318 DIY Science: Illustrated Guide to Home Chemistry Experiments
Phase of colloidContinuous phaseDispersed phaseColloid typesolid solid solid solid solsolid solid liquid solid emulsionsolid solid gas solid foamliquid liquid solid solliquid liquid liquid emulsionliquid liquid gas foamgas gas solid solid aerosolgas gas liquid aerosolgas gas gas n/aTABLE 18-1:
Types of colloids
What differentiates a colloid from a solution or a suspension is
the size of the dispersed particles. In a solution, the dispersed
particles are individual molecules, if the solute is molecular, or
ions, if the solute is ionic. Particles in solution are no larger than
one nanometer (nm), and usually much smaller. In a colloid, the
dispersed particles are much larger, with at least one dimension
on the close order of 1 nm to 200 nm (= 0.2 micrometer, μm). In
some colloids, the dispersed particles are individual molecules
of extremely large size, such as some proteins, or tightly bound
aggregates of smaller molecules. In a suspension, the dispersed
particles are larger than 100 nm.
These differing particle sizes affect the physical characteristics of
solutions, colloids, and suspensions, as follows:
- Solutions and (usually) colloids, do not separate under the
influence of gravity; suspensions eventually settle out. In a
colloid, the interactions among the tiny particles of the dispersed
phase with each other and/or with the continuous phase are
sufficient to overcome the force exerted by gravity on the tiny
particles of the dispersed phase. In a suspension, the force of
gravity on the more massive particles of the dispersed phase
is sufficient to cause them to settle out eventually, although it
may take a long time for that to occur. (If the particles of the
dispersed phase are less dense than those of the continuous
phase, as in a mixture of oil dispersed in water, for example, the
dispersed phase “settles” out on top of the continuous phase,
but the concept is the same.) - Solutions do not separate when centrifuged; neither do colloids,
except those that contain the largest (and most massive)
dispersed particles, which may sometimes be separated in
an ultracentrifuge.- The particles in solutions and colloids cannot be separated with
filter paper, but suspensions can be separated by filtering. - Solutions pass unchanged through semipermeable
membranes—which are, in effect, filters with extremely tiny
pores—while suspensions and all colloids except those with
the very smallest particle sizes can be separated by membrane
filtration. - Flocculants are chemicals that encourage particulate
aggregation by physical means. Adding a flocculant to a solution
has no effect on the dispersed particles (unless the flocculent
reacts chemically with the solute) but adding a flocculant to
a colloid or suspension causes precipitation by encouraging
the dispersed particles to aggregate into larger groups and
precipitate out. - The particles in a solution affect the colligative properties of the
solution, and the particles in a colloid or suspension have no
effect on colligative properties. - Solutions do not exhibit the Tyndall Effect, while colloids and
suspensions do. The Tyndall Effect describes the scattering
effect of dispersed particles on a beam of light. Particles in
solution are too small relative to the wavelength of the light to
cause scattering, but the particles in colloids and suspensions
are large enough to cause the light beam to scatter, making it
visible as it passes through the colloid or suspension.
- The particles in solutions and colloids cannot be separated with
Figure 18-1 shows the Tyndall Effect in a beaker of water to which
a few drops of milk had been added. I used a green laser pointer
for this image, because the much dimmer red laser pointer that
I used when I actually did the lab session proved impossible to