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To this point we have concentrated on the laws thatgovern motion in the universe, using our knowledge ofthe larger objects around us to illustrate these laws andconfirm their validity. Now we begin a study of matteritself: What is it? What is it made of? How does itbehave? Can we understand its behavior in terms of thelaws we already know? Do the laws that govern themotions of large chunks of matter also explain thedetailed properties and behavior of matter itself?

Our probing will take us deep within matter to dis-cover molecules, atoms, electrons, the constituents ofatomic nuclei, and even the smaller particles that pro-tons and neutrons are made of. We can understandmuch in terms of the laws already described. Yet, thesmallest objects seem not to obey these laws—at leastnot in the same way as before. The study of matter will,therefore, lead to a deeper understanding of the laws ofnature.

Before a new science has developed a fundamentalunderstanding of the laws governing it, its foundersbegin by classifying things. Aristotle (384-322 B.C.),using this strategy centuries ago, became recognized asan influential originator of many of the modern disci-plines: physics, biology, politics, psychology, andethics. The names he gave the disciplines are the onesused to this day.

For 20 years Aristotle worked at the side of theaging Plato in the Academy at Athens; for seven, helived in Macedonia as tutor to Alexander the Great.Aristotle was interested in nature and compiled obser-vations of biological facts, many of which were fur-nished from Alexander’s expeditions. He classified liv-ing things into genera and species. What are the simi-larities in things? What are the differences? What arethe opposites? What are the extremes? Where is themean? In De Partibus Animalium, for example, hesays: “The course of exposition must be, first, to statethe attributes common to whole groups of animals, andthen to attempt to give their explanation.” With theattributes in place, Aristotle’s goal was to find a unify-ing theory that explained them. He applied theapproach not only to biology but also to physics, poli-tics, psychology, ethics, and other disciplines.

As Aristotle taught us, classification is a prelimi-nary activity in a science. In this chapter, in a very lim-

ited way, we will discuss some of the properties of mat-ter by which it can be classified. But classification is notto be confused with understanding. In later chapters wewill see how some of these characteristics can be under-stood in terms of unifying theories and laws.

The Continuous Model of Matter

Matter as we encounter it seems to be fairly uni-form. We do not perceive it as being made of tiny,invisible particles. Water, for example, shows no visi-ble evidence of any internal structure, even with themost powerful optical microscope. The model thatassumes matter to be uniform is called the ContinuousModel of Matter (Fig. 10.1).

Figure 10.1. Matter as we deal with it seems uniformand continuous.

10. The Physical Properties of Matter

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Descriptions of matter in common experience usu-ally assume a continuous model. The details of thestructure of matter seem unimportant in the design ofbridges, buildings, automobiles, and kitchen appliances.We do not think of atoms and molecules when we cook,clean, use a pencil, or burn gasoline. Generally we treatmatter as we find it—uniform and continuous—andclassify it according to the properties that it displayswhen found in large chunks (i.e., large enough to beseen under optical microscopes).

This concept of continuous composition of mattersuits most of our purposes. Although we can explainmost of the properties of matter in terms of more funda-mental models, scientists are still not able to predict accu-rately many important properties of specific kinds of mat-ter. On the other hand, great progress is being made inthis direction, as evidenced by the large number of syn-thetic, or man-made, materials now available.

The States of Matter

Most of the matter we encounter occurs in one ofthree states: solid, liquid, or gas. Solids are rigid andresist changes in size or shape. Liquids flow to assume

the shape of their container, but resist changes in vol-ume. Gases expand to assume both the shape and sizeof their container. Liquids and gases are both referredto as fluids, defined most simply as materials that flow.

A fourth state of matter, plasma, also occurs in cer-tain situations. A plasma is a gas in which both positiveand negative charges are free to move independently ofeach other. When this occurs, the gas is said to be ion-ized. Plasmas occur naturally in the earth’s upperatmosphere, where ionization is caused by radiationfrom the sun. Lightning discharges create a plasma.The gases inside fluorescent lights and neon advertisingsigns are partly ionized when the tubes are emittinglight. Very hot flames are also plasmas.

Some materials, like water, change from one state toanother at common temperatures; others require extremetemperatures for such changes. All materials, includingair and other gases, can be solidified if their temperatureis low enough. They melt into liquids as their tempera-ture increases, and then vaporize into gases at even high-er temperatures (Fig. 10.2). They finally become plas-mas if their temperature continues to increase. (Thissimple view is slightly complicated by the fact that somematerials decompose as their temperature increases.)

Below 0°C Above 0°C�& below 100°C

Above 100°C

Figure 10.2. Most materials, such as water, change from one state to another as temperature increases.

MELTING TEMPERATURE BOILING TEMPERATURE SPECIFIC(°C) (°C) GRAVITY

Nitrogen -210 -196 0.0013 gas0.81 liquid

Ammonia -78 -33 0.0008 gas0.82 liquid

Water 0 100 0.92 solid1.00 liquid

Carbon tetrachloride -23 77 1.6(dry cleaning solvent)Ethyl alcohol -117 79 0.79Table salt 801 1413 2.2Lead 327 1613 11.3Iron 1535 3000 7.9

Table 10.1. Melting temperature, boiling temperature, and specific gravity of common materials.

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One way to classify a given sample of matter is toidentify its state at various temperatures and to note thetemperature at which it changes state. The temperaturesat which changes of state occur, called the melting andboiling temperatures, vary widely among differentmaterials. For example, hydrogen melts at -259 °C andboils at -253 °C. Gold, on the other hand, melts at 1063°C and boils at 2600 °C. We normally encounter hydro-gen only as a gas and gold only as a solid. Table 10.1shows the melting and boiling temperatures for severalcommon materials.

Density

Matter may also be classified according to its den-sity. Density is the mass per unit volume (density =mass/volume). Density is a property of a material,whereas mass is a property of a specific object. Forexample, a ball bearing and a cannonball that are bothmade of the same material have the same density, eventhough their masses are quite different. On the otherhand, two balls with exactly the same mass, one madeof aluminum and the other of iron, would have differentdensities. The aluminum ball would be significantlylarger than the iron ball, even though both have thesame mass (Fig. 10.3).

Figure 10.3. These balls have the same mass. Whichhas the greater density? How do you know?

Densities of materials vary widely. The density ofhydrogen is only 0.089 kilogram per cubic meter,whereas that of gold, a solid, is over 19,000 kilogramsper cubic meter, more than 200,000 times larger.

The densities of all materials change with tempera-ture, usually (but not always) decreasing as temperatureincreases. Changes of state are always accompanied byabrupt changes in density. Liquids are usually less densethan solids of the same material; gases are always lessdense than the corresponding liquids. Water is an impor-tant exception to this general rule, because ice is slightlyless dense than water. Table 10.1 lists the specific gravity(relative density) for several common materials.

Color

The colors of materials provide interesting clues totheir internal structure. By “color,” we usually meanwhat happens to the light that falls on material. When

white light, which contains a rainbow of colors, falls onthe surface of a material, some colors are absorbed andsome are diffusely reflected. The reflected light reach-es our eyes, where its color is identified. An apple is redbecause it reflects only the red part of white light andabsorbs the rest. A leaf is green because it reflects onlygreen and absorbs everything else. The sky is bluebecause more of the blue light than the red light fromthe sun is scattered toward the earth (Fig. 10.4). Whatcauses a material to absorb some colors of light andreflect others? The answer, as we will see in later chap-ters, provides important information about the structureof materials as well as the nature of light.

Figure 10.4. Why is an apple red?

Some materials emit light under certain circum-stances. The light from the sun, electric lamps, hot elec-tric stoves, lightning, and melted iron are commonexamples. Actually, everything radiates some energy;however, it is not always apparent, since the radiation isusually infrared rather than visible light. The differentcolors of this emitted radiation are an important charac-teristic of a material, and they depend on the tempera-ture and properties of the material.

We have long known that light separates into itscomponent colors when passed through atriangular-shaped piece of glass called a prism. Whenwhite light from the sun passes through such a prism, itbreaks into all the colors of the rainbow. Light from anelectric lamp also displays all the visible colors, butblue is comparatively dimmer than the blue from sun-light.

The pattern of colors emitted by a material is calledthe spectrum of its emission. A spectrum that coversthe entire rainbow, such as sunlight, is said to be con-tinuous. Some materials, particularly in the plasmastate, emit only a few of the possible colors. The spec-trum is then said to be a discrete (or line) spectrum.

The specific colors that are emitted in a discretespectrum are characteristic of the material and can beused to identify the material even when it is not other-wise accessible. For example, light from the sun andstars reveals a great deal about the materials of whichthese objects are composed. Details of emitted and

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reflected light have been an important part in under-standing the detailed structure and behavior of matterand of light itself (Fig. 10.5).

Response to Force

Another way to identify materials is to describetheir response to external forces. Materials vary great-ly in their hardness (or softness) and their ability toreturn to their original shape and size once externalforces are removed.

Materials that permanently change shape under theinfluence of outside forces are said to be plastic. Softclay and putty are common examples. Plasticitydepends on the temperature of the material. Iron, forexample, is not plastic at ordinary temperatures but canbe formed into almost any shape when very hot (but stillconsiderably below its melting temperature).

Materials that deform when forces are applied, butreturn to their original size and shape when the forcesare removed, are said to be elastic. A rubber band iselastic, because it stretches when forces are applied andreturns to its original shape and size when the stretchingforces are removed.

Almost all the materials we deal with in ordinarycircumstances are elastic. We often do not notice thedeflections that occur, so we tend not to be aware of

them. For example, when we sit on a chair, the chairbends slightly because of the force we apply. When westand, the force on the chair is removed and the chairreturns to its original shape. A bridge made of thestrongest steel bends slightly when we walk on it. Suchdeflections occur whenever forces are applied, no mat-

ter how small the force or how strong the material.Stronger forces always cause larger deflections.

Civil and mechanical engineers must take such dis-tortions into account in the design of buildings, bridges,automobile engines, and almost every other project. Acommon technique, for example, is to form buildingmaterials so that they will assume a more desired shapewhen they carry their designed load. A prestressed con-crete beam is curved upward when placed into position,but it is straight when carrying its load as part of a build-ing or bridge (Fig. 10.6). If the beam were straight tobegin with, it would sag downward when carrying theload.

The stiffness or elasticity of a material is character-ized by its elastic constant. This number simply mea-sures the amount of force needed to cause a particulardeformation and is defined roughly by the relationship

elastic constant ! force .deformation

Rubber has a small elastic constant, since a smallforce produces a comparatively large deformation.Steel, on the other hand, deforms only slightly when fair-ly large forces are applied; thus, it has a large elastic con-stant. In this sense, steel is far more elastic than rubber.

Elastic constants are associated with three differentkinds of deformations: tension, compression, and shear.A material is said to be under tension when the forcesact to stretch it (Fig. 10.7); the forces would pull thematerial apart if it did not resist. If the forces pushinward on the material, it is said to be compressed; theforces cause the material to occupy a smaller volumethan before. Shearing forces cause the material tochange shape. These might be applied parallel to thesurface of the material, perhaps by frictional forces.

Fluids have little resistance to changes in shape, sotheir elastic constants for shear would be nearly zero. Thecompressional elastic constant of a liquid, however, isusually quite high. For example, water is only slightlymore dense (more compressed) at the bottom of the oceanthan near the surface. Compressional elastic constants forgases are considerably lower. A comparatively smallforce on the handle of an automobile tire pump can cause

Figure 10.5. Light from an incandescent bulb contains all the colors. A plasma emits a discrete spectrum that containsonly a few colors.

Figure 10.6. A prestressed concrete beam straightenswhen a load is applied.

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a significant reduction in the volume of the air inside. If the forces applied to an elastic material are too

large, it will not return to its original size and shapewhen the force is removed. Instead, the material willbecome permanently deformed just as if it were plastic.The maximum force that can be sustained without per-manent deformation is a measure of the elastic limit ofthe material. This is another important property bywhich materials can be classified and identified.Builders and engineers clearly need to be concernedabout the elastic limits of the materials they use.

The elastic properties of materials are a result ofelectric forces acting within each piece of matter.Again, detailed explanations of these properties andtheir variations require significant understanding of thesubmicroscopic structure of matter.

Electrical Properties

Electrical properties provide some of the mostprovocative clues about the internal structure of matter.Solid materials can be roughly divided into two classes,conductors and nonconductors. An electric current canflow through conductors if an electrical force acts on thecharged particles within the material. Electric currentdoes not flow through nonconductors. (Fig. 10.8).

All matter contains electrically charged particles.The presence of an electric current means that at least

some of the charged particles are free to move aboutinside the conductor. Conversely, the absence of currentin the nonconductors implies that the charged particlesin these materials are not free to move, but are heldrigidly in place.

Metals are conductors—copper, aluminum, iron,sodium, and zinc. Nonconductors include the non-metals such as phosphorus, sulfur, water, sodium chlo-ride (table salt), and sugar. These are sometimes calledinsulators.

In addition, some materials, called semiconduc-tors, conduct small currents under appropriate circum-stances. They do not conduct as well as the conductors,but they do conduct better than nonconductors. Thismeans that only a small fraction of the charged particlessemiconductors are made of are free to move inside thematerial. Semiconductors made of silicon form thebasis of our modern computer technology.

Some nonconductors become conductors if they aremelted, or if they are dissolved in water or some otherliquid. Table salt, for example, is a nonconductor as asolid. It becomes a conductor when melted or when dis-solved in liquid. Other nonconductors do not exhibitthis behavior. Water is a nonconductor as ice or as liq-uid, but it will conduct an electric current if certainmaterials such as table salt are dissolved in it.Dissolved sugar, on the other hand, does not causewater to become a conductor (Fig. 10.9).

Figure 10.7. Three important ways that force can be applied to materials. Which of these is under tension? compres-sion? shear?

Figure 10.8. Which material is the conductor? Which is the nonconductor? How do you know?

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Nonconductors that become conductors as liquids,or that become conductors when dissolved in a noncon-ducting liquid such as water, are called ionic materials.Those that remain nonconductors under these circum-stances are nonionic (Fig. 10.10). Apparently the inter-nal structures of ionic and nonionic materials differ.The charged particles in ionic materials somehowbecome free in the liquid state, whereas those in non-ionic materials remain bound. These differences canonly be explained by a detailed model of the internalstructure of these materials.

Figure 10.10. The classification of materials accordingto their electrical properties.

Summary

In this chapter as we begin a study of matter, wehave followed Aristotle’s lead by classifying matter invarious ways. Classification is a preliminary exercisewe will follow in later chapters to find unifying theoriesthat attempt to explain the characteristics.

Matter can be classified according to its physicalstates: solid, liquid, gas, and plasma. Matter may alsobe classified according to its density, color, response toforces, and conduction of electricity. All of these givesome clues to the underlying structure of matter.

The spectrum of light emitted by matter is particu-larly important. Under some conditions, matter emits acontinuous spectrum; under others it emits a discrete

spectrum that is characteristic of the material. The emit-ted light is a direct probe into the atom.

In addition to the features of matter described in thischapter, matter participates in a variety of chemicalchanges. The chemical properties of matter are an impor-tant part of our description of nature and provide addi-tional evidence for our model of the structure of matter.

STUDY GUIDEChapter 10: The Physical Properties of Matter

A. FUNDAMENTAL PRINCIPLES: No new funda-mental principles.

B. MODELS, IDEAS, QUESTIONS, OR APPLICA-TIONS

1. The Continuous Model of Matter: That model ofmatter which sees matter as smoothly divisiblewithout limit, i.e., matter that is not made up of dis-crete particles (molecules).

2. What are the four states of matter?3. It is useful to describe matter in terms of density, color,

responsiveness to force, and electrical properties.

C. GLOSSARY1. Color: A characteristic of matter imparted to it by

the nature of the reflected light which it transmits toan observer.

2. Compression Force: A force which is applied insuch a way as to compress a material.

3. Conductor (specifically, of electricity): A sub-stance that readily allows an electric current to flowthrough it. The opposite of an insulator (noncon-ductor). Copper wire is a conductor.

4. Continuous Spectrum: A spectrum in which thecolors blend gradually together without noticeableabrupt changes or missing colors. The opposite ofa discrete spectrum.

5. Density: Mass per unit volume.6. Discrete (or Line) Spectrum: A spectrum of sep-

Figure 10.9. (a) Water by itself is a nonconductor. (b) Adding salt to water causes it to conduct. (c) Adding sugar towater does not cause it to conduct an electric current.

All Materials

Conductors(As Solids)

Nonconductors(As Solids)

(Conductors As Liquids)Ionic Nonionic

(Nonconductors As Liquids)

arate and distinct colors. The opposite of a contin-uous spectrum.

7. Elastic: An adjective describing materials thatdeform when forces are applied but return to theiroriginal size and shape when the forces areremoved.

8. Elastic Constant: A quantitative measure of elas-ticity formed by taking the ratio of the forceapplied to a material to a measure of the resultingdeformation.

9. Elastic Limit: The maximum force that a materialcan sustain without sustaining a permanent changein shape.

10. Fluid: Matter that flows readily. Gases and liquidsare fluids.

11. Gas: A physical state of matter that readilychanges both shape and volume to match its con-tainer.

12. Ionic Material: A material that is a nonconductorof electricity as a solid but that conducts electricitywhen melted or dissolved in water.

13. Ionized Matter: Matter in which at least some ofthe atoms have been altered from their ordinary neu-tral state by the addition or subtraction of electrons.

14. Liquid: A physical state of matter that readilychanges shape to match its container but that resistschanges in volume.

15. Nonconductor (specifically, of electricity): Aninsulator. A substance that does not readily allowan electric current to flow through it. The oppositeof a conductor. Glass is a nonconductor.

16. Nonionic Material: A material lacking in someway the characteristics of an ionic material.

17. Plasma: A physical state of matter characterizedby fluid properties but in which positive and nega-tive charges move independently.

18. Plastic: An adjective describing materials that per-manently change shape under the influence ofexternal forces.

19. Semiconductor: A class of materials with electricalconducting properties somewhere between conduc-tors and nonconductors. Silicon is a semiconductor.

20. Shearing Force: A force that is applied in such away as to be tangential to the surface on which itacts.

21. Solid: A physical state of matter that is character-ized by rigidity and resistance to changes in sizeand shape.

22. Spectrum: A display of the amounts and colors oflight emitted by a particular source in which onesees the colors separated from one another.

23. Tension Force: A force that is applied in such away as to stretch a material.

D. FOCUS QUESTIONS: None.

E. EXERCISES10.1. Describe what is meant by a fluid and give

examples of substances that are fluids at ordinary tem-peratures.

10.2. Describe how materials change from one stateto another. How do materials like gasoline and copperdiffer from one another in this respect?

10.3. Which has greater density, an ice cube or aniceberg? Explain your answer.

10.4. Name several materials that are solids at ordi-nary temperatures.

10.5. Name several materials that are liquids atordinary temperatures.

10.6. Name several materials that are gases at ordi-nary temperatures.

10.7. Describe what is meant by a “plasma” andgive an example of a plasma that occurs in nature.

10.8. Explain the meaning of the word “density.”

10.9. Choose several materials and list them inorder of increasing density.

10.10. Why do people look different when seenunder mercury vapor lamps than when seen by sun-light? In particular, red objects seem almost blackunder such lamps.

10.11. Explain how it might be possible to knowsomething about the materials in a planet without goingthere.

10.12. Explain what is meant by a “continuous”spectrum.

10.13. Explain what is meant by a “discrete” spec-trum.

10.14. A certain wire stretches 0.1 millimeter whena particular force is applied. Another wire in the samesize and shape but made of a different metal stretches0.01 millimeter with the same force. Which has thelarger elastic constant? Explain your answer.

10.15. Explain why the elastic properties of materi-als must be taken into account when designing build-ings, bridges, and other structures.

10.16. Explain what is meant by the term “elasticconstant.”

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10.17. Show how forces can be applied to a mater-ial to produce compression, tension, and shear.

10.18. In what way are the elastic constants of flu-ids different than those for solids?

10.19. What conditions must be met if a current isto flow through any material?

10.20. What is the difference between the internalstructures of conductors and nonconductors?

10.21. What is the difference between the internalstructures of ionic and nonionic materials?

10.22. Define “conductor,” “nonconductor,”“ionic,” and “nonionic.”

10.23. Suppose you are given an unfamiliar sampleof material, say a rock. Describe the experiments youmight perform that allow you to classify the materialaccording to its electrical properties.

10.24. Why is tap water a conductor while purewater is not?

10.25. Do you think the human body is a conduc-tor or nonconductor? Why? What does this implyabout the structure of the body?

10.26. Which of the following processes does notproduce a change of state?

(a) melting ice(b) dissociating gas atoms(c) freezing water(d) boiling water(e) falling object

10.27. Is “classifying” the same as “understand-ing”? In modern testing practice one often sees multi-ple-choice questions for which the available choices arethe names of things. What is being tested in suchinstances: classification or understanding? What didAristotle think was the objective of classification?

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