40 CHAPTER 2 Science, Matter, Energy, and Systems
In a chemical change, or chemical reaction,
there is a change in the arrangement of atoms or ions
within molecules of the substances involved. Chemists
use chemical equations to represent what happens in a
chemical reaction. For example, when coal burns com-
pletely, the solid carbon (C) in the coal combines with
oxygen gas (O 2 ) from the atmosphere to form the gas-
eous compound carbon dioxide (CO 2 ).+ +EnergyBlack solid Colorless gas Colorless gasC
OO C OOReactant(s) Product(s)EnergyEnergyCarbon + Oxygen Carbon dioxideC+O 2 CO 2 ++In addition to physical and chemical changes, mat-
ter can undergo three types of nuclear changes, or
changes in the nuclei of its atoms (Figure 2-7). In the
first type, called natural radioactive decay, isotopes
spontaneously emit fast-moving subatomic particles,
high-energy radiation such as gamma rays, or both
(Figure 2-7, top). The unstable isotopes are called ra-
dioactive isotopes or radioisotopes.
Nuclear fission is a nuclear change in which the
nuclei of certain isotopes with large mass numbers
(such as uranium-235) are split apart into lighter nuclei
when struck by neutrons; each fission releases two or
three neutrons plus energy (Figure 2-7, middle). Each
of these neutrons, in turn, can trigger an additional fis-
sion reaction. Multiple fissions within a certain amount
of mass produce a chain reaction, which releases an
enormous amount of energy.Nuclear fusion is a nuclear change in which two
isotopes of light elements, such as hydrogen, are forced
together at extremely high temperatures until they fuse
to form a heavier nucleus (Figure 2-7, bottom). A tre-
mendous amount of energy is released in this process.
Fusion of hydrogen nuclei to form helium nuclei is the
source of energy in the sun and other stars.We Cannot Create or
Destroy Matter
We can change elements and compounds from one
physical, chemical, or nuclear form to another, but we
can never create or destroy any of the atoms involved
in any physical or chemical change. All we can do is
rearrange the atoms, ions, or molecules into different
spatial patterns (physical changes) or combinations
(chemical changes). These statements, based on many
thousands of measurements, describe a scientific law
known as the law of conservation of matter: when
a physical or chemical change occurs, no atoms are cre-
ated or destroyed (Concept 2-3).
This law means there is no “away” as in “to throw
away.” Everything we think we have thrown away remains
here with us in some form. We can reuse or recycle some
materials and chemicals, but the law of conservation of
matter means we will always face the problem of what
to do with some quantity of the wastes and pollutants
we produce.
We talk about consuming matter as if matter is
being used up or destroyed, but the law of conserva-
tion of matter says that this is impossible. What is
meant by matter consumption, is not destruction of mat-
ter, but rather conversion of matter from one form to
another.2-4 What Is Energy and How Can It Be Changed?
CONCEPT 2-4A When energy is converted from one form to another in a
physical or chemical change, no energy is created or destroyed (first law of
thermodynamics).
CONCEPT 2-4B Whenever energy is changed from one form to another, we end
up with lower-quality or less usable energy than we started with (second law of
thermodynamics).▲▲
Energy Comes in Many Forms
Energy is the capacity to do work or transfer heat. Work
is done when something is moved. The amount of work
done is the product of the force applied to an object to
move it a certain distance (work force distance).For example, it takes a certain amount of muscular
force to lift this book to a certain height.
There are two major types of energy: moving energy
(called kinetic energy) and stored energy (called potential
energy). Moving matter has kinetic energy because
it has mass and velocity. Examples are wind (a mov-