pesticideAny chemical compound
used to kill pests that destroy agricul-
tural production or are in some way
harmful to humans. Pesticides in-
clude herbicides(such as *2,4-D and
*Paraquat), which kill unwanted
plants or weeds; insecticides(such as
pyrethrum), which kill insect pests;
fungicides, which kill fungi; and ro-
denticides(such as *warfarin), which
kill rodents. The problems associated
with pesticides are that they are very
often nonspeciÜc and may there-
fore be toxic to organisms that are
not pests; they may also be non-
biodegradable, so that they persist in
the environment and may accumu-
late in living organisms (see bioaccu-
mulation). Organophosphorus
insecticides, such as malathion and
parathion, are biodegradable but can
also damage the respiratory and ner-
vous systems in humans as well as
killing useful insects, such as bees.
They act by inhibiting the action of
the enzyme cholinesterase. Organo-
chlorine insecticides, such as dield-
rin, aldrin, and *DDT, are very
persistent and not easily biodegrad-
able.peta-Symbol P. A preÜx used in the
metric system to denote one thou-
sand million million times. For exam-
ple, 10^15 metres = 1 petametre (Pm).petrochemicals Organic chemicals
obtained from petroleum or natural
gas.petrolatum See petroleum jelly.
petroleum A naturally occurring
oil that consists chieÛy of hydrocar-
bons with some other elements, such
as sulphur, oxygen, and nitrogen. In
its unreÜned form petroleum is
known as crude oil (sometimes rock
oil). Petroleum is believed to have
been formed from the remains of liv-
ing organisms that were deposited,
together with rock particles and bio-chemical and chemical precipitates,
in shallow depressions, chieÛy in ma-
rine conditions. Under burial and
compaction the organic matter went
through a series of processes before
being transformed into petroleum,
which migrated from the source rock
to become trapped in large under-
ground reservoirs beneath a layer of
impermeable rock. The petroleum
oftenÛoats above a layer of water
and is held under pressure beneath a
layer of *natural gas.
Petroleum reservoirs are discov-
ered through geological exploration:
commercially important oil reserves
are detected by exploratory narrow-
bore drilling. The major known re-
serves of petroleum are in Saudi
Arabia, Russia, China, Kuwait, Iran,
Iraq, Mexico, USA, United Arab Emi-
rates, Libya, and Venezuela. The oil is
actually obtained by the sinking of
an oil well. Before it can be used it is
separated by fractional distillation in
oil reÜneries. The main fractions ob-
tained are:
(1) ReÜnery gas A mixture of
methane, ethane, butane, and
propane used as a fuel and for mak-
ing other organic chemicals.
(2) GasolineA mixture of hydrocar-
bons containing 5 to 8 carbon atoms,
boiling in the range 40–180°C. It is
used for motor fuels and for making
other chemicals.
(3) Kerosine (or parafÜn oil) A mix-
ture of hydrocarbons having 11 or 12
carbon atoms, boiling in the range
160–250°C. Kerosine is a fuel for jet
aircraft and for oil-Üred domestic
heating. It is also cracked to produce
smaller hydrocarbons for use in
motor fuels.
(4) Diesel oil (or gas oil) A mixture
of hydrocarbons having 13 to 25
carbon atoms, boiling in the range
220–350°C. It is a fuel for diesel en-
gines.
The residue is a mixture of higherpesticide 406p