excess acidity because it gives a measure of the differ-
ence between the acidity of a solution and that of an
ideal solution of the same concentration. In practice X=
–(Ho + lg[H+]) and m* = 1– Φ.
cracking The process in which large molecules are
broken down into smaller molecules. Used especially in
the petroleum refining industry.
critical mass The amount of fissionable material nec-
essary for a CHAIN REACTIONto sustain itself.
critical micelle concentration(cmc) There is a rel-
atively small range of concentrations separating the
limit below which virtually no MICELLEs are detected
and the limit above which virtually all additional sur-
factant molecules form micelles. Many properties of
surfactant solutions, if plotted against the concentra-
tion, appear to change at a different rate above and
below this range. By extrapolating the loci of such a
property above and below this range until they inter-
sect, a value may be obtained known as the critical
micellization concentration (critical micelle concentra-
tion), symbol cM, abbreviated as cmc (or c.m.c.). As
values obtained using different properties are not quite
identical, the method by which the cmc is determined
should be clearly stated.
See alsoINVERTED MICELLE.
critical point The combination of critical tempera-
ture and critical pressure. The temperature and pres-
sure at which two phases of a substance in equilibrium
become identical and form a single phase.
critical pressure The pressure required to liquefy a
gas at its critical temperature.
critical temperature The temperature above which a
gas cannot be liquefied, regardless of the amount of
pressure applied.
cross conjugation In a system XC 6 H 4 GY, cross con-
jugation involves the substituent X, the benzene ring,
and the side-chain connective-plus-reaction site GY,
i.e., either X is a +R group and GY is a –R group, or X
is a –R group and GY is a +R group. In Hammett cor-
relations this situation can lead to the need to apply
exalted substituent constants σ+or σ–, respectively, as
in electrophilic or nucleophilic aromatic substitution.
The term through resonanceis synonymous. Cross con-
jugation has also been used to describe the interactions
occurring in 2-phenylallyl and and similar systems.cross reactivity The ability of an IMMUNOGLOBULIN,
specific for one ANTIGEN, to react with a second anti-
gen. A measure of relatedness between two different
antigenic substances.crown A MOLECULAR ENTITY comprising a mono-
cyclic LIGAND assembly that contains three or more
BINDING SITEs held together by covalent bonds and
capable of binding a GUESTin a central (or nearly cen-
tral) position. The ADDUCTs formed are sometimes
known as “coronates.” The best known members of
this group are macrocyclic polyethers, such as the below
“18-crown-6,” containing several repeating units
–CR 2 –CR 2 O– (where R is most commonly H), and
known as crown ethers.
See alsoHOST.cryoscopy The depression of the freezing point for a
material in solution; the study or practice of determin-
ing the freezing point of liquids.cryptand A MOLECULAR ENTITYcomprising a cyclic
or polycyclic assembly of BINDING SITEs that containscryptand 63