Science, Matter, and Energy - Environmental Science - page 27

Persistence is a measure of how long the pollu-
tant stays in the air, water, soil, or body. Pollutants
can be classified into four categories based on their
persistence:
■ Degradable, or nonpersistent, pollutants are bro-
ken down completely or reduced to acceptable levels
by natural physical, chemical, and biological processes.
■ Biodegradable pollutants are complex chemical
pollutants that living organisms (usually specialized
bacteria) break down into simpler chemicals. Human
sewage in a river, for example, is biodegraded fairly
quickly by bacteria if the sewage is not added faster
than it can be broken down.
■ Slowly degradable, or persistent, pollutants take
decades or longer to degrade. Examples include the
insecticide DDT and most plastics.
■ Nondegradable pollutants are chemicals that nat-
ural processes cannot break down. Examples include
the toxic elements lead, mercury, and arsenic. Ideally,
we should try not to use these chemicals. If we do, we
should figure out ways to keep them from getting into
the environment.
We can make the environment cleaner and convert
some potentially harmful chemicals into less harmful
physical or chemical forms. Nevertheless, the law of
conservation of matter means we will always face the
problem of what to do with some quantity of wastes
and pollutants.

mate how long a sample of a radioisotope must be
stored before it decays to a safe level. A rule of thumb is
that such decay takes about 10 half-lives. Thus people
must be protected from radioactive waste containing
iodine-131 (which concentrates in the thyroid gland
and has a half-life of 8 days) for 80 days (10 8 days).

Plutonium-239, which is produced in nuclear reactors
and used as the explosive in some nuclear weapons,
can cause lung cancer when its particles are inhaled in
even minute amounts. Its half-life is 24,000 years. Thus
it must be stored safely for 240,000 years (10 24,000

years)—about four times longer than our species

( Homo sapiens sapiens ) has existed.
Exposure to alpha particles, beta particles, or
gamma rays can alter DNA molecules in cells and in
some cases lead to genetic defects in the next genera-
tion of offspring or several generations later. Such ex-
posure can also damage body tissues and cause burns,
miscarriages, eye cataracts, and certain cancers.

Learn more about half-lives and how radioactive particles
can be used by doctors to help us at Environmental
ScienceNow.

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 more neutrons plus energy (Figure 2-6, p. 28).
Each of these neutrons, in turn, can trigger an addi-
tional fission reaction. For multiple fissions to take
place, enough fissionable nuclei must be present to
provide the critical mass needed for efficient capture
of these neutrons.
Multiple fissions within a critical mass produce a
chain reaction, which releases an enormous amount of
energy (see Figure 2-6). This is somewhat like a room
in which the floor is covered with spring-loaded
mousetraps, each topped by a Ping-Pong ball. Open
the door, throw in a single Ping-Pong ball, and watch
the action in this simulated chain reaction of snapping
mousetraps and balls flying around in every direction.
In an atomic bomb, an enormous amount of en-
ergy is released in a fraction of a second in an uncon-
trolled nuclear fission chain reaction. In the reactor of a
nuclear power plant, the rate at which the nuclear fis-
sion chain reaction takes place is controlled. The heat
released produces high-pressure steam to spin tur-
bines, thereby generating electricity.
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. A tremendous
amount of energy is released in this process. In fact,
fusion of hydrogen nuclei to form helium nuclei is the
source of energy in the sun and other stars.

Nuclear Changes: Radioactive Decay, Fission,
and Fusion
Nuclei of some atoms can spontaneously lose particles
or give off high-energy radiation, split apart, or fuse
together.
In addition to physical and chemical changes, matter
can undergo a third type of change known as a nuclear
change. There are three types of nuclear change: natural
radioactive decay, nuclear fission, and nuclear fusion.
Natural radioactive decay is a nuclear change
in which unstable isotopes spontaneously emit fast-
moving chunks of matter (alpha particles or beta parti-
cles), high-energy radiation (gamma rays), or both at a
fixed rate. The unstable isotopes are called radioactive
isotopes or radioisotopes. Radioactive decay of these
isotopes into other isotopes continues until it produces
a stable isotope that is not radioactive.
Each type of radioisotope spontaneously decays at
a characteristic rate into a different isotope. This rate of
decay can be expressed in terms of half-life: the time
needed for one-half of the nuclei in a given quantity of a
radioisotope to decay and emit their radiation to form
a different isotope.
An isotope's half-life cannot be changed by tem-
perature, pressure, chemical reactions, or any other
known factor. As a consequence, it can be used to esti-