lutions in which the solute molecules
are large and have an afÜnity for the
solvent. Starch in water is an example of such a system. Association colloids are systems in which the
dispersed phase consists of clusters
of molecules that have lyophobic and
lyophilic parts. Soap in water is an association colloid (see micelle).
Emulsions are colloidal systems in
which the dispersed and continuous
phases are both liquids, e.g. oil-inwater or water-in-oil. Such systems
require an emulsifying agent to stabilize the dispersed particles.
Gels are colloids in which both dispersed and continuous phases have a
three-dimensional network throughout the material, so that it forms a
jelly-like mass. Gelatin is a common
example. One component may sometimes be removed (e.g. by heating) to
leave a rigid gel (e.g. silica gel).
Other types of colloid include
aerosols (dispersions of liquid or
solid particles in a gas, as in a mist
or smoke) and foams (dispersions of
gases in liquids or solids).
colophony See rosin
colorimetric analysis Quantitative analysis of solutions by estimating their colour, e.g. by comparing it
with the colours of standard solutions.
colour centre A defect in a crystal
that changes the way in which it absorbs light or other electromagnetic
radiation. Impurities in the crystal affect the bind structure and allow
transitions in different regions of the
spectrums. Impurity colour centres
are responsible for the characteristic
colours of many gemstones. A particular type of colour centre is an Fcentre. This is a missing negative ion
in an ionic crystal, where the overall
charge neutrality occurs by trapping
an electron in the vacancy. The electron has energy levels similar to
those of a particle in a box. F-centres
can be produced by chemical activity
or by irradiation.
colour index (CI) A list, regarded as
deÜnitive, of dyes and pigments,
which includes information on their
commercial names, method of application, colour fastness, etc.
colour photography Any of various methods of forming coloured
images on Ülm or paper by photographic means. One common process
is a subtractive reversal system that
utilizes a Ülm with three layers of
light-sensitive emulsion, one responding to each of the three primary colours. On development a
black image is formed where the
scene is blue. The white areas are
dyed yellow, the complementary colour of blue, and the blackened areas
are bleached clean. A yellow Ülter
between this emulsion layer and
the next keeps blue light from the
second emulsion, which is greensensitive. This is dyed magenta
where no green light has fallen. The
Ünal emulsion is red-sensitive and is
given a cyan (blue-green) image on
the negative after dying. When white
light shines through the three dye
layers the cyan dye subtracts red
where it does not occur in the scene,
the magenta subtracts green, and the
yellow subtracts blue. The light projected by the negative therefore reconstructs the original scene either
as a transparency or for use with
printing paper.
columbium A former name for the
element *niobium.
column chromatography See
chromatography.
combinatorial chemistry A technique in which very large numbers
of related compounds are formed in
135
combinatorial chemistry
c
www.AzShimi.ir www.AzShimi.com
are large and have an afÜnity for the
solvent. Starch in water is an example of such a system. Association colloids are systems in which the
dispersed phase consists of clusters
of molecules that have lyophobic and
lyophilic parts. Soap in water is an association colloid (see micelle).
Emulsions are colloidal systems in
which the dispersed and continuous
phases are both liquids, e.g. oil-inwater or water-in-oil. Such systems
require an emulsifying agent to stabilize the dispersed particles.
Gels are colloids in which both dispersed and continuous phases have a
three-dimensional network throughout the material, so that it forms a
jelly-like mass. Gelatin is a common
example. One component may sometimes be removed (e.g. by heating) to
leave a rigid gel (e.g. silica gel).
Other types of colloid include
aerosols (dispersions of liquid or
solid particles in a gas, as in a mist
or smoke) and foams (dispersions of
gases in liquids or solids).
colophony See rosin
colorimetric analysis Quantitative analysis of solutions by estimating their colour, e.g. by comparing it
with the colours of standard solutions.
colour centre A defect in a crystal
that changes the way in which it absorbs light or other electromagnetic
radiation. Impurities in the crystal affect the bind structure and allow
transitions in different regions of the
spectrums. Impurity colour centres
are responsible for the characteristic
colours of many gemstones. A particular type of colour centre is an Fcentre. This is a missing negative ion
in an ionic crystal, where the overall
charge neutrality occurs by trapping
an electron in the vacancy. The electron has energy levels similar to
those of a particle in a box. F-centres
can be produced by chemical activity
or by irradiation.
colour index (CI) A list, regarded as
deÜnitive, of dyes and pigments,
which includes information on their
commercial names, method of application, colour fastness, etc.
colour photography Any of various methods of forming coloured
images on Ülm or paper by photographic means. One common process
is a subtractive reversal system that
utilizes a Ülm with three layers of
light-sensitive emulsion, one responding to each of the three primary colours. On development a
black image is formed where the
scene is blue. The white areas are
dyed yellow, the complementary colour of blue, and the blackened areas
are bleached clean. A yellow Ülter
between this emulsion layer and
the next keeps blue light from the
second emulsion, which is greensensitive. This is dyed magenta
where no green light has fallen. The
Ünal emulsion is red-sensitive and is
given a cyan (blue-green) image on
the negative after dying. When white
light shines through the three dye
layers the cyan dye subtracts red
where it does not occur in the scene,
the magenta subtracts green, and the
yellow subtracts blue. The light projected by the negative therefore reconstructs the original scene either
as a transparency or for use with
printing paper.
columbium A former name for the
element *niobium.
column chromatography See
chromatography.
combinatorial chemistry A technique in which very large numbers
of related compounds are formed in
135
combinatorial chemistry
c
www.AzShimi.ir www.AzShimi.com