Name: __________________________
Date: ___________________________
Flynt - ___Period
___th Grade Science
Introducing Colloids
In 1860, the Scottish chemist Thomas Graham discovered that certain
substances (e.g., glue, gelatin, or starch) could be separated from certain other
substances (e.g., sugar or salt) by dialysis (the movement of fluids through a
semi-permeable membrane). He gave the name colloid to substances that do
not diffuse through a semi-permeable membrane (like parchment, cellophane,
or cell walls) and the name crystalloid to those substances which do diffuse
and which are therefore a true solution. The Greek word for glue is kola,
leading Graham to choose the name “colloid” for these glue-like substances
(kola also appears as a root in other words like "protocol" and "collagen"). While
Graham followed the conventional practice for scientific nomenclature, it’s a
shame that such a cool phenomenon should receive such a dull name!
So what makes a colloid
different from a solution or a
suspension? Again, the
differences here are not nearly
as exciting as the substances
themselves. Remember that
colloids are mixtures of pure
substances. Colloidal particles
are larger than molecules in
a solution, but too small to be
observed directly with a
microscope; however, their
shape and size can be
determined by electron
microscopy.
In a true solution, the particles of dissolved substance are of
molecular size (tiny, tiny) and are thus smaller than colloidal particles; in fact,
in a solution, the particles of solute and solvent are approximately the same size
and in the same phase and all particles can diffuse across the semi-permeable
membrane.
In a coarse mixture
(e.g., a suspension) the
particles are much larger than
colloidal particles and settle
over time due to gravity.
Although there are no precise
boundaries of size between the
particles in mixtures, colloids, or
solutions, colloidal particles are
usually on the order of 10-7 to 105
cm in size.
Classification of Colloids
One way of classifying colloids is
to group them according to the
phase (solid, liquid, or gas) of the
dispersed substance and of the
medium of dispersion. A gas may
be dispersed in a liquid to form a
foam (e.g., shaving lather or
beaten egg white) or in a solid to
form a solid foam (e.g.,
Styrofoam or marshmallow). A
liquid may be dispersed in a gas
to form an aerosol (e.g., fog or
aerosol spray), in another liquid
to form an emulsion (e.g.,
homogenized milk or mayonnaise), or in a solid to form a gel (e.g., jellies or
cheese). A solid may be dispersed in a gas to form a solid aerosol (e.g., dust or
smoke in air), in a liquid to form a sol (e.g., ink or muddy water), or in a solid
to form a solid sol (e.g., certain alloys).
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 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.
Thixotropy
Thixotropy is a property
exhibited by certain gels, specifically
the semisolid, jelly-like colloids. A
thixotropic gel appears to be solid
and maintains a shape of its own
until it is subjected to a shearing
(lateral) force or some other
disturbance, such as the violent
shaking from an earthquake. It then
acts as a sol (a semifluid colloid –
basically a semi-liquid) and flows freely. To see “solid” ground
suddenly start flowing like a liquid can be quite shocking, especially
if the liquefaction occurs beneath buildings. Thixotropic behavior is
reversible, and when allowed to stand undisturbed, the sol slowly
reverts to a gel. Common thixotropic gels include oil well drilling
mud, ketchup, silly putty, and certain clays. Quick clay (or quick
sand), which is thixotropic, has caused landslides in permafrost
regions such as Scandinavia and Canada.
Summary: