The Scientific Method
There are many ways to obtain knowledge. Modern sci-
entists tend to obtain knowledge about the world by
making systematic observations. This principle is called
empiricism and is the basis of the scientific method. The
scientific method is a set of rules for asking and answer-
ing questions about science. Most scientists use the
scientific method loosely and often unconsciously.
However, the key concepts of the scientific method are
the groundwork for scientific study, and we will review
those concepts in this section.
The scientific method involves:
■
asking a specific question about a process or phe-
nomenon that can be answered by performing
experiments
■
formulating a testable hypothesis based on obser-
vations and previous results
■
designing an experiment, with a control, to test
the hypothesis
■
collecting and analyzing the results of the
experiment
■
developing a model or theory that explains the
phenomenon and is consistent with experimental
results
■
making predictions based on the model or theory

in order to test it and designing experiments that
could disprove the proposed theory
THE QUESTION
In order to understand something, a scientist must first
focus on a specific question or aspect of a problem. In
order to do that, the scientist has to clearly formulate the
question. The answer to such a question has to exist and
the possibility of obtaining it through experiment must
exist. For example, the question “Does the presence of
the moon shorten the life span of ducks on Earth?” is not
valid because it can not be answered through experi-
ment. There is no way to measure the life span of ducks on
Earth in the absence of the moon, since we have no way of
removing the moon from its orbit. Similarly, asking a
general question, such as “How do animals obtain food?”
is not very useful for gaining knowledge. This question is
too general and broad for one person to answer.
Better questions are more specific—for example,
“Does each member of a wolf pack have a set responsi-
bility or job when hunting for food?” A question that is
too general and not very useful is “Why do some people
have better memories than others?” A better, more spe-
cific question, along the same lines, is “What parts of the
brain and which brain chemicals are involved in recol-
lection of childhood memories?”
A good science question is very specific and
can be answered by performing experiments.
THE
HYPOTHESIS
After formulating a question, a scientist gathers the

information on the topic that is already available or pub-
lished, and then comes up with an educated guess or a
tentative explanation about the answer to the question.
Such an educated guess about a natural process or phe-
nomenon is called a hypothesis.
A hypothesis doesn’t have to be correct, but it should
be testable. In other words, a testable hypothesis can be
disproved through experiment, in a reasonable amount
of time, with the resources available. For example, the
statement, “Everyone has a soul mate somewhere in the
world,”is not a valid hypothesis. First, the term soul mate
is not well defined, so formulating an experiment to
determine whether two people are soul mates would be
difficult. More importantly, even if we were to agree on
what soul mate means and how to experimentally deter-
mine whether two people are soul mates, this hypothe-
sis could never be proved wrong. Any experiment
conceived would require testing every possible pair of
human beings around the world, which, considering the
population and the population growth per second, is just
not feasible.
A hypothesis doesn’t need to be correct. It only
has to be testable.
Disproving a hypothesis is not a failure. It casts away
illusions about what was previously thought to be true,
and can cause a great advance, a thought in another
direction that can bring about new ideas. Most likely, in
the process of showing that one hypothesis is wrong, a
– SCIENCE AS INQUIRY–
220

scientist may gain an understanding of a better hypoth-
esis. Disproving a hypothesis serves a purpose. Science
and our understanding of nature often advance through
tiny incremental pieces of information. Eliminating a
potential hypothesis narrows down the choices, and
eliminating the wrong answers sometimes leads to find-
ing the correct one.
THE EXPERIMENT
In an experiment, researchers manipulate one or more
variables and examine their effect on another variable or
variables. An experiment is carefully designed to test the
hypothesis. The number of variables in an experiment
should be manageable and carefully controlled. All vari-
ables and procedures are carefully defined and described,
as is the method of observation and measurement.
Results of a valid experiment are reproducible, meaning
that another researcher who follows the same procedure
should be able to obtain the same result.
A good experiment also includes one or more con-
trols. Experimental controls are designed to get an
understanding of the observed variables in the absence of
the manipulated variables. For example, in pharmaceu-
tical studies, three groups of patients are examined. One
is given the drug, one is given a placebo (a pill contain-
ing no active ingredient), and one is not given anything.
This is a good way to test whether the improvement in
patient condition (observed variable) is due to the active
ingredient in the pill (manipulated variable). If the
patients in the group that was given the placebo recover
sooner or at the same time as those who were given the

drug, the effect of pill taking can be attributed patient
belief that a pill makes one feel better, or to other ingre-
dients in the pill. If the group that was not given any pill
recovers faster or just as fast as the group that was given
the drug, the improvement in patient condition could be
a result of the natural healing processes.
An experimental control is a version of the
experiment in which all conditions and variables
are the same as in other versions of the exper-
iment, but the variable being tested is elimi-
nated or changed. A good experiment should
include carefully designed controls.
T
HE ANALYSIS
Analysis of experimental results involves looking for
trends in the data and correlation among variables. It
also involves making generalizations about the results,
quantifying experimental error, and correlating the
variable being manipulated to the variable being tested.
A scientist who analyzes results unifies them, interprets
them, and gives them meaning. The goal is to find a pat-
tern or sense of order in the observations and to under-
stand the reason for this order.
MODELS AND
THEORIES
After collecting a sufficient amount of consistently
reproducible results under a range of conditions or in dif-
ferent kinds of samples, scientist often seek to formulate
a theory or a model. A model is a hypothesis that is suffi-
ciently general and is continually effective in predicting

facts yet to be observed. A theory is an explanation of the
general principles of certain observations with extensive
experimental evidence or facts to support it.
Scientific models and theories, like hypotheses, should
be testable using available resources. Scientists make pre-
dictions based on their models and theories. A good the-
ory or model should be able to accurately predict an
event or behavior. Many scientists go a step beyond and
try to test their theories by designing experiments that
could prove them wrong. The theories that fail to make
accurate predictions are revised or discarded, and those
that survive the test of a series of experiments aimed to
prove them wrong become more convincing. Theories
and models therefore lead to new experiments; if they
don’t adequately predict behavior, they are revised
through development of new hypotheses and experi-
ments. The cycle of experiment-theory-experiment con-
tinues until a satisfactory understanding that is
consistent with observations and predictions is obtained.
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221

The Structure of Atoms
You and everything around you are composed of tiny particles called atoms. The book you are reading, the neu-
rons in your brain, and the air you are breathing can all be described as a collection of various atoms.
History of the Atom
The term atom, which means indivisible, was coined by Greek philosopher Democritus (460–370 B.C.). He dis-
agreed with Plato and Aristotle—who believed that matter could infinitely be divided into smaller and smaller
pieces—and postulated that matter is composed of tiny indivisible particles. In spite of Democritus, the belief that

matter could be infinitely divided lingered until the early 1800s, when John Dalton formulated a meaningful
atomic theory. It stated:
■
Matter is composed of atoms.
■
All atoms of a given element are identical.
■
Atoms of different elements are different and have different properties.
■
Atoms are neither created nor destroyed in a chemical reaction.
CHAPTER
Physical Science
PHYSICAL SCIENCE includes the disciplines of chemistry (the
study of matter) and physics (the study of energy and how energy
affects matter). The questions on the physical science section of the
GED will cover topics taught in high school chemistry and physics
courses. This chapter reviews the basic concepts of physical-
science—the structure of atoms, the structure and properties of mat-
ter, chemical reactions, motions and forces, conservation of energy,
increase in disorder, and interactions of energy and matter.
23
223
■
Compounds are formed when atoms of more
than one element combine.
■
A given compound always has the same relative
number and kind of atoms.
These postulates remain at the core of physical science
today, and we will explore them in more detail in the fol-

lowing sections.
Protons, Neutrons, and Electrons
An atom is the smallest unit of matter that has the prop-
erties of a chemical element. It consists of a nucleus sur-
rounded by electrons. The nucleus contains positively
charged particles called protons, and uncharged neu-
trons. Each neutron and each proton has a mass of about
1 atomic mass unit, abbreviated amu. An amu is equiv-
alent to about 1.66 × 10
−24
g. The number of protons in
an element is called the atomic number. Electrons are
negatively charged and orbit the nucleus in electron
shells.
Electrons in the outermost shell are called valence
electrons. Valence electrons are mostly responsible for
the properties and reaction patterns of an element. The
mass of an electron is more than 1,800 times smaller
than the mass of a proton or a neutron. When calculat-
ing atomic mass, the mass of electrons can safely be neg-
lected. In a neutral atom, the number of protons and
electrons is equal. The negatively charged electrons are
attracted to the positively charged nucleus. This attrac-
tive force holds an atom together. The nucleus is held
together by strong nuclear forces.
A representation of a lithium atom (Li). It has 3 protons (p)
and 4 neutrons (n) in the nucleus, and 3 electrons (e) in the
two electron shells. Its atomic number is 3 (p). Its atomic
mass is 7 amu (p + n). The atom has no net charge because
the number of positively charged protons equals the number

of negatively charged electrons.
Charges and Masses
of Atomic Particles
Proton Neutron Electron
Charge +1 0 –1
Mass 1 amu 1 amu
ᎏ
18
1
00
ᎏ
amu
Isotopes
The number of protons in an element is always the same.
In fact, the number of protons is what defines an ele-
ment. However, the number of neutrons in the atomic
nucleus, and thus the atomic weight, can vary. Atoms
that contain the same number of protons and electrons,
but a different number of neutrons, are called isotopes.
The atomic masses of elements in the periodic table are
weighted averages for different isotopes. This explains
why the atomic mass (the number of protons plus the
number of neutrons) is not a whole number. For exam-
ple, most carbon atoms have 6 protons and 6 neutrons,
giving it a mass of 12 amu. This isotope of carbon is
called “carbon twelve” (carbon-12). But the atomic mass
of carbon in the periodic table is listed as 12.011. The
mass is not simply 12, because other isotopes of carbon
have 5, 7, or 8 neutrons, and all the isotopes and their
abundance are considered when the average atomic mass

is reported.
Ions
An atom can lose or gain electrons and become charged.
An atom that has lost or gained one or more electrons is
called an ion. If an atom loses an electron, it becomes a
positively charged ion. If it gains an electron, it becomes
a negatively charged ion. For example, calcium (Ca), a
biologically important element, can lose two electrons to
become an ion with a positive charge of +2 (Ca
2+
). Chlo-
rine (Cl) can gain an electron to become an ion with a
negative charge of −1 (Cl
−
).
The Periodic Table
The periodic table is an organized list of all known ele-
ments, arranged in order of increasing atomic number,
such that elements with the same number of valence
electrons, and therefore similar chemical properties, are
found in the same column, or group. For example, the
last column in the periodic table lists the inert (noble)
gases, such as helium and neon—highly unreactive ele-
ments. A row in the periodic table is called a period.
3 p
4 n
e
e
e
Nucleus

Electron
shells
– PHYSICAL SCIENCE–
224
Elements that share the same period have the same num-
ber of electron shells.
Common Elements
Some elements are frequently encountered in biologi-
cally important molecules and everyday life. Below you
will find a list of common elements, their symbols, and
common uses.
H—Hydrogen: involved in the nuclear process that
produces energy in the sun
He—Helium: used to make balloons fly
C—Carbon: found in all living organisms; pure car-
bon exists as graphite and diamonds
N—Nitrogen: used as a coolant to rapidly freeze
food
O—Oxygen: essential for respiration (breathing)
and combustion (burning)
Si—Silicon: used in making transistors and solar
cells
Cl—Chlorine: used as a disinfectant in pools and as
a cleaning agent in bleach
Ca—Calcium: necessary for bone formation
Fe—Iron: used as a building material; carries oxygen
in the blood
Cu—Copper: a U.S. penny is made of copper; good
conductor of electricity
I—Iodine: lack in the diet results in an enlarged thy-

roid gland, or goiter
Hg—Mercury: used in thermometers; ingestion can
cause brain damage and poisoning
Pb—Lead: used for X-ray shielding in a dentist
office
Some elements exist in diatomic form (two atoms of
such an element are bonded), and are technically mole-
cules. These elements include hydrogen (H
2
), nitrogen
(N
2
), oxygen (O
2
), fluorine (F
2
), chlorine (Cl
2
), bromine
(Br
2
), and iodine (I
2
).

Structure and Properties
of Matter
Matter has weight and takes up space. The building
blocks of matter are atoms and molecules. Matter can
interact with other matter and with energy. These inter-

actions form the basis of chemical and physical
reactions.
Molecules
Molecules are composed of two or more atoms. Atoms
are held together in molecules by chemical bonds.
Chemical bonds can be ionic or covalent. Ionic bonds
form when one atom donates one or more electrons to
another. Covalent bonds form when electrons are shared
between atoms. The mass of a molecule can be calculated
by adding the masses of the atoms of which it is com-
posed. The number of atoms of a given element in a
molecule is designated in a chemical formula by a sub-
script after the symbol for that element. For example, the
glucose (blood sugar) molecule is represented as
C
6
H
12
O
6.
This formula tells you that the glucose mole-
cule is contains six carbon atoms (C), twelve hydrogen
atoms (H), and six oxygen atoms (O).
Organic and Inorganic Molecules
Molecules are often classified as organic or inorganic.
Organic molecules are those that contain both carbon
and hydrogen. Examples of organic compounds are
methane (natural gas, CH
4
), glycine (an amino acid,

NH
2
CH
2
COOH), and ethanol (an alcohol, C
2
H
5
OH).
Inorganic compounds include sodium chloride (table
salt, NaCl), carbon dioxide (CO
2
), and water (H
2
O).
States of Matter
Matter is held together by intermolecular forces—forces
between different molecules. Three common states of
matter are solid, liquid, and gas. Matter is an atom, a
molecule (compound), or a mixture. Examples of mat-
ter in solid form are diamonds (carbon atoms), ice
(water molecules), and metal alloys (mixtures of differ-
ent metals). A solid has a fixed shape and a fixed volume.
The molecules in a solid have a regular, ordered arrange-
ment and vibrate in place, but are unable to move far.
Examples of matter in liquid form are mercury (mer-
cury atoms), vinegar (molecules of acetic acid), and per-
fume (a mixture of liquids made of different molecules).
Liquids have a fixed volume, but take the shape of the
container they are in. Liquids flow, and their density

(mass per unit volume) is usually lower than the density
of solids. The molecules in a liquid are not ordered and
can move by sliding past one another through a process
called diffusion.
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225