Data Sheet

KEYWORD: COLLOID
The colloidal gold particles don’t
settle to the bottom of the vial the
way you would expect from larger
particles. Instead, they bump against
one another without accumulating.
The reason has to do with electrostatic
forces (forces of repulsion between
equal electrical charges) and
surfactants acting as “chemical
spacers.”
EXPERIMENT 32
The Tyndall Effect

Making the laser
beam visible
YOU WILL NEED
› Colloidal gold vial
› Laser pointer
› -mL measuring cup
› Clear apple juice
HERE’S HOW
››› Pour a little juice into the measuring cup
and set the colloidal gold vial next to it.
››› Shine the pointer’s laser beam through
both liquids.
››› In which liquid do you see the beam? In
which don’t you? What might be the
explanation?
WHAT’S HAPPENING
In the last experiment, you learned how light can be scattered
by small particles. So there must be a difference between the
two liquids. In one case (the colloidal gold) the laser’s light is
scattered, and in the other it’s not.
The reason has to do with the particles in the liquid and their
size. Apple juice is mostly water, plus substances from the
fruit: sugar, acids, and pigments. All these materials consist
of relatively small molecules. For example, a sugar
molecule, the structure of which you learned in Experiment
8, has a size of about 1 nm. Acid and pigment molecules
also occur in about this size. They won’t have any influence
on the laser beam because they are too small in relation to
the laser light’s wavelength (650 nm).
The colloidal gold contains particles 50 nanometers in
size. That makes them large enough to reflect the laser’s
light.
So a direct comparison shows how the fruit juice particles
are too small to influence the laser, while the nanocolloid
contains larger particles that scatter the light.