Subtract the two masses to find the mass of air in the syringe. Fit a stopper and funnel into an Erlenmeyer flask. The equip-ment must be airtight. Quickly pour water through the funnel.

Expected Outcome The mass of the air will be about 0.06 g. Some water will run into the flask (air can be com-pressed), but not very much, because air takes up the space in the flask.

Identifying SubstancesUse VisualsTable 2.1 Ask, What is the relation-ship between the state of a sub-stance at room temperature and its boiling point? (Substances that are gases at room temperature have boiling points below 20°C; liquids and solids have boiling points above 20°C. ) Are the properties listed in the table intensive or extensive? (intensive)

FYISome texts use substance to describe all materials and pure substance to describe elements and compounds. In this text, substance and pure substance are synonyms. A sample of matter can be described as pure if the amount of impurities in the sample is negligible.

Download a worksheet on Physical Properties of Matter for students to complete, and find additional teacher support from NSTA SciLinks.

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Gifted and TalentedAsk students to identify two states of matter other than solid, liquid, and gas. Have stu-dents prepare an oral or written report describing the general characteristics of these states and the conditions under which matter is likely to exist in these states.

(Plasmas are likely at extremely high tem-peratures; Bose-Einstein condensates are possible at extremely low temperatures.) Students should explain why these states of matter are not common on Earth.

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40 Chapter 2

Table 2.1

Physical Properties of Some Substances

Identifying SubstancesEach object in Figure 2.2 has a different chemical makeup, or composition.The sculpture of a falcon is mainly gold. The kettles are mainly copper.Matter that has a uniform and definite composition is called a substance.Gold and copper are examples of substances, which are also referred to aspure substances. Every sample of a given substance has identicalintensive properties because every sample has the same composition.

Gold and copper have some properties in common, but there are dif-ferences besides their distinctive colors. Pure copper can scratch the sur-face of pure gold because copper is harder than gold. Copper is better thangold as a conductor of heat or electric current. Both gold and copper aremalleable, which means they can be hammered into sheets without break-ing. But gold is more malleable than copper. Hardness, color, conductivity,and malleability are examples of physical properties. A physical property isa quality or condition of a substance that can be observed or measuredwithout changing the substance’s composition.

Table 2.1 lists physical properties for some substances. The states ofthe substances are given at room temperature. (Although scientists useroom temperature to refer to a range of temperatures, in this book it will beused to refer to a specific temperature, 20°C.) Physical properties can helpchemists identify substances. For example, a colorless substance that wasfound to boil at 100°C and melt at 0°C would likely be water. A colorlesssubstance that boiled at 78°C and melted at �117°C would most certainlynot be water. Based on Table 2.1, it would likely be ethanol.

Checkpoint Which is a better conductor of electric current—gold or copper?

Figure 2.2 This gold falcon standard from Egypt is about 3000 years old. The copper kettles are about 150 years old. Analyzing Data Which of the properties listed in Table 2.1 could not be used to distinguish copper from gold?

For: Links on Physical Properties of Matter

Visit: www.SciLinks.orgWeb Code: cdn-1021

Substance State Color

Melting

point (°C)

Boiling

point (°C)

Neon gas colorless �249 �246

Oxygen gas colorless �218 �183

Chlorine gas greenish-yellow �101 �34

Ethanol liquid colorless �117 78

Mercury liquid silvery-white �39 357

Bromine liquid reddish-brown �7 59

Water liquid colorless 0 100

Sulfur solid yellow 115 445

Sodium chloride solid white 801 1413

Gold solid yellow 1064 2856

Copper solid reddish-yellow 1084 2562

Differentiated Instruction


States of Matter

CLASS ActivityCLASS

Comparing Solids and LiquidsPurpose Students construct models of a solid and liquid.

Materials foam balls or miniature marshmallows

Procedure Use Figure 2.3 as a guide for the three-dimensional models.

Expected Outcome The particles in each model should be closely packed, but the arrangement in the solid should be orderly.


States of ChocolatePurpose Students observe three states of matter when chocolate is heated.

Materials Piece of solid chocolate, small beaker, large beaker, boiling water

Safety Use a thermal mitt while hand-ling the boiling water.

Procedure Show students pieces of solid chocolate. Place them in a small beaker surrounded by boiling water in a larger beaker. After a few minutes, ask, How many physical states can you discern in the mixture? (three)

Expected Outcome Three states can be discerned as the chocolate melts: solid, liquid, and vapor. Remind students that their ability to smell the chocolate means that some components of the mixture must have vaporized.

FYIThere is a formal definition of compressibility in Section 14.1.

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Answers to...Figure 2.2 State; both are solids.Figure 2.3 Solids have a definite shape and volume because their par-ticles are in a rigid, orderly arrange-ment; gases have no definite shape or volume because their particles are rel-atively far apart and can move freely.

Checkpoint

copper

Thixotropic MaterialsChanges of state are typically associated with changes in temperature. However, the state of matter can also be affected by other variables. Thixotropic materials are solidlike materials that liquefy when subjected to shearing forces. For example, many paints are thixotropic; they thin out when brushed on a surface and thicken when the brush

strokes stop, thus keeping the paint from sliding off the wall! A shearing force has an opposite effect on quicksand. Quick move-ments “thicken” the quicksand and make it much more difficult for a person or animal trapped in it to move. (A shearing force causes two layers to slide in opposite direc-tions along a plane of contact.)

Section 2.1 Properties of Matter 41

States of MatterDepending on the circumstances, you use three different words to refer towater—water, ice, and steam. Water, which is a common substance, existsin three different physical states. So can most other substances. Threestates of matter are solid, liquid, and gas. Certain characteristics that candistinguish these three states of matter are summarized in Figure 2.3.

Solids A solid is a form of matter that has a definite shape and volume.The shape of a solid doesn’t depend on the shape of its container. Theparticles in a solid are packed tightly together, often in an orderly arrange-ment, as shown in Figure 2.3a. As a result, solids are almost incompressible;that is, it is difficult to squeeze a solid into a smaller volume. In addition,solids expand only slightly when heated.

Liquids Look at Figure 2.3b. The particles in a liquid are in close contactwith one another, but the arrangement of particles in a liquid is not rigid ororderly. Because the particles in a liquid are free to flow from one locationto another, a liquid takes the shape of the container in which it is placed.However, the volume of the liquid doesn’t change as its shape changes. Thevolume of a liquid is fixed or constant. Thus, a liquid is a form of matter thathas an indefinite shape, flows, yet has a fixed volume. Liquids are almostincompressible, but they tend to expand slightly when heated.

Figure 2.3 The arrangement of particles is different in solids, liquids, and gases. In a solid, the particles are packed closely together in a rigid arrangement. In a liquid, the particles are close together, but they are free to flow past one another. In a gas, the particles are relatively far apart and can move freely.Relating Cause and Effect Use the arrangements of their particles to explain the general shape and volume of solids and gases.

a

bc

Solid Liquid GasDefinite shape

Definite volumeNot easily compressed

Indefinite shapeDefinite volume

Not easily compressed

Indefinite shapeIndefinite volumeEasily compressed

b ca

withChemASAP

Animation 1 Relate the states of matter to the arrangements of their particles.

Facts and Figures

42 Chapter 2


Physical ChangesUse VisualsFigure 2.4 Have students study the photograph. Refer them to page R14 of the Elements Handbook for data on the properties of gallium. Ask, Is the melting of gallium a reversible or irreversible change? (reversible) What is the physical state of gallium at room temperature? (solid)

ASSESSEvaluate UnderstandingTo assess students’ knowledge of states of matter, ask, How are three states of matter involved when a candle burns? Tell students to include the definitions for each state. (Solid wax has a definite shape and volume; the melted liquid waxis shapeless and runs down the side of the candle; the vapor that forms above the wick has no shape.)

ReteachIf possible, bring samples of some of the substances listed in Table 2.1 to class. Describe the physical properties of selected samples and have students practice identifying them by referring to Table 2.1.

Elements Handbook

The melting point of indium is 157°C. Both indium and gallium (30°C) have relatively low melting points com-pared with gold (1064°C).

with ChemASAP

If your class subscribes to the Interactive Textbook, use it to review key concepts in Section 2.1.

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Section 2.1 Assessment1. intensive and extensive properties 2. Every sample of a given substance has

the same chemical composition.3. solid, liquid, gas4. Physical changes are either reversible or

irreversible. Reversible changes can be “undone,” or reversed. Irreversible changes cannot be undone.

5. Color; sodium chloride is the only white solid listed.

6. Liquids and gases have an indefinite shape. The shape of a solid is definite; the shape of a liquid is indefinite.

7. The freezing of mercury is reversible because solid mercury can be melted.

8. Samples of gold and copper can have the same mass and volume (extensive proper-ties). They cannot have the same set of intensive properties because they have different chemical compositions.

42 Chapter 2

Handbook

Read about the metal indium on page R16. What is the melting point of indium? Which other metal has a similar melting point—gallium or gold? Provide data to support your answer.

1. Key Concept Name two categories used to classify properties of matter.

2. Key Concept Explain why all samples of a given substance have the same intensive properties.

3. Key Concept Name three states of matter.

4. Key Concept Describe the two categories used to classify physical changes.

5. Which property in Table 2.1 can most easily distin-guish sodium chloride from the other solids?

6. In what way are liquids and gases alike? In what way are liquids and solids different?

7. Is the freezing of mercury a reversible or irrevers-ible physical change? Explain your answer.

8. Explain why samples of gold and copper can have the same extensive properties, but not the same intensive properties.

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Gases Like a liquid, a gas takes the shape of its container. But unlike a liq-uid, a gas can expand to fill any volume. A gas is a form of matter that takesboth the shape and volume of its container. Look back at Figure 2.3c. Asshown in the model, the particles in a gas are usually much farther apartthan the particles in a liquid. Because of the space between particles, gasesare easily compressed into a smaller volume.

The words vapor and gas are sometimes used interchangeably. Butthere is a difference. The term gas is used for substances, like oxygen, thatexist in the gaseous state at room temperature. (Gaseous is the adjectiveform of gas.) Vapor describes the gaseous state of a substance that is gener-ally a liquid or solid at room temperature, as in water vapor.

Checkpoint When should the term vapor be used instead of gas?

Physical ChangesThe melting point of gallium metal is 30°C. Figure 2.4 shows how heat froma person’s hand can melt a sample of gallium. The shape of the samplechanges during melting as the liquid begins to flow, but the composition ofthe sample does not change. Melting is an example of a physical change.During a physical change, some properties of a material change, but thecomposition of the material does not change.

Words such as boil, freeze, melt, and condense are used to describephysical changes. So are words such as break, split, grind, cut, and crush.However, there is a difference between these two sets of words. Each setdescribes a different type of physical change. Physical changes can beclassified as reversible or irreversible. Melting is an example of a reversiblephysical change. If a sample of liquid gallium is cooled below its meltingpoint, the liquid will become a solid. All physical changes that involve achange from one state to another are reversible. Cutting hair, filing nails,and cracking an egg are examples of irreversible physical changes.

Assessment 2.1 Test yourself on the concepts in Section 2.1.

Figure 2.4 The silvery substance in the photograph is gallium, which has a melting point of 30°C.Inferring What can you infer about the temperature of the hand holding the gallium?

2.1 Section Assessment


Hanging By a Thread

PurposeThis Technology & Society feature builds on the discussion of technology and biotechnology in Chapter 1, where an operational definition of a gene was given, and on the discussion of proper-ties in Section 2.1.

Background• Only about one-third of spider species

spin webs, but all spiders produce silk. Different types of silk glands produce silk used for wrapping prey, silk used for wrapping eggs, non-sticky silk used for draglines, and sticky strands or droplets used for trapping prey.

• Silk is a composite of proteins, which explains why the silk can be strong, tough, and elastic.

• Semi-permanent webs of golden orb spiders are large (40–80 cm diameter). A cable of golden orb dragline silk that is slightly thicker than a garden hose would be strong enough to support two jet planes full of passengers and cargo without breaking.

• Historically, people have collected spi-der silk to catch fish or birds and to use as bandages to stop blood flow from wounds. Possible commercial applica-tions of recombinant spider silk are for sutures, biodegradable fishing lines, soft body armor, and in composites.

Answers to...Interpreting Diagrams The silk glands are in the spider’s abdomen.Figure 2.4 Because gallium is shown in the liquid state, the temperature of the hand must be greater than 30°C.

Checkpoint

The term vapor is used to refer to the gaseous state of a substance that is usually a liquid or solid at room temperature.

Heart

Eyes

MouthStomach

LungSilk glands

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Hanging by a Thread

Strands in a spider web are about one tenth the diameter

of a human hair. Yet a golden orb spider web can withstand

the impact of a insect, or even a small bird, flying at high

speed because the silk in the web’s frame and spokes is

stronger than steel, more elastic than nylon, and tougher

than rubber. Scientists are always looking for lightweight

materials with these properties, but they cannot set up

farms to harvest spider silk because a spider will fight to

defend its territory. Instead, scientists use biotechnology

to produce spider silk. Interpreting Diagrams Where are

the silk glands located in a spider?

Spinnerets(magnification: 110,0003)

Spider anatomy A spider releases its silk through spinnerets at the tip of its abdomen. Inside each spinneret are tens or hundreds of spigots. Silk travels from a silk gland through a duct to a spigot. As the silk is released from a spigot, it changes from a liquid to a solid.

Female Golden Orb SpiderNephila clavipesBody length: 24 mm to 40 mm

Spider silk from goat’s milkScientists have identified the spider genes that contain the instructions for producing silk. When these genes are transferred to goats, the goats produce milk containing spider silk. Scientists separate the silk from the milk, purify it, and spin it into fibers.

44 Chapter 2

Print• Guided Reading and Study Workbook,


Section 2.2 Review• Transparencies, T12–T14


Problem-Solving 2.10, Assessment 2.2

2.2

FOCUSObjectives2.2.1 Categorize a sample of matter

as a substance or a mixture.2.2.2 Distinguish between homoge-

neous and heterogeneous sam-ples of matter.

2.2.3 Describe two ways that com-ponents of mixtures can be separated.

Guide for Reading

Build VocabularyWord Forms Have students look up the term homogenize and explain how this process could be used to turn a heterogeneous mixture into one that is more homogeneous.

Reading StrategyUsing Context Clues Tie the Reading Strategy to the Build Vocabulary strat-egy. Have students use the definitions of homogeneous and heterogeneous mixtures to answer the question posed in the Reading Strategy.

INSTRUCT

After students read the introduction, ask What is the result of panning? (Gold is separated from a mixture of materials.) What property is used to separate the mixture? (heaviness of the materials) Heavier and lighter are used to compare the materials because density isn’t discussed until Chapter 3.

Classifying Mixtures

DiscussBring orange juice or liquid salad dressing to class and compare the properties of these mixtures to those of pure water or pure NaCl. Explain that mixtures are variable in composition.

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44 Chapter 2

2.2 Mixtures

In 1848, gold was discovered in California. This discovery led to a massive migration, or rush, of people to California. Panning is one way to separate gold from a mixture of gold and materials such as sand or gravel. A pan containing the mixture is placed under-water and shaken vigorously from left to right. This motion causes heavier materials, such as gold, to move to the bottom of the pan and lighter materials, such as sand, to move to the top where they can be swept away. In this section, you will learn how to classify and separate mixtures.

Guide for Reading

Key Concepts• How can mixtures be classified?• How can mixtures be

separated?

Vocabularymixture

heterogeneous mixture

homogeneous mixture

solution

phase

filtration

distillation

Reading StrategyBuilding Vocabulary After you read this section, explain the dif-ference between homogeneous and heterogeneous mixtures.

Classifying MixturesA salad bar, like the one in Figure 2.5, provides a range of items, such ascucumbers and hot peppers. Customers choose which items to use in theirsalads and how much of each item to use. So each salad has a differentcomposition. A mixture is a physical blend of two or more components.

Most samples of matter are mixtures. Some mixtures are easier to rec-ognize than others. You can easily recognize chicken noodle soup as a mix-ture of chicken, noodles, and broth. Recognizing air as a mixture of gases ismore difficult. But the fact that air can be drier or more humid shows thatthe amount of one component of air—water vapor—can vary. Chickennoodle soup and air represent two different types of mixtures. Basedon the distribution of their components, mixtures can be classified asheterogeneous mixtures or as homogeneous mixtures.

Figure 2.5 You can choose the amount of each item you select from a salad bar. So your salad is unlikely to have the same composition as other salads containing the same items.


Quick LABQuick LAB

Separating Mixtures

ObjectiveAfter completing this activity, students will be able to:• separate the components of a mix-

ture using paper chromatography.Skills Focus Observing, inferring, drawing conclusions

Prep Time 15 minutes

Advance Prep • Cut the paper strips in advance to

save time. Strips of paper toweling can be used in place of filter paper.

Class Time 25 minutes

Safety Rubbing alcohol is poisonous and flammable. It is also an irritant when inhaled. Keep containers cov-ered and away from heat. If the room is not well ventilated, use a fume hood.

Expected Outcome Bands of color will separate on the filter paper.

Extension Students could repeat the lab using different brands of markers, different colors, or water in place of rubbing alcohol.

Analyze and Conclude 1. Bands of colors appear as the alco-

hol moves up the paper.2. The bands of colors indicate that

green ink is a mixture.3. The color pattern of the unknown

ink can be compared with color pat-terns from known types of green ink.

For EnrichmentStudents can research Rf values of dif-ferent dyes and how they are calcu-lated (ratio of distance traveled by dye to distance traveled by solvent). They can then quantify this lab by calculat-ing the Rf values of the dyes in the ink.

FYIIn Section 15.3, solutions, suspensions, and colloids are compared.

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Answers to...

Checkpoint

one

English LearnersEncourage English-learner students to compile a glossary in which they define each key term in English and in their native language. Suggest that students also include illustrations when appropriate.

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Filter paper

Ink line

Alcohol

Section 2.2 Mixtures 45

Quick LABQuick LAB

Separating Mixtures

PurposeTo separate a mixture using paper chromatography.

Materials

• green marking pen

• filter paper strip

• metric ruler

• clear plastic tape

• pencil

• rubbing alcohol

• clear plastic drinking cup

• clear plastic wrap

Procedure 1. Use the marking pen to draw a line

across a strip of filter paper, as shown in the drawing. The line should be 2 cm from one end of the strip.

2. Tape the unmarked end of the filter paper to the center of a pencil so that the strip hangs down when the pencil is held horizontally.

3. Working in a well-ventilated room, pour rubbing alcohol into a plastic cup to a depth of 1 cm.

4. Rest the pencil on the rim of the cup so that the ink end of the strip touches the rubbing alcohol, but does not extend below its surface. Use plastic wrap to cover the top of the cup.

5. Observe the setup for 15 minutes.

Analyze and Conclude

1. How did the appearance of the filter paper change during the procedure?

2. What evidence is there that green ink is a mixture?

3. How could you use this procedure to identify an unknown type of green ink?

Heterogeneous Mixtures In chicken noodle soup, the ingredients arenot evenly distributed throughout the mixture. There is likely to be morechicken in one spoonful than in another spoonful. A mixture in which thecomposition is not uniform throughout is a heterogeneous mixture.

Homogeneous Mixtures The substances in the olive oil and vinegar inFigure 2.6 are evenly distributed throughout these mixtures. So olive oildoesn’t look like a mixture. The same is true for vinegar. Vinegar is a mixtureof water and acetic acid, which dissolves in the water. Olive oil and vinegarare homogeneous mixtures. A homogeneous mixture is a mixture in whichthe composition is uniform throughout. Another name for a homogeneousmixture is a solution. Many solutions are liquids. But some are gases, likeair, and some are solids, like stainless steel, which is a mixture of iron, chro-mium, and nickel.

The term phase is used to describe any part of a sample with uniformcomposition and properties. By definition, a homogeneous mixture con-sists of a single phase. A heterogeneous mixture consists of two or morephases. When oil and vinegar are mixed, they form layers, or phases, asshown in Figure 2.6. The oil phase floats on the water phase.

Checkpoint How many phases are there in a homogeneous mixture?

Figure 2.6 Olive oil and vinegar are homogeneous mixtures. The substances in these mixtures are evenly distributed. When olive oil is mixed with vinegar, they form a heterogeneous mixture with two distinct phases.

Olive oil Vinegar

Oil & vinegar

Differentiated InstructionChromatographyThe basis of chromatography is the partitioning of components between a stationary phase and a moving phase based on differences in solubility. The components travel with different speeds in the moving solvent.

Facts and Figures

46 Chapter 2


CONCEPTUAL PROBLEM 2.1

Aluminum nails are used on trail mark-ers so that if a tree is harvested, the nail can be sliced through at the lumber mill without causing injury to a worker.

Answers9. Iron is magnetic; table salt is not.

Table salt will dissolve in water; iron will not.

10. By lowering the temperature to below the boiling point of each gas, you could condense each sub-stance and separate the gases.

Practice Problems PlusClassify the following mixtures as homogeneous or heterogeneous.a. granite rock (heterogeneous)b. salt water (homogeneous)c. paint (heterogeneous)d. a silver ring (homogeneous)

Separating Mixtures


Metallic BreakfastPurpose Students will observe the separation of iron filings from iron-fortified breakfast cereal.

Materials iron-fortified breakfast cereal, 400-mL beaker, distilled water, magnetic stirrer with stirring bar

Safety Remind students not to eat the cereal.

Procedure Place a stirring bar in a400-mL beaker. Add about 30 g of cereal to the beaker and add distilled water until the beaker is about half full. Using a magnetic stirrer, mix gently for about 20 minutes. Retrieve the stirring bar and observe the black iron filings attached to it.

Expected Outcome Iron filings will cover the stirring bar. They are added to cereal as an iron supplement. Explain that stomach acid changes the iron into a form the body can use.

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Distillation A working distillation apparatus was described in the writings of Maria of Alexan-dria, an alchemist who lived and worked nearly two thousand years ago. The city of Alexandria, located on the Nile River in North

Africa, was a world center of science and cul-ture at that time. Maria of Alexandria is also credited with inventing other chemical appa-ratus, such as the water bath, which to this day bears her name: the bain marie.

46 Chapter 2

Practice Problems

1 2


Separating a Heterogeneous MixtureSometimes plastic signs are used to mark trails used by hikers orvehicles. The sign in the photo is used to mark locations along atrail where an all terrain vehicle (ATV) is permitted. Aluminumnails are used to attach signs at eye level to trees or posts. Howcould a mixture of aluminum nails and iron nails be separated?

Analyze Identify the relevant concepts.

List properties of aluminum and iron.

Aluminum:

• metal

• gray color

• doesn’t dissolve in water

• not attracted to magnet

Iron:

• metal

• gray color

• doesn’t dissolve in water

• attracted to magnet

Solve Apply concepts to this situation.

Identify a property that can be used to separate iron and aluminum objects. The ability to be attracted by a magnet is a property that iron and aluminum do not share. You could use a magnet to remove the iron nails from a mixture of iron and aluminum nails.

Practice Problems

9. What physical properties could be used to separate iron filings from table salt?

10. Air is mainly a mixture of nitrogen and oxygen, with small amounts of other gases such as argon and carbon dioxide. What property could you use to separate the gases in air?

Separating MixturesIf you have a salad containing an ingredient you don’t like, you can use afork to remove the pieces of the unwanted ingredient. Many mixtures arenot as easy to separate. To separate a mixture of olive oil and vinegar, forexample, you could decant, or pour off, the oil layer. Or you might coolthe mixture until the oil turned solid. The first method takes advantage ofthe fact that oil floats on water. The second method takes advantage of adifference in the temperatures at which the olive oil and vinegar freeze.

Differences in physical properties can be used to separate mixtures.

Filtration The colander in Figure 2.7 can separate cooked pasta fromthe cooking water. The water passes through the holes in the colander, but thepasta does not. The holes, or pores, in a coffee filter are smaller than theholes in a colander to retain coffee grains. But the holes are not smallenough to retain the particles in water. The process that separates a solidfrom the liquid in a heterogeneous mixture is called filtration.

Figure 2.7 A colander is used to separate pasta from the water in which it was cooked. This process is a type of filtration.

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Problem Solving 2.10 Solve Problem 10 with the help of an interactive guided tutorial.

Facts and Figures


Use VisualsFigure 2.8 Review the distillation pro-cess, and describe the components of the apparatus. Ask, What might be an advantage to having a long con-denser in a distillation apparatus? (There would be more surface area on which the vapor can condense.) Is dis-tilled water that contains dissolved gases a pure substance? (No, distilled water is not pure unless the dissolved gases are removed.)

ASSESSEvaluate UnderstandingHave students identify five items that fit each of the following categories.a. substanceb. homogeneous mixturec. heterogeneous mixtured. solutionHave students select three items (one each from categories b, c, and d). For each item selected, students should outline a method for separating the components.

ReteachExplain that distillation can be used to separate a mixture of gases. Show stu-dents the drawing of the fractional dis-tillation of liquid air on R24 of the Elements Handbook. Ask, What physi-cal property is used to separate the gases? (boiling point)[icon] Writing Activity

Possible evidence includes the fact that only some substances in the tea leaves dissolve in water, and tea is sold both with and without caffeine.

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Answers to...Figure 2.8 They are much higher than the boiling point of water.

Section 2.2 Assessment11. as heterogeneous or homogeneous 12. differences in physical properties13. A phase is any part of a sample with uni-

form composition. There is one phase in a homogeneous mixture and two or more phases in a heterogeneous mixture.

14. a. homogeneous b. heterogeneousc. homogeneous d. heterogeneous

15. Both have a uniform composition through-out. A substance has a definite composi-tion; a solution has a variable composition.

16. Filtration separates solids from a liquid in a heterogeneous mixture. Distillation can separate a liquid from substances dis-solved in the liquid.

17. Add water to dissolve the salt. Pour the resulting mixture onto a piece of closely woven cloth. The sand will remain on the cloth, and the salt solution will pass through. Use evaporation to remove the water from the salt solution, leaving solid salt behind.

Section 2.2 Mixtures 47

100°C

Steam at100°C

Cold water out

Cold water in

Condenser

ReceiverflaskTap water

(mixture)

Distilledwater

withChemASAP

Distillation Tap water is a homogeneous mixture of water and sub-stances that dissolved in the water. One way to separate water from theother components in tap water is through a process called distillation. Dur-ing a distillation, a liquid is boiled to produce a vapor that is then con-densed into a liquid. Figure 2.8 shows an apparatus that can be used toperform a small-scale distillation.

As water in the distillation flask is heated, water vapor forms, rises in theflask, and passes into a glass tube in the condenser. The tube is surroundedby cold water, which cools the vapor to a temperature at which it turns backinto a liquid. The liquid water is collected in a second flask. The solid sub-stances that were dissolved in the water remain in the distillation flaskbecause their boiling points are much higher than the boiling point of water.


11. Key Concept How are mixtures classified?

12. Key Concept What type of properties can be used to separate mixtures?

13. Explain the term phase as it relates to homoge-neous and heterogeneous mixtures.

14. Classify each of the following as a homogeneous or heterogeneous mixture.

a. food coloring b. ice cubes in liquid water c. mouthwash d. mashed, unpeeled potatoes

15. How are a substance and a solution similar? How are they different?

16. In general, when would you use filtration to sepa-rate a mixture? When would you use distillation to separate a mixture?

17. Describe a procedure that could be used to separate a mixture of sand and table salt.

Writing to Persuade Write a paragraph in support of this statement: Dry tea is a mixture, not a sub-stance. Include at least two pieces of evidence to support your argument.


Figure 2.8 A distillation can be used to remove impurities from water. As liquid water changes into water vapor, substances dissolved in the water are left behind in the distillation flask.Inferring What can you infer about the boiling points of substances dissolved in the impure water?

48 Chapter 2


Section 2.3• Core Teaching Resources, Section 2.3

Review, Interpreting Graphics• Transparencies, T15–T17


Problem-Solving 2.19, Assessment 2.3• Go Online, Section 2.3

2.3

FOCUSObjectives2.3.1 Explain the difference between

an element and a compound.2.3.2 Distinguish between a sub-

stance and a mixture.2.3.3 Identify the chemical symbols

of elements, and name ele-ments, given their symbols.

Guide for Reading

Build VocabularyWord Forms Compare the common meanings of element, elemental, and elementary in relation to the specific use of element in chemistry. In their broadest sense, the terms refer to fun-damentals, first principles, or basics.

Reading StrategyPreview Before students read the sec-tion, have them preview the headings to get an overall sense or the content.

INSTRUCT

Ask students if they have ever eaten cotton candy. Ask those who have to describe its properties. Explain that its sweet taste is due to a sugar called sucrose, which is a compound.

Distinguishing Elements and CompoundsUse VisualsCompare the chemical change of sugar shown on R29 with the one shown in Figure 2.9. Ask, What is similar about the changes? (Sugar breaks down into solid carbon and water vapor.) What is different? (In one case, heating causes the change; in the other, the addition of an acid.) You can also compare the electrolysis of water to the distillation of water (Section 2.2).

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Section Resources

48 Chapter 2

2.3 Elements and Compounds

Guide for Reading

Key Concepts• How are elements and

compounds different?• How can substances and

mixtures be distinguished?• What do chemists use to

represent elements and compounds?

Vocabularyelement

compound

chemical change

chemical symbol

Reading StrategyRelating Text and Visuals As you read, look at Figure 2.10. Explain how this illustration helps you understand the relationship between different kinds of matter.

Distinguishing Elements and CompoundsSubstances can be classified as elements or compounds. An element is thesimplest form of matter that has a unique set of properties. Oxygen andhydrogen are two of the more than 100 known elements. A compound is asubstance that contains two or more elements chemically combined in afixed proportion. For example, carbon, oxygen, and hydrogen are chemi-cally combined in the compound sucrose, the sugar in spun sugar. (Some-times sucrose is referred to as table sugar to distinguish it from other sugarcompounds.) In every sample of sucrose there are twice as many hydrogenparticles as oxygen particles. The proportion of hydrogen particles tooxygen particles in sucrose is fixed. There is a key difference between ele-ments and compounds. Compounds can be broken down into sim-pler substances by chemical means, but elements cannot.

Breaking Down Compounds Physical methods that are used to sepa-rate mixtures cannot be used to break a compound into simpler substances.Boil liquid water and you get water vapor, not the oxygen and hydrogen thatwater contains. Dissolve a sugar cube in water and you still have sucrose, notoxygen, carbon, and hydrogen. This result does not mean that sucrose orwater cannot be broken down into simpler substances. But the methodsmust involve a chemical change. A chemical change is a change that pro-duces matter with a different composition than the original matter. Heating

is one of the processes used to break down compounds into simpler sub-stances. The layer of sugar in Figure 2.9 is heated in a skillet until it breaks

down into solid carbon and water vapor. Can the substances that areproduced also be broken down?

Figure 2.9 When table sugar is heated, it goes through a series of chemical changes. The final products of these changes are solid carbon and water vapor.

Take two pounds of sugar, two cups of boiling water, and one-quarter teaspoon of cream of tartar. You have the ingredients to make spun sugar. Add food coloring and you have the sticky, sweet concoction sold at baseball games and amusement parks as cotton candy. Sugar is a substance that contains three other substances—carbon, hydrogen, and oxygen. In this section, you will learn how substances are classified as elements or compounds.


Section 2.3 Elements and Compounds 49

Word OriginsThere is no chemical process that will break down carbon into simplersubstances because carbon is an element. Heating will not cause water tobreak down, but electricity will. When an electric current passes throughwater, oxygen gas and hydrogen gas are produced. The following diagramsummarizes the overall process.

Chemical change

Compound CompoundCarbonElement

Waterheat�Sugar

Chemical change

CompoundOxygenElementElement

Water electricityHydrogen �

¡

¡¬

Properties of Compounds In general, the properties of compoundsare quite different from those of their component elements. Sugar is asweet-tasting, white solid, but carbon is a black, tasteless solid. Hydrogen isa gas that burns in the presence of oxygen—a colorless gas that supportsburning. The product of this chemical change is water, a liquid that canstop materials from burning. Figure 2.10 shows samples of table salt(sodium chloride), sodium, and chlorine. When the elements sodium andchlorine combine chemically to form sodium chloride, there is a change incomposition and a change in properties. Sodium is a soft, gray metal. Chlo-rine is a pale yellow-green poisonous gas. Sodium chloride is a white solid.

Checkpoint What process can be used to break down water? Figure 2.10 Compounds and the elements from which they form have different properties. Observing Based on the photographs, describe two physical properties of sodium and two of chlorine.

Sodium chloride(commonly known as table salt) is used to season or preserve food.

Chlorine is used to kill harmful organisms in swimming pools.

Sodium is stored under oil to keep it from reacting with oxygen or water vapor in air. Sodium vapor produces the light in some street lamps.

Compound comes from a Latin word componere,meaning “to put together.” Elements are put together, or chemically combined, in compounds. What items are put together in a compound sentence?


Decomposition of SugarPurpose Students will observe the decomposition of sugar by acid.

Materials 100-mL beaker, powdered sugar, 18M sulfuric acid, glass stirring rod

Safety Perform the demo in a fume hood or in a well-ventilated area and be sure to wear safety goggles. CAUTION This is an extremely exo-thermic reaction.

Procedure Place about 25 g of powdered sugar in a 100-mL beaker. Carefully add about 10–15 mL of 18M sulfuric acid, and stir rapidly with a glass rod.

Disposal Place the cooled beaker and its contents in a large beaker and add water. Neutralize with NaOH and pour the liquid down the drain, flushing with excess water. Throw the solid resi-due in the trash.

Expected Outcome The mixture turns dark and a column of carbon rises out of the beaker.

Word OriginsA compound sentence contains two independent clauses, each with a subject and a verb. The two clauses are joined by a conjunction.

RelateRefer students to Salt of the Earth on page R9 of the Elements Handbook for a discussion of the historic importance of table salt.

FYIIn the Small-Scale lab in Chapter 21, students will learn that the electrolysis of water doesn’t work with pure water.

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Answers to...Figure 2.10 sodium: gray color, shiny, solid at room temperature; chlorine: yellow-green color, gas at room temperature.

Checkpoint

passing an elec-tric current through water

Gifted and TalentedOther forms of energy besides electricity can be used to bring about the decomposition of water into hydrogen and oxygen. Ask stu-dents to consider whether heat could be used for this purpose. Tell them that early chemists considered water to be an element

because it would not decompose when heated. Have students do research on the conditions necessary to decompose water using heat. What hazards, if any, might pre-clude the routine use of this method?

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50 Chapter 2


Distinguishing Substances and MixturesUse VisualsFigure 2.11 Have students study the figure. Then ask, Can compounds be separated into their component ele-ments by physical processes? (No, they must be separated by chemical processes.)

CLASS ActivityCLASS

SubstancesPurpose To establish the definition of substance, using an inductive approach.

Materials 20 index cards, masking tape

Procedure Make signs with the names of each of these items: 1 oxygen, 2 neon, 3 apple, 4 sand, 5 iron, 6 water, 7 air, 8 paint, 9 sodium chloride, 10 sucrose, 11 carbon dioxide, 12 granite, 13 laun-dry detergent, 14 citric acid, 15 cereal, 16 salad, 17 salad dressing, 18 copper, 19 ocean water, 20 gold. Using masking tape and the board or wall, create two columns. Place items 1 and 2 under Column A and items 3 and 4 under Column B. Ask students to think about the criteria for each column. Then place item 5 in Column A. Students may have to change their criteria. Place item 6 in Column A. Ask students what they think the criteria are now. Continue placing one item up at a time in a column until students understand the criteria. Every-thing in Column A has a uniform and definite composition.

Expected Outcome Column A is for substances. Column A: 1, 2, 5, 6, 9, 10, 11, 14, 18, 20. Column B is for mixtures. Column B: 3, 4, 7, 8, 12, 13, 15, 16, 17, 19. Make sure students understand the difference between the uniform com-position of a substance and of a solu-tion. The composition of the substance is fixed. The composition of the mix-ture can vary.

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Facts and FiguresOrigins of Element NamesMany elements were named for the people who discovered them or the places where they were discovered. Some elements were given descriptive names taken from classical Latin or Greek. Others were named for figures in mythology. Polonium is named for Poland, the native land of Marie Curie, who discovered radium. Californium was discovered at the

University of California. The name chlorine comes from the Greek chloros, meaning greenish-yellow. Chlorine is a greenish-yellow gas. The name calcium is derived from the Latin calx, meaning lime. Calcium is a major component of limestone. Students can find the origins of element names by consulting a dictionary or encyclopedia.

50 Chapter 2

Silver

Matter

Element

Substance

Definite composition(homogeneous)

Can be separated

physically

Can be separated

chemically

Compound

Mixture of

substances

Variable composition

Uniform; alsocalled a solution

Homogeneous

mixtureHeterogeneous

mixtureNonuniform;distinct phases

BerylSilver GraniteStainless Steel

Figure 2.11 The flow chart summarizes the process for classifying matter. Any sample of matter is either an element, a compound, or a mixture. Interpreting Diagrams What is the key difference between a substance and a solution?

Distinguishing Substances and MixturesDeciding whether a sample of matter is a substance or a mixture basedsolely on appearance can be difficult. After all, homogeneous mixtures andsubstances will both appear to contain only one kind of matter. Sometimesyou can decide by considering whether there is more than one version ofthe material in question. For example, you can buy whole milk, low-fatmilk, no-fat milk, light cream, and heavy cream. From this information, youcan conclude that milk and cream are mixtures. You might infer that thesemixtures differ in the amount of fat they contain. Most gas stations offer atleast two blends of gasoline. The blends have different octane ratings anddifferent costs per gallon, with premium blends costing more than regularblends. So gasoline must be a mixture.

You can use their general characteristics to distinguish substancesfrom mixtures. If the composition of a material is fixed, the materialis a substance. If the composition of a material may vary, the material isa mixture. Figure 2.11 summarizes the general characteristics of elements,compounds, and mixtures.


Answers to...Figure 2.11 The composition of a substance is fixed; the composition of a solution may vary.

Less Proficient Readers/English LearnersHave students make flash cards to help learn the names and symbols of at least 30 ele-ments. If an element has a different common name in the student’s native country, have the student include that information on the flash card.

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Answers18. Liquid A is probably a substance.

Liquid B is a mixture.19. The liquid was not an element

because a solid was left when the liquid evaporated. A physical pro-cess, such as evaporation, cannot be used to break down a com-pound. Therefore, the liquid was a mixture.

Practice Problems PlusClassify the following materials as an element, compound, or mixture. Give reasons for your answers.a. table salt (NaCl) (compound; its for-

mula shows that it is composed of at least two elements)

b. salt water (mixture; its name sug-gests that it is composed of at least two substances)

c. sodium (Na) (element; its symbol shows that it is composed of only one kind of matter)

Symbols and FormulasDiscussPoint out that the use of chemical sym-bols is an example of the “international language of chemistry.” Discuss why people all over the world use the same set of chemical symbols. Show how symbols are used in chemical formulas. Emphasize that a compound is always made up of the same elements in the same proportions.

CLASS ActivityCLASS

Timeline of DiscoveryTo help students explore the history of science, have the class work as a group to make a timeline for the discovery of elements. Refer them to the data on dates of discovery in the Elements Handbook. Students could use the timeline to detect shifts in naming conventions.

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Section 2.3 Elements and Compounds 51


Classifying MaterialsWhen the blue-green solid in the photograph is heated, a colorless gas and a black solid form. All three materials are substances. Is it possible to classify these substances as elements or compounds?

Analyze Identify the relevant concepts.

List the known facts and relevant concepts.• A blue-green solid is heated.• A colorless gas and a black solid appear.• A compound can be broken down into simpler

substances by a chemical change, but an element cannot.

• Heating can cause a chemical change.

Solve Apply concepts to this situation.

Determine if the substances are elements or compounds. Before heating, there was one sub-stance. After heating there were two substances. The blue-green solid must be a compound. Based on the information given, it isn’t possible to know if the colorless gas or black solid are elements or compounds.

Practice Problems

18. Liquid A and Liquid B are clear liquids. They are placed in open containers and allowed to evaporate. When evaporation is complete, there is a white solid in container B, but no solid in container A. From these results, what can you infer about the two liquids?

19. A clear liquid in an open container is allowed to evaporate. After three days, a solid is left in the container. Was the clear liquid an element, a compound, or a mixture? How do you know?

Symbols and FormulasThe common names water and table salt do not provide information aboutthe chemical composition of these substances. Also, words are not ideal forshowing what happens to the composition of matter during a chemicalchange. Chemists use chemical symbols to represent elements, andchemical formulas to represent compounds.

Using symbols to represent different kinds of matter is not a new idea.Figure 2.12 shows some symbols that were used in earlier centuries. Thesymbols used today for elements are based on a system developed by aSwedish chemist, Jöns Jacob Berzelius (1779–1848). He based his symbolson the Latin names of elements. Each element is represented by a one- ortwo-letter chemical symbol. The first letter of a chemical symbol is alwayscapitalized. When a second letter is used, it is lowercase.

Figure 2.12 The symbols used to represent elements have changed over time. Alchemists and the English chemist John Dalton (1766–1844) both used drawings to represent chemical elements. Today, elements are represented by one- or two-letter symbols.

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Problem Solving 2.19 Solve Problem 19 with the help of an interactive guided tutorial.

Dalton‘s Symbols

Gold Oxygen Zinc

Carbon

Alchemy Symbols

Gravel Tin Clay

Lead Salt Zinc


52 Chapter 2


Download a worksheet on Element Names for students to complete, and find additional teacher support from NSTA SciLinks.

ASSESSEvaluate UnderstandingAsk students to explain in their own words the difference between an element and a compound.

ReteachSet up a display of elements in the classroom to help students relate abstract chemical symbols to actual samples. Have students present oral reports about the physical properties of elements and refer to the display.[icon] Writing Activity

Elements and compounds are alike in that they both are substances of fixed composition. They are different in that elements cannot be sepa-rated into simpler substances through chemical changes.

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Section 2.3 Assessment20. Compounds can be broken down into

simpler substances by chemical means, but elements cannot.

21. A substance has a fixed composition. The composition of a mixture may vary.

22. Chemical symbols are used to represent elements. Chemical formulas are used to represent compounds.

23. heating or an electric current

24. a. compound b. mixturec. mixture d. element

25. a. H b. O c. Ag d. Na e. H f. Al 26. a. carbon b. calcium c. potassium

d. gold e. iron f. copper27. Carbon, hydrogen, oxygen, and nitrogen;

hydrogen is present in the greatest pro-portion by number of atoms.

52 Chapter 2

Table 2.2

Symbols and Latin Names for Some Elements

withChemASAP

Name Symbol Latin name

Sodium Na natriumPotassium K kaliumAntimony Sb stibiumCopper Cu cuprumGold Au aurumSilver Ag argentumIron Fe ferrumLead Pb plumbumTin Sn stannum

If the English name and the Latin name of an element are similar, thesymbol will appear to have been derived from the English name. Examplesinclude Ca for calcium, N for nitrogen, and S for sulfur. Table 2.2 showsexamples of elements where the symbols do not match the English names.

Chemical symbols provide a shorthand way to write the chemical for-mulas of compounds. The symbols for hydrogen, oxygen, and carbon areH, O, and C. The formula for water is H2O. The formula for sucrose, or tablesugar, is C12H22O11. Subscripts in chemical formulas are used to indicate therelative proportions of the elements in the compound. For example, thesubscript 2 in H2O indicates that there are always two parts of hydrogen foreach part of oxygen in water. Because a compound has a fixed composi-tion, the formula for a compound is always the same.


20. Key Concept How is a compound different from an element?

21. Key Concept How can you distinguish a substance from a mixture?

22. Key Concept What are chemical symbols and chemical formulas used for?

23. Name two methods that can be used to break down compounds into simpler substances.

24. Classify each of these samples of matter as an element, a compound, or a mixture.

a. table sugar b. tap water c. cough syrup d. nitrogen

25. Write the chemical symbol for each element. a. hydrogen b. oxygen c. silver d. sodium e. hydrogen f. aluminum

26. Name the chemical elements represented by the following symbols.

a. C b. Ca c. K d. Au e. Fe f. Cu

27. What elements make up the pain reliever acetaminophen, chemical formula C8H9O2N?Which element is present in the greatest proportion by number of particles?

Compare and Contrast Paragraph Compare and contrast elements and compounds. Compare them by saying how they are alike. Contrast them by describing how they are different.


For: Links on Element Names





Section 2.4 Review• Transparencies, T18–T19• Laboratory Manual, Labs 2–3• Laboratory Practicals, 2-1• Small-Scale Chemistry Laboratory

Manual, Lab 2


Assessment 2.4• Go Online, Section 2.4

2.4

FOCUSObjectives2.4.1 Describe what happens during

a chemical change.2.4.2 Identify four possible clues

that a chemical change has taken place.

2.4.3 Apply the law of conservation of mass to chemical reactions.

Guide for Reading

Build VocabularyCompare and Contrast Table Have students make a table to compare physical and chemical changes.

Reading StrategyPredict Before students read the sec-tion, have them predict the meanings of reactant and product. Ask students what they are basing their predictions on.

INSTRUCT

What evidence do you see in the photo that a chemical reaction has occurred? (The metal has changed color.)

Chemical ChangesUse VisualsFigure 2.13 Discuss the difference between physical and chemical changes. Ask, What substances are present before and after the change in photo a? (iron and sulfur) What sub-stances are present before and after the change in photo b? (iron and sul-fur before, iron sulfide after )

FYIThe Roman numerals were omitted from iron(III) oxide and iron(II) sulfide to avoid having to explain the Stock system, which is explained on p. 254.

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Answers to...Figure 2.13 b; A new substance is formed.

Section Resources

Connecting to Your World

Section 2.4 Chemical Reactions 53

b

a

2.4 Chemical Reactions

Iron is an element with many desirable properties. It is abundant, easy to shape when heated, and relatively strong, especially when mixed with carbon in steel.

Iron has one main disadvantage. Over time, objects made of iron will rust if they are

left exposed to air. The brittle layer of rust that forms on the surface of the object flakes off, exposing more iron to the air.

In this section, you will learn to recognize chemical changes and to distinguish them

from physical changes.

Guide for Reading

Key Concepts• What always happens during a

chemical change?• What are four possible clues

that a chemical change has taken place?

• How are the mass of the reactants and the mass of the products of a chemical reaction related?

Vocabularychemical property

chemical reaction

reactant

product

precipitate

law of conservation of mass

Reading Strategy Predicting Before you read, predict what will happen to the mass of a sample of matter that burns. After you read, check the accuracy of your prediction and correct any misconceptions.

Chemical ChangesThe compound formed when iron rusts is iron oxide (Fe2O3). Words such asburn, rot, rust, decompose, ferment, explode, and corrode usually signify achemical change. The ability of a substance to undergo a specific chemicalchange is called a chemical property. Iron is able to combine with oxygen toform rust. So the ability to rust is a chemical property of iron. Chemicalproperties can be used to identify a substance. But chemical properties canbe observed only when a substance undergoes a chemical change.

Figure 2.13 compares a physical change and a chemical change thatcan occur in a mixture of iron and sulfur. When a magnet is used to sepa-rate iron from sulfur, the change is a physical change. The substancespresent before the change are the same substances present after thechange, although they are no longer physically blended. Recall that duringa physical change, the composition of matter never changes. During achemical change, the composition of matter always changes. When the mix-ture of iron and sulfur is heated, a chemical change occurs. The sulfur andiron react and form iron sulfide (FeS).

A chemical change is also called a chemical reaction. One or more sub-stances change into one or more new substances during a chemical reaction.A substance present at the start of the reaction is a reactant. A substanceproduced in the reaction is a product. In the reaction of iron and sulfur,iron and sulfur are reactants and iron sulfide is a product.

Figure 2.13 A mixture of iron filings and sulfur can be changed. A magnet separates the iron from the sulfur. Heat combines iron and sulfur in a compound. Classifying Which change is a chemical change? Explain.

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54 Chapter 2


Recognizing Chemical Changes


Identifying a Chemical ChangePurpose Students will practice identifying chemical changes.

Materials Bunsen burner, match, tongs, magnesium ribbon, 4 test tubes, solutions of 0.1M AgNO3, 0.1M NaCl, 0.1M K2CrO4, and 3M H2SO4, mossy zinc, marble chip, spatulas, cobalt blue glass filters

Safety For Step 3, wear gloves to avoid stains on your skin from silver nitrate. CAUTION Students should not look at burning magnesium without the cobalt blue glass filters.

Procedure As you do each step, have students identify the clue for chemical change. Emphasize that the clues point to a chemical change but do not confirm that a change has taken place.a. Light a Bunsen burner. (heat and

light produced)

b. Using tongs, hold a piece of magne-sium ribbon in a burner flame until it ignites. Remove from heat and observe. (The product is a white pow-der; heat and light are produced.)

c. Put 5 mL of silver nitrate solution in each of two test tubes. Add a small amount of sodium chloride solution to one tube and potassium chro-mate solution to the other. (color change, formation of a precipitate)

d. Put 5 mL of the sulfuric acid in each of two test tubes. Add a piece of mossy zinc to one tube and a marble chip (CaCO3) to the other. (formation of a gas)

Download a worksheet on Chemi-cal and Physical Changes for stu-dents to complete, and find additional teacher support from NSTA SciLinks.

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Urinalysis Test Strips A test-strip urinalysis is semi-quantitative, meaning that the test provides information about the level of a material present in the urine, but not a specific quantity. The strips can also be used to test for ketones, protein, blood, nitrite, bilirubin, urobilinogen, pH, white blood cells, and specific gravity. All the tests involve a color change.

In one glucose test, the hydrogen peroxide produced when glucose is oxidized reacts with potassium iodide. The oxygen produced in the second reaction binds with a dye to produce colors that range from blue to dark brown. You may want to relate what happens with test strips to what happens in the Small-Scale lab on p. 56.

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54 Chapter 2

Recognizing Chemical ChangesHow can you tell whether a chemical change has taken place? There arefour clues that can serve as a guide. Possible clues to chemical changeinclude a transfer of energy, a change in color, the production of a gas, orthe formation of a precipitate.

Every chemical change involves a transfer of energy. For example,energy stored in natural gas is used to cook food. When the methane in nat-ural gas combines with oxygen in the air, energy is given off in the form ofheat and light. Some of this energy is transferred to and absorbed by foodthat is cooking over a lit gas burner. The energy causes chemical changes totake place in the food. The food may brown as it cooks, which is anotherclue that chemical changes are occurring.

You can observe two other clues to chemical change while cleaning abathtub. The ring of soap scum that can form in a bathtub is an example ofa precipitate. A precipitate is a solid that forms and settles out of a liquidmixture. Some bathroom cleaners that you can use to remove soap scumstart to bubble when you spray them on the scum. The bubbles are a prod-uct of a chemical change that is taking place in the cleaner.

If you observe a clue to chemical change, you cannot be certain that achemical change has taken place. The clue may be the result of a physicalchange. For example, energy is always transferred when matter changesfrom one state to another. Bubbles form when you boil water or open acarbonated drink. The only way to be sure that a chemical change hasoccurred is to test the composition of a sample before and after the change.Figure 2.14 shows examples of practical situations in which different cluesto chemical change are visible.

Checkpoint What energy transfer takes place when food cooks?

For: Links on Chemical and Physical Changes


Figure 2.14 Clues to chemical change often have practical applications. Bubbles of carbon dioxide gas form when a geologist puts acid on a rock that contains compounds called carbonates. When a test strip is dipped in urine, the color change is used to estimate the level of the sugar glucose in urine. One step in the pro-duction of cheese is a reaction that causes milk to separate into solid curds and liquid whey.

a

b

c

Facts and Figures


Conservation of Mass

Use VisualsFigure 2.15 Explain that the photos are before and after views of a reaction. Ask, Why are the containers covered? (The covers prevent evaporation of the solu-tions and loss of mass.)

ASSESS

Evaluate UnderstandingWrite this word equation on the board.

methane + oxygen → carbon dioxide + water + energy

Have students identify the reactants and products and one possible clue for a chemical change.

ReteachWrite a word equation on the board for the reaction of magnesium and oxygen to produce magnesium oxide. Ask stu-dents to explain how mass is conserved in this reaction. (The difference between the mass of the magnesium and the mass of the magnesium oxide is the mass of the oxygen with which magnesium combines.)

Connecting Concepts

Lavoisier would have done a series of experiments in which he measured the masses of reactants and products before he proposed the law of con-servation of mass.

with ChemASAP

If your class subscribes to the interactive Textbook, use it to review key concepts in Section 2.4.

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Answers to...Figure 2.15 change of color and formation of a precipitate; The bal-ance reading did not change.

Checkpoint

The burning of methane produces heat that is transferred to the food.

Section 2.4 Assessment28. The composition of matter always

changes during a chemical change.29. a transfer of energy, a change in color, the

production of a gas, the formation of a precipitate

30. The mass of the products is always equal to the mass of the reactants.

31. In a physical change, the chemical com-position of a substance does not change.

In a chemical change, the chemical com-position of the reactants changes as one or more products form.

32. a. physical b. physicalc. chemical d. chemical

33. Mass is conserved in every physical change or chemical reaction.

34. 43.2 g

Section 2.4 Chemical Reactions 55

withChemASAP

Conservation of MassWhen wood burns, substances in the wood combine with oxygen from theair. As the wood burns, a sizable amount of matter is reduced to a small pileof ashes. The reaction seems to involve a reduction in the amount of mat-ter. But appearances can be deceiving. During any chemical reaction,the mass of the products is always equal to the mass of the reactants. Twoof the products of burning wood—carbon dioxide gas and water vapor—are released into the air. When the mass of these gases is considered, theamount of matter is unchanged. Careful measurements show that the totalmass of the reactants (wood and the oxygen consumed) equals the totalmass of the products (carbon dioxide, water vapor, and ash).

Mass also holds constant during physical changes. For example, when10 grams of ice melt, 10 grams of liquid water are produced. Similar obser-vations have been recorded for all chemical and physical changes studied.The scientific law that reflects these observations is the law of conservationof mass. The law of conservation of mass states that in any physical changeor chemical reaction, mass is conserved. Mass is neither created nordestroyed. The conservation of mass is more easily observed when achange occurs in a closed container, as in Figure 2.15.


28. Key Concept How does a chemical change affect the composition of matter?

29. Key Concept Name four possible clues that a chemical change has taken place.

30. Key Concept In a chemical reaction, how does the mass of the reactants compare with the mass of the products?

31. What is the main difference between physical changes and chemical changes?

32. Classify the following changes as physical or chemical changes.

a. Water boils. b. Salt dissolves in water. c. Milk turns sour. d. A metal rusts.

33. According to the law of conservation of mass, when is mass conserved?

34. Hydrogen and oxygen react chemically to form water. How much water would form if 4.8 grams of hydrogen reacted with 38.4 grams of oxygen?

The Scientific Method Lavoisier proposed the law of conservation of mass in 1789. Write a paragraph describing, in general, what Lavoisier must have done before he proposed this law. Use what you learned about the scientific method in Section 1.3.


a b

Figure 2.15 When the liquids in photograph A are mixed, they react. None of the products are gases. Analyzing Data How do you know that a reaction took place and that mass was con-served during the reaction?

56 Chapter 2

Small-ScaleLAB

Small-ScaleLAB

1 + 2 + 3 = Black!Objective In this activity, students will begin to:• familiarize themselves with small-

scale equipment and methodology.• observe and analyze some chemical

reactions.

Skills Focus Observing, Classifying

Prep Time 1 hourMaterials Paper, metric ruler, reaction surface, pipette, dropper, spatulas, KI, liquid starch, cereal, NaClO, H2O2, CuSO4, foods, iodized and non-iodized salt, antacid tablets, color marker pens

Advance Prep

Class Time 40 minutes

Teaching Tips• Demonstrate how to use a pipette as a

stirrer. This information, as well as a dis-cussion of reaction surfaces, can be found in the Small-Scale Chemistry Laboratory Manual.

• Explain that using a mixture of KI and NaClO to test for starch is an example of qualitative analysis.

Expected Outcome

For EnrichmentHave students design and carry out an experiment to quantify the amount of KI in iodized salt.

Solution Preparation

0.1M KI 4.2 g KI in 250 mL water

0.2M CuSO4 12.5 g CuSO4•5H2O in 250 mL water

1% NaClO 50 mL household bleach in 200 mL water

3% H2O2 Use undiluted house-hold hydrogen peroxide

20% starch 50 mL liquid starch in 200 mL water

NaClO H2O2 CuSO4

KI yellow yellow brown ppt

KI + starch black black black

KI + paper black black black

KI + cereal black black black

L2

L3

56 Chapter 2

Small-ScaleLAB

Small-ScaleLAB

1 � 2 � 3 � Black!

PurposeTo make macroscopic observations of chemical reactions and use them to solve problems.

Materials

• paper

• metric ruler

• reaction surface

• materials shown in grid

• pipette, medicine droppers, and spatulas

Procedure1. Draw two copies of the grid on separate sheets

of paper. Make each square in the grid 2 cm on each side.

2. Place a reaction surface over one of the grids. Use the second grid as a data table to record your observations.

3. Use the column and row labels to determine which materials belong in each square. Depending on the material, add one drop, one piece, or a few grains.

4. Stir each mixture by forcing air from an empty pipette as directed by your teacher.

KI

NaClO H2O2 CuSO4

KI �

Starch

KI �

Paper

KI �

Cereal

Analyze and ConcludeUsing your experimental data, record the answers to the following questions below your data table.

1. What color is a mixture of sodium hypochlorite (NaClO) and potassium iodide (KI)?

2. What happens when you mix NaClO, KI, and starch?

3. What do NaClO, H2O2, and CuSO4 have in common?

4. What substance is found in both paper and cereal? How do you know?

5. If you used NaClO instead of CuSO4 in reactions other than the reaction with KI and starch, would you expect the results to always be identical? Explain your answer.

You’re The ChemistThe following small-scale activities allow you to develop your own procedures and analyze the results.

1. Design It! Design and carry out an experiment to see which foods contain starch.

2. Design It! Read the label on a package of iodized salt. How much KI does iodized salt contain? Design an experiment to demonstrate the presence of KI in iodized salt and its absence in salt that is not iodized.

3. Design It! Antacid tablets often contain starch as a binder to hold the ingredients in the tablet together. Design and carry out an experiment to explore various antacid tablets to see if they contain starch.

4. Analyze It! NaClO is a bleaching agent. Such agents are used to whiten clothes and remove stains. Use different color marker pens to draw several lines on a piece of white paper. Add one drop of NaClO to each line. What happens? Try inventing a technique that you can use to make “bleach art.”

Analyze and Conclude1. yellow2. The mixture turns a blue-black color.3. They all turn a mixture of KI and starch black. 4. Starch; both turn blue-black, which suggests

the presence of starch.5. The results may be the same in reactions that

are similar to the one with KI and starch, but different in other reactions.

You’re The Chemist1. Add KI + NaClO to various foods. A black

color indicates the presence of starch.2. Most table salt contains 0.01% KI. Wet only a

portion of a small pile of salt with starch. Add CuSO4 or H2O2. A black color indicates the presence of KI.

3. If an antacid tablet contains starch, it will turn black when treated with KI + NaClO.

4. The color fades. A picture can be drawn with colored ink. Areas can be treated with NaClO to bleach parts of the picture.

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