(e.g. ionic contributions to the bond-
ing).CPMG sequence (Carr–Purcell–
Meiboom–Gill sequence)A sequence
of pulses used for spin-echo experi-
ments in nuclear magnetic resonance
(NMR) in which the initial pulse is
90 °followed by a series of 180°
pulses. A CPMG sequence is designed
so that the spin echoes die away ex-
ponentially with time. Spin–spin re-
laxation occurs characterized by a
time constant T 2 , which can be deter-
mined from the decay signal.crack cocaineSee cocaine.crackingThe process of breaking
down chemical compounds by heat.
The term is applied particularly to
the cracking of hydrocarbons in the
kerosine fraction obtained from *pe-
troleum reÜning to give smaller hy-
drocarbon molecules and alkenes. It
is an important process, both as a
source of branched-chain hydrocar-
bons suitable for gasoline (for motor
fuel) and as a source of ethene and
other alkenes. Catalytic cracking is a
similar process in which a catalyst is
used to lower the temperature re-
quired and to modify the products
obtained.
cream of tartar See potassium hy-
drogentartrate.
creosote 1.(wood creosote) An al-
most colourless liquid mixture of
phenols obtained by distilling tar ob-
tained by the destructive distillation
of wood. It is used medically as an
antiseptic and expectorant. 2.(coal-
tar creosote) A dark liquid mixture of
phenols and cresols obtained by dis-
tilling coal tar. It is used for preserv-
ing timber.
cresols See methylphenols.CRG process (catalytic rich gas
process) An industrial process for
producing fuel gas from naphthaand other hydrocarbon sources. It in-
volves a nickel-based catalyst, pres-
sures of up to 70 bar, and
temperatures between 250°C and
650 °C depending on the feedstock.
The reactions are:
CnH 2 n+2+ nH 2 O →nCO + (2n +1)H 2CO + 3H 2 →CH 4 + H 2 OCO + H 2 O →CO 2 + H 2
The result is a mixture of methane,
carbon monoxide, carbon dioxide,
and trace amounts of ethane and
other hydrocarbons. With partial car-
bon dioxide removal it is possible to
produce town gas with medium
caloriÜc value containing about 30%
CH 4 , 30% H 2 , and 2% CO. The process
can be used to produce SNG. In this
case there are multiple methanation
stages and complete removal of CO 2
to give a product containing about
98.5% CH 4 , 0.9% H 2 , and 0.1% CO.
cristobaliteA mineral form of *sil-
icon(IV) oxide, SiO 2.critical pressure The pressure of a
Ûuid in its *critical state; i.e. when it
is at its critical temperature and criti-
cal volume.critical stateThe state of aÛuid in
which the liquid and gas phases both
have the same density. TheÛuid is
then at its critical temperature, criti-
cal pressure, and critical volume.critical temperature 1.The tem-
perature above which a gas cannot
be liqueÜed by an increase of pres-
sure. See also critical state. 2.See
transition point.critical volumeThe volume of a
Üxed mass of aÛuid in its *critical
state; i.e. when it is at its critical tem-
perature and critical pressure. The
critical speciÜc volume is its volume
per unit mass in this state: in the
past this has often been called the
critical volume.CPMG sequence 148c