Kelter P., Mosher M., Scott A. Chemistry. The Practical Science

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its elements appear in the substance). A chemical change results in a change in the

chemical composition of a substance. Therefore, as we look at each process, we

think about whether changes in chemical composition occur.

Solution

a. The fuel reacts with O

and is converted into carbon dioxide and water vapor

(and some other gases as well). The reaction is accompanied by the release of

energy. This is a chemical change.

b. Melting ice changes the physical condition of the water from solid to liquid. It

is still water, so this is a physical change.

c. Dissolving sugar in your tea is a physical change. The sugar is still there; it is

just mixed intimately with your tea.

d. Your body does a good job of utilizing the sugar in a chocolate bar. This is an

example of a chemical change that releases energy as the sugar is converted

into other substances.

Further Insights

As we learn more about how atoms are joined, or bonded, to other atoms, we will

discover that many physical properties are based on the type and the ratio of the

elements that make up a compound. In other words, physical properties such as

color, density, and melting point are based on chemical composition. For example,

we will learn why, at room temperature, water is a liquid, whereas methane, a com-

pound of similar mass containing the elements carbon and hydrogen, is found as

“natural gas.”

PRACTICE 1.1

Indicate whether each process involves a chemical or a physical change.

a. The rusting of a copper statue

b. The yellowing of an old newspaper

c. Boiling water on the stove

d. Shaping a piece of wood into a post

e. Sharpening your lawn mower’s blade

f. Developing a photograph

See Problems 19, 20, and 109–111.

Classiﬁcation of Matter

We’ve discussed matter as anything that takes up space and possesses mass. How-

ever, chemists are often concerned with more speciﬁc classiﬁcations of matter. By

classifying matter into groups with similar characteristics, we can reduce the

number of questions that we need to ask in order to determine the chemical or

physical properties of a substance.

Substances include elements and compounds. Chemists are often concerned

with

mixtures of substances, rather than pure substances. For example, the bottle

of pure, vacuum-distilled water on the chemist’s shelf contains only water. In

contrast, tap water, well water, river water, and seawater contain mixtures of water

and other substances (such as salt, dirt, small amounts of oxygen, and so on).

Mixtures themselves can be categorized as either homogeneous or heteroge-

neous (Figure 1.9). A

homogeneous mixture,orsolution, is uniform all the way

through. Consider a bottle of tap water. Any part of the tap water sample is the

same as any other part. A

heterogeneous mixture, on the other hand, contains a

different composition in different places. For example, if we throw some salt into

a bucket of sand and stir the contents around for a short while, we will ﬁnd that

8 Chapter 1 The World of Chemistry

1.2 The Chemist’s Shorthand 9

FIGURE 1.9

Examples of homogeneous mixtures (tap water, gold

ring, and grape juice) and examples of heterogeneous

mixtures (concrete and salad dressing).

Homogeneous Heterogeneous Elements

Matter

Compounds

SubstanceMixtures

FIGURE 1.10

The classiﬁcation of matter.

some parts of the mixture have more salt than sand, whereas some parts are

mostly sand. The mixture is heterogeneous. Grape juice is a homogeneous mix-

ture; it is the same throughout. The same is true of a 14-karat gold wedding

ring—a homogeneous mixture of the elements gold, silver, and copper. On the

other hand, cement is a heterogeneous mixture because some parts contain more

sand and rock than other parts. Oil-and-vinegar salad dressing is also a heteroge-

neous mixture, no matter how hard you shake it. The relationships among the

classiﬁcations of matter are shown in Figure 1.10.

EXERCISE 1.2 Describing the Chemistry of the Shuttle

Figure 1.8 shows the propulsion system provided by the main engines of the space

shuttle. Can you write a summary of this system that includes all these terms used

in an appropriate way?

❚ physical change ❚ element(s)

❚ chemical change ❚ compounds(s)

❚ atom(s) ❚ mixture

Visualization: Comparison of a

Compound and a Mixture

Visualization: Comparison of a

Solution and a Mixture

Visualization: Homogeneous

Mixtures: Air and Brass

Solution

There are many possible ways of phrasing a suitable answer. Here is one example:

“The space shuttle’s main fuel tank contains the elements oxygen and hydrogen.

The oxygen is in the form of molecules, each composed of two oxygen atoms. The

hydrogen is in the form of molecules, each composed of two hydrogen atoms. The

oxygen and hydrogen are combined to form a mixture that, when ignited, reacts ex-

plosively to form molecules of water. This is a chemical change because the elements

oxygen and hydrogen are being changed into the compound water. Each water mol-

ecule contains one oxygen atom and two hydrogen atoms. The water is formed as a

gas, but as it leaves the back of the shuttle, it cools and undergoes a change into

droplets of liquid water—an example of a physical change.”

PRACTICE 1.2

Classify each as either a homogeneous or a heterogeneous mixture.

a. Hot tea d. Gasoline

b. A milkshake e. Bronze

c. A root beer ﬂoat f. A chef’s salad

See Problems 15–18 and 21–22.

Physical Separations

Pure water can be separated out of the homogeneous mixture called seawater by

a process known as

desalination, the removal of salts (Figure 1.11). Desalination

employs physical, rather than chemical, changes. In Saudi Arabia, fresh water is so

sparse that desalination of seawater has become essential to life. Just over 50% of

Saudi Arabia’s usable water supply is generated via desalination, and about 30%

of the world’s desalination capacity is in Saudi Arabia. The United States comes

in second, with about 12%, and many other countries in the Middle East, North

Africa, Asia, and the Caribbean account for the remainder. Desalination is a

growing industry, and many companies are making a considerable investment in

research to exploit signiﬁcant improvements in the technology.

Figure 1.12 shows that the ﬁrst crucial step in one important type of desali-

nation process is to use heat to cause some of the water molecules in seawater to

escape from the liquid into a gas. In fact, most liquids can be converted into gases

if you supply enough heat. This process involves changing the

state of the water

sample. The common states of matter, which you know to be solid, liquid, and

gas, are among the most vital physical characteristics of matter. Cooling can re-

verse the changes of state that can be driven forward by heating. This is what hap-

pens in the desalination plant. The gaseous water

(water vapor) is cooled and changes back into liquid

water for drinking water supplies.

This desalination process is known as heat desali-

nation because heat is used to drive the water mole-

cules out of seawater. Two other forms of desalina-

tion illustrate other ways of separating a component

from a homogenous mixture (see Figure 1.13). In

desalination by

electrodialysis, the seawater is passed

though a tube lined with a special porous membrane

and surrounded by electrically charged plates. The

salts in the seawater mixture are drawn through

the membrane. The water molecules remain within

the membrane and are not affected by the electric

charge. In desalination by

reverse osmosis, the

seawater is pumped at high pressure through

porous membranes. The high pressure forces water

10 Chapter 1 The World of Chemistry

Application

HEMICAL ENCOUNTERS:

Desalination

FIGURE 1.11

A desalination plant 120 kilometers

south of Jeddah, on Saudi Arabia’s Red

Sea coast.

Visualization: Structure of a

Solid

Visualization: Structure of a

Liquid

Visualization: Structure of a Gas

Video Lesson: States of Matter

Thermometer

Cold

water out

Cold

water in

Distillation

flask

Condenser

Receiver

Seawater

Pure

water

Heating

mantle

molecules through tiny pores in the membrane through which the salts are

largely unable to pass. Both electrodialysis and reverse osmosis are likely to be-

come increasingly viable methods of commercial desalination in the ﬁrst few

decades of the twenty-ﬁrst century. We will have much more to say about these

techniques in Chapter 11.

Desalination demonstrates that many important processes that include

chemicals do not actually involve chemical changes. Chemistry can be about

separating and changing the state of chemicals, as well as about using existing

chemicals to make other chemicals. Chemists use their understanding of the way

substances interact to interpret and change our world in predictable ways.

1.2 The Chemist’s Shorthand 11

FIGURE 1.12

The desalination of seawater is the source of

drinking water for many people in the world. In

heat desalination, the salt water is changed

into steam. The steam, which contains no salt,

rises out of the ﬂask, travels to the condenser,

and is converted back into liquid water.

++++++++

Flow of

water

Membraned

tubing

= Na

= Cl

–

Key:

Demineralize

water

Electrodialysis Reverse Osmosis

Concentrated

solution

Electrode

––––––––

Membraned

tubing

Membraned

tubing

Demineralized

water

Concentrated

solution

Membrane

= Na

= Cl

–

Key:

FIGURE 1.13

Separating pure drinking

water from a homogeneous

mixture of seawater. The

separation of homogeneous

mixtures is often more

involved than the separation

of heterogeneous mixtures.

1.3 The Scientiﬁc Method

As we have noted, chemists such as Judith Fairclough-Baity ask questions in order

to discover and explore nature. This is how all scientists perform their craft. The

process of science is all about asking questions and making careful observations.

For example, the question“Is hydrogen gas explosive?”can be answered by prepar-

ing a little pure hydrogen, igniting it (from a safe distance!) in the presence of

oxygen, and watching what happens. Although the question that Ms. Fairclough-

Baity asks is a little more complex (“Can a silicone tube be created that doesn’t

fracture when it is being ﬂexed?”), the process is the same as that which we use to

answer all scientiﬁc questions (Figure 1.14).

We call this rational approach to knowing the

scientiﬁc method. The scientiﬁc

method is not a rigid set of rules, because the human mind is an intellectually

ﬂexible place. Rather, it is a way of asking questions and ﬁnding answers. Think-

ing about our world in this rational way ensures that we get meaningful results

and steadily learn more about the topics we investigate. The method does not al-

ways provide the answer to a question immediately. Instead, many days, months,

or even years may be needed to arrive at an adequate answer. The scientiﬁc

method is often described as a series of steps; these steps are listed in Table 1.1.

Figure 1.15, which illustrates the general procedure used to learn in science, con-

veys the same approach graphically.

The scientiﬁc method is a reasonable outcome of people’s desire to learn

about how nature works, what nature can and cannot do, and how we can chem-

ically manipulate what nature has provided for us. There are two crucial points

here. One is that the scientiﬁc method is the approach we use to test our proposed

explanations and ideas about nature by looking for evidence, and by discarding

or modifying explanations for which there is no evidence. The other point is that

you can apply this way of understanding to any subject or topic, whether in the

classroom or in life after college.

Step 5 in Table 1.1 shows us that the scientiﬁc method is a cyclical process.

Each iteration of the steps allows the scientist to reﬁne the hypothesis until it

completely accounts for his or her observations. Many times, this reﬁned and

well-tested hypothesis provides the answer to the question of concern formulated

in step 1.

12 Chapter 1 The World of Chemistry

FIGURE 1.14

Asking questions and experimenting are the start of the scientiﬁc questioning process.

Video Lesson: The Scientiﬁc

Method

Sometimes, patterns emerge in the data collected through experi-

mentation. When these patterns are consistently noted across a series of

different systems under study, a statement called a

scientiﬁc law can be

made. The scientiﬁc law summarizes the patterns that are observed in the

data. They are succinct descriptions of the behavior of the natural world.

Yet the scientiﬁc law does not attempt to explain these observations. For

example, one scientiﬁc law that we will discuss later describes the behav-

ior of a gas as the temperature, pressure, and volume of the gas are

changed. This scientiﬁc law does not explain why the gas behaves the

way it does; it simply tells us what to expect.

Once a law related to a particular phenomenon has been established

using the scientiﬁc method, a theory can be constructed. A theory is

based on the observations made from a variety of experiments, so it pro-

vides a tested explanation, developed from hypotheses that have been

supported by the results of experiments. In short, a

theory is an attempt,

based on scientiﬁcally acquired evidence, to explain observations that

have been described as fact. For example, the theory that an asteroid collided

with Earth some 65 million years ago is an attempt to explain the demise of the

dinosaurs.

A theory is a powerful statement not to be taken lightly. It is not guesswork or

supposition. It is based on thoughtful and structured questions, experiments, and

data analysis. Some theories become trusted to such an extent that they almost

acquire the status of certainties, because they are repeatedly conﬁrmed by many

different experiments. Other theories last for only a short time, until they are

modiﬁed or even swept away by new evidence. For example, early theories on the

origins of chemicals suggested that compounds inherent in living creatures, such

as the molecule urea found in urine, contained a “life force” and that they could

not be made from inanimate things such as dirt or rocks. The later discovery of

evidence that this is not true refuted that early theory. All theories that stand

the test of time do so because they continue to be supported by experimental

evidence.

1.3 The Scientiﬁc Method 13

Design/perform

experiments

Create a

hypothesis

Theory

Make

observations

Find previous

information

Formulate a

question

The Scientiﬁc Method

Step 1: Formulate a question that you would like answered.

Step 2: Find out what is already known about your question. This is

typically done by searching the scientiﬁc literature, which

contains the work of other scientists.

Step 3: Make observations—in other words, examine things. The

observations involve the collection of both qualitative (non-

numeric) and quantitative (derived from measurement) data

about the system under study. Recording the data in a

systematic way is vital to this step.

Step 4: Create a

hypothesis, or possible explanation that would account

for your observations. In other words, the hypothesis is a

statement about what you think might be going on.

Step 5: Design and perform experiments that are carefully and

deliberately set up to test your hypothesis. Speciﬁc testing of

the hypothesis is needed to ensure that the possible

explanation “holds up” to scrutiny. These experiments yield

further observations and data for you to consider, and the

method is joined again at step 3.

TABLE 1.1

FIGURE 1.15

The scientiﬁc method.

A theory that explains the extinction of dinosaurs.

14 Chapter 1 The World of Chemistry

To ensure that the scientiﬁc method is working well, scientists perform their

experiments several times. Repeating experiments time after time allows the sci-

entist to prove that the data were collected properly, that the observations are cor-

rect, and that the new information he or she has discovered is real. By publishing

the results of their experiments, scientists also expose their work to other people’s

scrutiny and criticism. Other scientists then have the opportunity to check the ex-

periments, results, and hypotheses to make sure these are valid. Communication

of all types, whether via classroom teaching, writing books, or publishing journal

articles, is as vital to science as performing experiments in the laboratory. There are

nearly 600,000 chemistry-related articles published each year worldwide; each

article is intended to communicate ideas for others to consider.

The scientiﬁc method is powerful because it works. Over time, it has yielded

hypotheses, theories, and laws that describe, explain, and predict much of the be-

havior of the natural world. It has enabled us to make use of ﬁre and to extract

The intensive studying that goes on in college can occa-

sionally give anyone a headache. A most beneﬁcial result

of the application of the scientiﬁc method is our ability

to control the pain with readily available medicines. The

systematic study of pain control began several thousand

years ago, and now, centuries later, new words are still

being added to our vocabulary as analgesic (pain-killing)

drugs are discovered and tested. A look at the history of

pain control helps us learn about the power and the limi-

tations of the scientiﬁc method.

The question was“Can anything in the world around

us be used to control pain?”Asian manuscripts from

2400 years ago offer some of the ﬁrst examples of useful

answers to that question, including the observation that

infusions of willow tree bark can relieve pain and treat

fevers.

This observation was the result of performing exper-

iments, gathering data, and interpreting results in order

to identify which naturally available plant materials

might be useful in human health. The ancient manu-

scripts about willow bark are very early instances of the

publication of results, allowing other people to know

what experiments had been done, to check the results,

and to make use of them. One example of such checking

that results are replicable occurred in the 1830s, when a

Scottish physician was able to conﬁrm that extracts of

willow bark relieved the pain of acute rheumatism.

As our understanding of chemistry and our ability to

make better measurements developed, investigations on

willow bark led scientists to create the hypothesis that

one particular compound within the willow bark was re-

sponsible for the control of rheumatism pain. This led to

the question “Which compound?” and to experiments

designed to isolate the different substances in the willow

bark. The answer came in the 1840s, when chemists and

physicians determined that salicin (see Figure 1.16), iso-

lated from both willow bark and a plant known as spirea,

was the ingredient responsible for the pain control. In

1870, Professor von Necki in Basle, Switzerland, per-

formed experiments and gathered data showing that

salicin, when ingested, is converted into the salicylic

acid shown in Figure 1.16. This led to the manufacture of

salicylic acid. It was given directly to patients to test the

hypothesis that it would relieve pain and fevers. One out-

come of these experiments (some of the ﬁrst examples of

“controlled clinical

trials”) was that the

salicylic acid, although

effective, severely irri-

tated the lining of the

mouth, throat, and

stomach. This led var-

ious chemists to form

the hypothesis that

modifying the sali-

cylic acid in some way

might yield com-

pounds that were

effective painkillers

without having the

nasty side effects of

salicylic acid. In the

1890s, Felix Hoffman

(see Figure 1.17) was

working for the Bayer

Company in Germany

How do we know?

How to control pain:

An example of the scientiﬁc method in action

The ancient Egyptians used reasoning

to develop medicines for pain.

1.3 The Scientiﬁc Method 15

when he synthesized acetylsalicylic acid. This modiﬁca-

tion of salicylic acid became known as aspirin, today one

of the most effective and widely used analgesic drugs.

In the twenty-ﬁrst century we have many different

analgesic drugs at our disposal. The development of each

one involved the application of the scientiﬁc method to

discover, test, and improve our ability to control pain.

The powerful analgesic known as morphine, for exam-

ple, is a natural product of the opium poppy. Heroin is a

synthetic derivative of morphine, made by treating mor-

phine with the chemical acetic anhydride. In experi-

ments, heroin is observed to be up to eight times as ef-

Acetylsalicylic acid

CC OH

fective at relieving pain as morphine. Unfortunately, the

side effects of these compounds include chemical depen-

dence and the associated hazards of abuse. Therefore,

clinically, they are used only under controlled conditions

to alleviate acute pain.

FIGURE 1.17

Felix Hoffman and the

wonder drug, aspirin.

HC CH

C C

Salicylic acidSalicin

FIGURE 1.16

Salicin can be converted into salicylic acid.

pure minerals from the Earth. Through it we have learned about the interaction

of individual molecules and have applied this understanding to form the massive

three-dimensional molecules we call plastics. And it has allowed us both to un-

derstand the chemistry of life and to leave the Earth’s surface to explore our

moon, the other planets, and beyond.

EXERCISE 1.3 A Theory or a Law?

Earth’s atmosphere is a mixture of gases—largely nitrogen (N

), oxygen (O

), and

water vapor (H

O), with much smaller amounts of argon (Ar) and a wide variety of

other gases. Careful observation of these gases under controlled experimental con-

ditions has shown that when the temperature is held constant, halving the pressure

of the gas causes its volume to roughly double. As one goes down, the other goes up,

as though pressure and volume were at the two ends of a seesaw.

Similarly, when the pressure is doubled (“see . . .”), the volume

becomes half of the original value (“saw . . .”). We can describe this

behavior by saying that at constant temperature, the volume of a gas

is inversely proportional to its pressure. Most gases deviate slightly

from this generalized statement, but they all adhere to it quite

closely at the temperatures found in the atmosphere, and this be-

havior has been checked many times with many different gases. Is

this an example of a hypothesis, a theory, or a scientiﬁc law?

Solution

The statement that “at constant temperature, the volume of a gas is inversely

proportional to its pressure” is a description of the behavior of a gas. It is not an ex-

planation for the behavior being described. Hence, this statement is an example of a

scientiﬁc law rather than a theory. This particular law is actually known as Boyle’s

law, in honor of Robert Boyle (1627–1691), who discovered and publicized it.

PRACTICE 1.3

Describe each step a scientist would follow as he or she investigated the following

question using the scientiﬁc method. Carefully explain each step, describing any

speciﬁc experiments and observations the scientist would make. Will the result of

this application of the scientiﬁc method be a hypothesis, a theory, or a scientiﬁc law?

There is a dark liquid in my cup. Is the liquid coffee and who put it there?

See Problems 25–28 and 33–34.

1.4 Units and Measurement

Chemistry is a precise science that is based on our ability to properly measure the

interactions of chemicals. When Judith Fairclough-Baity conducts performance

experiments on a new piece of silicone tubing that she has constructed, she makes

observations. She collects data that include the length of the tube, the diameter of

the tube, the volume of blood it will hold, and the temperature range in which it

is usable. The data she collects about her experiments are known as quantitative

observations. In fact, making quantitative measurements of the results of an ex-

periment is fundamental to all scientiﬁc inquiries.

Recently, Fairclough-Baity examined the usable life of three different types of

silicone tubes. She set up an experiment that compressed and expanded the tube

continuously, mimicking the use of the tube in a heart–lung pump. Her results

were obtained on pieces of tubes with very well-deﬁned masses. She reported

those results to other chemists in an article she wrote. In order to communicate

her data and results effectively, she used units of measurement that are common

among the sciences. Before the eighteenth century, this was not done. Some scien-

tists measured masses in karats; some in drams, grains, ounces, pennyweights, or

pounds; and still others in scruples (an old pharmacist’s measurement, in which

288 scruples = 12 ounces). The multitude of different units made comparing the

results of experiments difﬁcult. Today, only two major systems for measuring

data are currently in use: the English system (used in several countries, including

the United States, where it is more formally known as the United States Custom-

ary System, or USCS) and the metric system. In the 1960s, the metric system was

modernized to what we use in scientiﬁc studies today.

16 Chapter 1 The World of Chemistry

VP VP

Pressure and volume are inversely proportional.

Video Lesson: The Measurement

of Matter

The Système International and Its Base Units

The modiﬁcation of the metric system in the 1960s gave rise to the Système Inter-

national d’Unites, also known as the

International System (SI). The SI deﬁnes seven

base units used to measure seven fundamental physical quantities. These physical

quantities, the corresponding base units, and their abbreviations are summarized

in Table 1.2. From these seven base units, scientists can report measurements of

every imaginable type and can effectively report those measurements to other sci-

entists. During our exploration of chemistry, we will use the ﬁrst ﬁve of these

units extensively.

Suppose, for example, that we are in charge of a water desalination plant. Part

of our daily routine at the plant would likely include checking the quality of the

water that the plant produces. Some tests that measure the quality of the water re-

quire a ﬁxed mass of the liquid. Using a balance, we can measure the mass of a

water sample. To report our scientiﬁc measurement, we will combine the base

unit for our measurement with a preﬁx. This preﬁx multiplies the number por-

tion of the measurement by a power of 10. For example, the base unit of mass is

the

kilogram (kg). The preﬁx kilo- in kilogram, one of the preﬁxes listed in

Table 1.3, indicates that 1 kilogram is equal to 1000 grams (g). The kilogram is the

only base unit that incorporates a preﬁx in its deﬁnition.

1 kg = 1 × 10

g = 1000 g

Unfortunately, the mass of an object in kilograms is often incorrectly referred

to as a weight. In fact, there is a sharp distinction between the concepts of mass

and weight.

Mass is a measure of the amount of matter in a body, determined by

measuring its inertia, or resistance to changes in its state of motion.

Weight is a mea-

sure of the downward force exerted on a body by gravity. The mass of an object is a

fundamental property of the object because it will not change, for instance, if we

1.4 Units and Measurement 17

The Fundamental “Base Units” of the International System (SI)

Physical Quantity Name of Unit Symbol

Mass kilogram kg

Length meter* m

Time second s

Temperature kelvin K

Amount of substance mole mol

Electric current ampere A

Luminous intensity candela cd

TABLE 1.2

*Meter is spelled metre in most countries except the United States.

The SI Preﬁxes

Multiple Preﬁx Name

Y yotta

Z zetta

Eexa

Ppeta

Ttera

G giga

Mmega

k kilo

da deka

−1

ddeci

−2

ccenti

−3

m milli

−6

µ micro

−9

n nano

−12

p pico

−15

f femto

−18

a atto

−21

zzepto

−24

yyocto

TABLE 1.3