source: The Columbia Encyclopedia: Sixth Edition. 2000.
http://www.bartelby.com/65/co/colloid.html
Properties of Colloids
One property of colloid systems that distinguishes them from
true solutions is that colloidal particles scatter light. If a beam
of light, such as that from a flashlight, passes through a colloid,
the light is reflected (scattered) by the colloidal particles and
the path of the light can therefore be observed. When a beam of
light passes through a true solution (e.g., salt in water) there is
so little scattering of the light that the path of the light cannot
be seen and the small amount of scattered light cannot be detected
except by very sensitive instruments. The scattering of light by
colloids, known as the Tyndall effect, was first explained by the
British physicist John Tyndall. When an ultramicroscope (see
microscope) is used to examine a colloid, the colloidal particles
appear as tiny points of light in constant motion; this motion,
called Brownian movement, helps keep the particles in suspension.
Absorption is another characteristic of colloids, since the finely
divided colloidal particles have a large surface area exposed. The
presence of colloidal particles has little effect on the colligative
properties (boiling point, freezing point, etc.) of a solution.
The particles of a colloid selectively absorb ions and acquire
an electric charge. All of the particles of a given colloid take on
the same charge (either positive or negative) and thus are repelled
by one another. If an electric
potential is applied to a colloid, the charged colloidal particles
move toward the oppositely charged electrode; this migration is
called electrophoresis. If the charge on the particles is
neutralized, they may precipitate out of the suspension. A colloid
may be precipitated by adding another colloid with oppositely
charged particles; the particles are attracted to one another,
coagulate, and precipitate out. Addition of soluble ions may
precipitate a colloid; the ions in seawater precipitate the
colloidal silt dispersed in river water, forming a delta. A method
developed by F. G. Cottrell reduces air pollution by removing
colloidal particles (e.g., smoke, dust, and fly ash) from exhaust
gases with electric precipitators. Particles in a lyophobic system
are readily coagulated and precipitated, and the system cannot
easily be restored to its colloidal state. A lyophilic colloid does
not readily precipitate and can usually be restored by the addition
of solvent.
---
Regards,
George Martin
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