10. The Physical Properties of Matter
To this point we have concentrated on the laws that
govern motion in the universe, using our knowledge of
the larger objects around us to illustrate these laws and
confirm their validity. Now we begin a study of matter
itself: What is it? What is it made of? How does it
behave? Can we understand its behavior in terms of the
laws we already know? Do the laws that govern the
motions of large chunks of matter also explain the
detailed properties and behavior of matter itself?
Our probing will take us deep within matter to discover molecules, atoms, electrons, the constituents of
atomic nuclei, and even the smaller particles that protons and neutrons are made of. We can understand
much in terms of the laws already described. Yet, the
smallest objects seem not to obey these laws—at least
not in the same way as before. The study of matter will,
therefore, lead to a deeper understanding of the laws of
nature.
Before a new science has developed a fundamental
understanding of the laws governing it, its founders
begin by classifying things. Aristotle (384-322 B.C.),
using this strategy centuries ago, became recognized as
an influential originator of many of the modern disciplines: physics, biology, politics, psychology, and
ethics. The names he gave the disciplines are the ones
used to this day.
For 20 years Aristotle worked at the side of the
aging Plato in the Academy at Athens; for seven, he
lived in Macedonia as tutor to Alexander the Great.
Aristotle was interested in nature and compiled observations of biological facts, many of which were furnished from Alexander’s expeditions. He classified living things into genera and species. What are the similarities in things? What are the differences? What are
the opposites? What are the extremes? Where is the
mean? In De Partibus Animalium, for example, he
says: “The course of exposition must be, first, to state
the attributes common to whole groups of animals, and
then to attempt to give their explanation.” With the
attributes in place, Aristotle’s goal was to find a unifying theory that explained them. He applied the
approach not only to biology but also to physics, politics, psychology, ethics, and other disciplines.
As Aristotle taught us, classification is a preliminary activity in a science. In this chapter, in a very lim-
ited way, we will discuss some of the properties of matter by which it can be classified. But classification is not
to be confused with understanding. In later chapters we
will see how some of these characteristics can be understood in terms of unifying theories and laws.
The Continuous Model of Matter
Matter as we encounter it seems to be fairly uniform. We do not perceive it as being made of tiny,
invisible particles. Water, for example, shows no visible evidence of any internal structure, even with the
most powerful optical microscope. The model that
assumes matter to be uniform is called the Continuous
Model of Matter (Fig. 10.1).
Figure 10.1. Matter as we deal with it seems uniform
and continuous.
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Descriptions of matter in common experience usually assume a continuous model. The details of the
structure of matter seem unimportant in the design of
bridges, 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 treat
matter as we find it—uniform and continuous—and
classify it according to the properties that it displays
when found in large chunks (i.e., large enough to be
seen under optical microscopes).
This concept of continuous composition of matter
suits most of our purposes. Although we can explain
most of the properties of matter in terms of more fundamental models, scientists are still not able to predict accurately many important properties of specific kinds of matter. On the other hand, great progress is being made in
this direction, as evidenced by the large number of synthetic, or man-made, materials now available.
the shape of their container, but resist changes in volume. Gases expand to assume both the shape and size
of their container. Liquids and gases are both referred
to as fluids, defined most simply as materials that flow.
A fourth state of matter, plasma, also occurs in certain situations. A plasma is a gas in which both positive
and negative charges are free to move independently of
each other. When this occurs, the gas is said to be ionized. Plasmas occur naturally in the earth’s upper
atmosphere, where ionization is caused by radiation
from the sun. Lightning discharges create a plasma.
The gases inside fluorescent lights and neon advertising
signs are partly ionized when the tubes are emitting
light. Very hot flames are also plasmas.
Some materials, like water, change from one state to
another at common temperatures; others require extreme
temperatures for such changes. All materials, including
air and other gases, can be solidified if their temperature
is low enough. They melt into liquids as their temperature increases, and then vaporize into gases at even higher temperatures (Fig. 10.2). They finally become plasmas if their temperature continues to increase. (This
simple view is slightly complicated by the fact that some
materials decompose as their temperature increases.)
The States of Matter
Most of the matter we encounter occurs in one of
three states: solid, liquid, or gas. Solids are rigid and
resist changes in size or shape. Liquids flow to assume
Above 0°C
& below 100°C
Below 0°C
Above 100°C
Figure 10.2. Most materials, such as water, change from one state to another as temperature increases.
Table 10.1. Melting temperature, boiling temperature, and specific gravity of common materials.
MELTING TEMPERATURE
(°C)
BOILING TEMPERATURE
(°C)
Nitrogen
-210
-196
Ammonia
-78
-33
Water
0
100
Carbon tetrachloride
(dry cleaning solvent)
Ethyl alcohol
Table salt
Lead
Iron
-23
77
-117
801
327
1535
79
1413
1613
3000
88
SPECIFIC
GRAVITY
0.0013
0.81
0.0008
0.82
0.92
1.00
1.6
0.79
2.2
11.3
7.9
gas
liquid
gas
liquid
solid
liquid
One way to classify a given sample of matter is to
identify its state at various temperatures and to note the
temperature at which it changes state. The temperatures
at which changes of state occur, called the melting and
boiling temperatures, vary widely among different
materials. For example, hydrogen melts at -259 °C and
boils at -253 °C. Gold, on the other hand, melts at 1063
°C and boils at 2600 °C. We normally encounter hydrogen only as a gas and gold only as a solid. Table 10.1
shows the melting and boiling temperatures for several
common materials.
white light, which contains a rainbow of colors, falls on
the surface of a material, some colors are absorbed and
some are diffusely reflected. The reflected light reaches our eyes, where its color is identified. An apple is red
because it reflects only the red part of white light and
absorbs the rest. A leaf is green because it reflects only
green and absorbs everything else. The sky is blue
because more of the blue light than the red light from
the sun is scattered toward the earth (Fig. 10.4). What
causes a material to absorb some colors of light and
reflect others? The answer, as we will see in later chapters, provides important information about the structure
of materials as well as the nature of light.
Density
Matter may also be classified according to its density. 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. For
example, a ball bearing and a cannonball that are both
made of the same material have the same density, even
though their masses are quite different. On the other
hand, two balls with exactly the same mass, one made
of aluminum and the other of iron, would have different
densities. The aluminum ball would be significantly
larger than the iron ball, even though both have the
same mass (Fig. 10.3).
Figure 10.4. Why is an apple red?
Some materials emit light under certain circumstances. The light from the sun, electric lamps, hot electric stoves, lightning, and melted iron are common
examples. Actually, everything radiates some energy;
however, it is not always apparent, since the radiation is
usually infrared rather than visible light. The different
colors of this emitted radiation are an important characteristic of a material, and they depend on the temperature and properties of the material.
We have long known that light separates into its
component colors when passed through a
triangular-shaped piece of glass called a prism. When
white light from the sun passes through such a prism, it
breaks into all the colors of the rainbow. Light from an
electric lamp also displays all the visible colors, but
blue is comparatively dimmer than the blue from sunlight.
The pattern of colors emitted by a material is called
the spectrum of its emission. A spectrum that covers
the entire rainbow, such as sunlight, is said to be continuous. Some materials, particularly in the plasma
state, emit only a few of the possible colors. The spectrum is then said to be a discrete (or line) spectrum.
The specific colors that are emitted in a discrete
spectrum are characteristic of the material and can be
used to identify the material even when it is not otherwise accessible. For example, light from the sun and
stars reveals a great deal about the materials of which
these objects are composed. Details of emitted and
Figure 10.3. These balls have the same mass. Which
has the greater density? How do you know?
Densities of materials vary widely. The density of
hydrogen is only 0.089 kilogram per cubic meter,
whereas that of gold, a solid, is over 19,000 kilograms
per cubic meter, more than 200,000 times larger.
The densities of all materials change with temperature, usually (but not always) decreasing as temperature
increases. Changes of state are always accompanied by
abrupt changes in density. Liquids are usually less dense
than solids of the same material; gases are always less
dense than the corresponding liquids. Water is an important exception to this general rule, because ice is slightly
less dense than water. Table 10.1 lists the specific gravity
(relative density) for several common materials.
Color
The colors of materials provide interesting clues to
their internal structure. By “color,” we usually mean
what happens to the light that falls on material. When
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Figure 10.5. Light from an incandescent bulb contains all the colors. A plasma emits a discrete spectrum that contains
only a few colors.
reflected light have been an important part in understanding the detailed structure and behavior of matter
and of light itself (Fig. 10.5).
ter how small the force or how strong the material.
Stronger forces always cause larger deflections.
Civil and mechanical engineers must take such distortions into account in the design of buildings, bridges,
automobile engines, and almost every other project. A
common technique, for example, is to form building
materials so that they will assume a more desired shape
when they carry their designed load. A prestressed concrete beam is curved upward when placed into position,
but it is straight when carrying its load as part of a building or bridge (Fig. 10.6). If the beam were straight to
begin with, it would sag downward when carrying the
load.
The stiffness or elasticity of a material is characterized by its elastic constant. This number simply measures the amount of force needed to cause a particular
deformation and is defined roughly by the relationship
Response to Force
Another way to identify materials is to describe
their response to external forces. Materials vary greatly in their hardness (or softness) and their ability to
return to their original shape and size once external
forces are removed.
Materials that permanently change shape under the
influence of outside forces are said to be plastic. Soft
clay and putty are common examples. Plasticity
depends on the temperature of the material. Iron, for
example, is not plastic at ordinary temperatures but can
be formed into almost any shape when very hot (but still
considerably below its melting temperature).
Materials that deform when forces are applied, but
return to their original size and shape when the forces
are removed, are said to be elastic. A rubber band is
elastic, because it stretches when forces are applied and
returns to its original shape and size when the stretching
forces are removed.
Almost all the materials we deal with in ordinary
circumstances are elastic. We often do not notice the
deflections that occur, so we tend not to be aware of
elastic constant !
force
.
deformation
Rubber has a small elastic constant, since a small
force produces a comparatively large deformation.
Steel, on the other hand, deforms only slightly when fairly large forces are applied; thus, it has a large elastic constant. In this sense, steel is far more elastic than rubber.
Elastic constants are associated with three different
kinds of deformations: tension, compression, and shear.
A material is said to be under tension when the forces
act to stretch it (Fig. 10.7); the forces would pull the
material apart if it did not resist. If the forces push
inward on the material, it is said to be compressed; the
forces cause the material to occupy a smaller volume
than before. Shearing forces cause the material to
change shape. These might be applied parallel to the
surface of the material, perhaps by frictional forces.
Fluids have little resistance to changes in shape, so
their elastic constants for shear would be nearly zero. The
compressional elastic constant of a liquid, however, is
usually quite high. For example, water is only slightly
more dense (more compressed) at the bottom of the ocean
than near the surface. Compressional elastic constants for
gases are considerably lower. A comparatively small
force on the handle of an automobile tire pump can cause
Figure 10.6. A prestressed concrete beam straightens
when a load is applied.
them. For example, when we sit on a chair, the chair
bends slightly because of the force we apply. When we
stand, the force on the chair is removed and the chair
returns to its original shape. A bridge made of the
strongest steel bends slightly when we walk on it. Such
deflections occur whenever forces are applied, no mat-
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Figure 10.7. Three important ways that force can be applied to materials. Which of these is under tension? compression? shear?
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 shape
when the force is removed. Instead, the material will
become permanently deformed just as if it were plastic.
The maximum force that can be sustained without permanent deformation is a measure of the elastic limit of
the material. This is another important property by
which materials can be classified and identified.
Builders and engineers clearly need to be concerned
about the elastic limits of the materials they use.
The elastic properties of materials are a result of
electric forces acting within each piece of matter.
Again, detailed explanations of these properties and
their variations require significant understanding of the
submicroscopic structure of matter.
some of the charged particles are free to move about
inside the conductor. Conversely, the absence of current
in the nonconductors implies that the charged particles
in these materials are not free to move, but are held
rigidly in place.
Metals are conductors—copper, aluminum, iron,
sodium, and zinc. Nonconductors include the nonmetals such as phosphorus, sulfur, water, sodium chloride (table salt), and sugar. These are sometimes called
insulators.
In addition, some materials, called semiconductors, conduct small currents under appropriate circumstances. They do not conduct as well as the conductors,
but they do conduct better than nonconductors. This
means that only a small fraction of the charged particles
semiconductors are made of are free to move inside the
material. Semiconductors made of silicon form the
basis of our modern computer technology.
Some nonconductors become conductors if they are
melted, or if they are dissolved in water or some other
liquid. Table salt, for example, is a nonconductor as a
solid. It becomes a conductor when melted or when dissolved in liquid. Other nonconductors do not exhibit
this behavior. Water is a nonconductor as ice or as liquid, but it will conduct an electric current if certain
materials such as table salt are dissolved in it.
Dissolved sugar, on the other hand, does not cause
water to become a conductor (Fig. 10.9).
Electrical Properties
Electrical properties provide some of the most
provocative clues about the internal structure of matter.
Solid materials can be roughly divided into two classes,
conductors and nonconductors. An electric current can
flow through conductors if an electrical force acts on the
charged particles within the material. Electric current
does not flow through nonconductors. (Fig. 10.8).
All matter contains electrically charged particles.
The presence of an electric current means that at least
Figure 10.8. Which material is the conductor? Which is the nonconductor? How do you know?
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Figure 10.9. (a) Water by itself is a nonconductor. (b) Adding salt to water causes it to conduct. (c) Adding sugar to
water does not cause it to conduct an electric current.
Nonconductors that become conductors as liquids,
or that become conductors when dissolved in a nonconducting liquid such as water, are called ionic materials.
Those that remain nonconductors under these circumstances are nonionic (Fig. 10.10). Apparently the internal structures of ionic and nonionic materials differ.
The charged particles in ionic materials somehow
become free in the liquid state, whereas those in nonionic materials remain bound. These differences can
only be explained by a detailed model of the internal
structure of these materials.
spectrum that is characteristic of the material. The emitted light is a direct probe into the atom.
In addition to the features of matter described in this
chapter, matter participates in a variety of chemical
changes. The chemical properties of matter are an important part of our description of nature and provide additional evidence for our model of the structure of matter.
STUDY GUIDE
Chapter 10: The Physical Properties of Matter
A. FUNDAMENTAL PRINCIPLES: No new fundamental principles.
B. MODELS, IDEAS, QUESTIONS, OR APPLICATIONS
1. The Continuous Model of Matter: That model of
matter which sees matter as smoothly divisible
without limit, i.e., matter that is not made up of discrete 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.
All Materials
Nonconductors
Conductors
(As Solids)
(As Solids)
Ionic
(Conductors As Liquids)
Nonionic
(Nonconductors As Liquids)
Figure 10.10. The classification of materials according
to their electrical properties.
C. GLOSSARY
1. Color: A characteristic of matter imparted to it by
the nature of the reflected light which it transmits to
an observer.
2. Compression Force: A force which is applied in
such a way as to compress a material.
3. Conductor (specifically, of electricity): A substance that readily allows an electric current to flow
through it. The opposite of an insulator (nonconductor). Copper wire is a conductor.
4. Continuous Spectrum: A spectrum in which the
colors blend gradually together without noticeable
abrupt changes or missing colors. The opposite of
a discrete spectrum.
5. Density: Mass per unit volume.
6. Discrete (or Line) Spectrum: A spectrum of sep-
Summary
In this chapter as we begin a study of matter, we
have followed Aristotle’s lead by classifying matter in
various ways. Classification is a preliminary exercise
we will follow in later chapters to find unifying theories
that attempt to explain the characteristics.
Matter can be classified according to its physical
states: solid, liquid, gas, and plasma. Matter may also
be classified according to its density, color, response to
forces, and conduction of electricity. All of these give
some clues to the underlying structure of matter.
The spectrum of light emitted by matter is particularly important. Under some conditions, matter emits a
continuous spectrum; under others it emits a discrete
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7.
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23.
arate and distinct colors. The opposite of a continuous spectrum.
Elastic: An adjective describing materials that
deform when forces are applied but return to their
original size and shape when the forces are
removed.
Elastic Constant: A quantitative measure of elasticity formed by taking the ratio of the force
applied to a material to a measure of the resulting
deformation.
Elastic Limit: The maximum force that a material
can sustain without sustaining a permanent change
in shape.
Fluid: Matter that flows readily. Gases and liquids
are fluids.
Gas: A physical state of matter that readily
changes both shape and volume to match its container.
Ionic Material: A material that is a nonconductor
of electricity as a solid but that conducts electricity
when melted or dissolved in water.
Ionized Matter: Matter in which at least some of
the atoms have been altered from their ordinary neutral state by the addition or subtraction of electrons.
Liquid: A physical state of matter that readily
changes shape to match its container but that resists
changes in volume.
Nonconductor (specifically, of electricity): An
insulator. A substance that does not readily allow
an electric current to flow through it. The opposite
of a conductor. Glass is a nonconductor.
Nonionic Material: A material lacking in some
way the characteristics of an ionic material.
Plasma: A physical state of matter characterized
by fluid properties but in which positive and negative charges move independently.
Plastic: An adjective describing materials that permanently change shape under the influence of
external forces.
Semiconductor: A class of materials with electrical
conducting properties somewhere between conductors and nonconductors. Silicon is a semiconductor.
Shearing Force: A force that is applied in such a
way as to be tangential to the surface on which it
acts.
Solid: A physical state of matter that is characterized by rigidity and resistance to changes in size
and shape.
Spectrum: A display of the amounts and colors of
light emitted by a particular source in which one
sees the colors separated from one another.
Tension Force: A force that is applied in such a
way as to stretch a material.
E.
EXERCISES
10.1. Describe what is meant by a fluid and give
examples of substances that are fluids at ordinary temperatures.
10.2. Describe how materials change from one state
to another. How do materials like gasoline and copper
differ from one another in this respect?
10.3. Which has greater density, an ice cube or an
iceberg? Explain your answer.
10.4. Name several materials that are solids at ordinary temperatures.
10.5. Name several materials that are liquids at
ordinary temperatures.
10.6. Name several materials that are gases at ordinary temperatures.
10.7. Describe what is meant by a “plasma” and
give 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 in
order of increasing density.
10.10. Why do people look different when seen
under mercury vapor lamps than when seen by sunlight? In particular, red objects seem almost black
under such lamps.
10.11. Explain how it might be possible to know
something about the materials in a planet without going
there.
10.12. Explain what is meant by a “continuous”
spectrum.
10.13. Explain what is meant by a “discrete” spectrum.
10.14. A certain wire stretches 0.1 millimeter when
a particular force is applied. Another wire in the same
size and shape but made of a different metal stretches
0.01 millimeter with the same force. Which has the
larger elastic constant? Explain your answer.
10.15. Explain why the elastic properties of materials must be taken into account when designing buildings, bridges, and other structures.
D. FOCUS QUESTIONS: None.
10.16. Explain what is meant by the term “elastic
constant.”
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10.17. Show how forces can be applied to a material to produce compression, tension, and shear.
10.18. In what way are the elastic constants of fluids different than those for solids?
10.19. What conditions must be met if a current is
to flow through any material?
10.20. What is the difference between the internal
structures of conductors and nonconductors?
10.21. What is the difference between the internal
structures of ionic and nonionic materials?
10.22. Define “conductor,” “nonconductor,”
“ionic,” and “nonionic.”
10.23. Suppose you are given an unfamiliar sample
of material, say a rock. Describe the experiments you
might perform that allow you to classify the material
according to its electrical properties.
10.24. Why is tap water a conductor while pure
water is not?
10.25. Do you think the human body is a conductor or nonconductor? Why? What does this imply
about the structure of the body?
10.26. Which of the following processes does not
produce 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 “understanding”? In modern testing practice one often sees multiple-choice questions for which the available choices are
the names of things. What is being tested in such
instances: classification or understanding? What did
Aristotle think was the objective of classification?
94