economic english 7 pdf

Protons, Neutrons, and Electrons An atom is the smallest unit of matter that has the prop-erties of a chemical element.. The number of protons in an element is called the atomic number..

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

T HE Q UESTION

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

T HE H YPOTHESIS

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

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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

T HE E XPERIMENT

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 contain-ingredient), and one is not given anythcontain-ing

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 A NALYSIS

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

M ODELS AND T HEORIES

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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 T h e S t r u c t u r e o f A t o m s

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

C H A P T E R

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

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■ 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 × 10−24g 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

Mass 1 amu 1 amu 18 1 00amu

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 (Ca2+) 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

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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 (H2), nitrogen

(N2), oxygen (O2), fluorine (F2), chlorine (Cl2), bromine

(Br2), and iodine (I2)

 S t r u c t u r e a n d P r o p e r t i e s

o f M a t t e r

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

C6H12O6.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, CH4), glycine (an amino acid,

NH2CH2COOH), and ethanol (an alcohol, C2H5OH) Inorganic compounds include sodium chloride (table salt, NaCl), carbon dioxide (CO2), and water (H2O)

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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