mixed copper–iron sulphide, CuFeS 2 ,
crystallizing in the tetragonal sys-
tem; the principal ore of copper. It is
similar in appearance to pyrite and
gold. It crystallizes in igneous rocks
and hydrothermal veins associated
with the upper parts of acid igneous
intrusions. Chalcopyrite is the most
widespread of the copper ores, occur-
ring, for example, in Cornwall (UK),
Sudbury (Canada), Chile, Tasmania
(Australia), and Rio Tinto (Spain).
chalk A veryÜne-grained white
rock composed of the skeletal re-
mains of microscopic sea creatures,
such as plankton, and consisting
largely of *calcium carbonate
(CaCO 3 ). It is used in toothpaste and
cosmetics. It should not be confused
with blackboard ‘chalk’, which is
made from calcium sulphate.
change of phase (change of state)
A change of matter in one physical
*phase (solid, liquid, or gas) into an-
other. The change is invariably ac-
companied by the evolution or
absorption of energy.
chaosUnpredictable and appar-
ently random behaviour arising in a
system that is governed by determin-
istic laws (i.e. laws that, given the
state of the system at a given time,
uniquely determine the state at any
other time). Chaotic dynamics are
widespread in nature: examples are
the turbulentÛow ofÛuids, the dy-
namics of planetary moons, oscilla-
tions in electrical circuits, and the
long-term unpredictability of the
weather. In such situations, chaos
arises because the mathematical
equations expressing the (determinis-
tic) laws in question are nonlinear
and extremely sensitive to the initial
conditions. It is impossible to specify
data at a given time to a sufÜcient de-
gree of precision to be able to predict
the future behaviour of the system
because two pieces of initial data dif-
fering by even a very small amount
will give widely different results at a
later time. Originally, the theory was
introduced to describe unpredictabil-
ity in meteorology, as exempliÜed by
the butterÛy effect. It has been sug-
gested that the dynamical equations
governing the weather are so sensi-
tive to the initial data that whether
or not a butterÛyÛaps its wings in
one part of the world may make the
difference between a tornado occur-
ring or not occurring in some other
part of the world. In chemistry,
chaos theory has been used to ex-
plain oscillatory (clock) reactions,
such as the *B–Z reaction, and
chaotic reactions.chaotic dynamicsSee chaos.chaotic reaction A type of chemi-
cal reaction in which the concentra-
tions of reactants show chaotic
behaviour. This may occur when the
reaction involves a large number of
complex interlinked steps. Under
such conditions, it is possible for the
reaction to display unpredictable
changes with time. See also bistabil-
ity; oscillating reaction.charcoal A porous form of carbon
produced by the destructive distilla-
tion of organic material. Charcoal
from wood is used as a fuel. All forms
of charcoal are porous and are used
for adsorbing gases and purifying
and clarifying liquids. There are sev-
eral types depending on the source.
Charcoal from coconut shells is a par-
ticularly good gas adsorbent. Animal
charcoal (or bone black) is made by
heating bones and dissolving out the
calcium phosphates and other min-
eral salts with acid. It is used in sugar
reÜning. Activated charcoal is char-
coal that has been activated for ad-
sorption by steaming or by heating
in a vacuum.113 charcoal
c