Download Examples of Colloids - Teachers TryScience

ENGINEERING ACTIVITY
MIXING IT UP!
Introduction
A colloid, in chemistry, is a substance in which one substance of microscopically dispersed
insoluble particles is suspended throughout another substance. Sometimes the dispersed
substance alone is called the colloid;[1] the term colloidal suspension refers unambiguously to
the overall mixture (although a narrower sense of the word suspension is contradistinguished from
colloids by larger particle size). Unlike a solution, whose solute and solvent constitute only
one phase, a colloid has a dispersed phase (the suspended particles) and a continuous phase
(the medium of suspension). To qualify as a colloid, the mixture must be one that does
not settle or would take a very long time to settle appreciably.
A colloid is a heterogeneous mixture in which the dispersed particles are intermediate in size
between those of a solution and a suspension. The particles are spread evenly throughout the
dispersion medium, which can be a solid, liquid, or gas. Because the dispersed particles of a
colloid are not as large as those of a suspension, they do not settle out upon standing. The
table below (Table below) summarizes the properties and distinctions between solutions,
colloids, and suspensions.
Properties of Solutions, Colloids, and Suspensions
Solutions
Colloids
Suspensions
Homogeneous
Heterogeneous
Heterogeneous
Particle size: 0.01-1
nm; atoms, ions, or
molecules
Particle size: 1-1000
Particle size: over 1000 nm, suspended;
nm, dispersed; large
large particles or aggregates
molecules or aggregates
Do not separate on
standing
Do not separate on
standing
Cannot be separated
by filtration
Cannot be separated by
Can be separated by filtration
filtration
Do not scatter light
Scatter light (Tyndall
effect)
Particles settle out
May either scatter light or be opaque
Tyndall Effect
Colloids are often confused with true homogeneous solutions because the individual dispersed
particles of a colloid cannot be seen. When light is passed through a true solution, the dissolved
particles are too small to deflect the light. However, the dispersed particles of a colloid, being
larger, do deflect light (Figure below). TheTyndall effect is the scattering of visible light by
colloidal particles. You have undoubtedly “seen” a light beam as it passes through fog, smoke,
or a scattering of dust particles suspended in air. All three are examples of colloids.
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Suspensions may scatter light, but if the number of suspended particles is sufficiently large, the
suspension may simply be opaque, and the light scattering will not occur.
(A) Light passes first through a colorless solution and is not scattered. When it passes through
a diluted milk solution, the light is scattered by colloidal particles, an observation called the
Tyndall effect. (B) The Tyndall effect allows sunlight to be seen as it passes through a fine mist.
Examples of Colloids
Listed below (Table below) are examples of colloidal systems, most of which are very familiar.
Some of these are shown below (Figure below). The dispersed phase describes the particles,
while the dispersion medium is the material in which the particles are distributed.
Classes of Colloids
Class of Dispersed
Colloid Phase
Dispersion
Medium
Examples
Sol and
gel
solid
liquid
paint, jellies, blood, gelatin, mud
Solid
aerosol
solid
gas
smoke, dust in air
Solid
liquid
emulsion
solid
cheese, butter
Liquid
liquid
emulsion
liquid
milk, mayonnaise
Liquid
aerosol
liquid
gas
fog, mist, clouds, aerosol spray
Foam
gas
solid
marshmallow
Foam
gas
liquid
whipped cream, shaving cream
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Some common colloids: gelatin dessert (gel); smoke (solid aerosol); butter (solid emulsion);
mayonnaise (liquid emulsion); fog (liquid aerosol); marshmallows (foam); whipped cream
(foam).
Another property of a colloidal system is observed when the colloids are studied under a light
microscope. The colloids scintillate, reflecting brief flashes of light because the colloidal
particles move in a rapid and random fashion. This phenomenon, called Brownian motion, is
caused by collisions between the small colloidal particles and the molecules of the dispersion
medium
Emulsions
Butter and mayonnaise are examples of a class of colloids called emulsions. Anemulsion is a
colloidal dispersion of a liquid in either a liquid or a solid. A stable emulsion requires an
emulsifying agent to be present. Mayonnaise is made in part of oil and vinegar. Since oil is
nonpolar, and vinegar is a polar aqueous solution, the two do not mix and would quickly
separate into layers. However, the addition of egg yolk causes the mixture to become stable
and not separate. Egg yolk is capable of interacting with both the polar vinegar and the
nonpolar oil. The egg yolk is called the emulsifying agent. Soap acts as an emulsifying agent
between grease and water. Grease cannot be simply rinsed off your hands or another surface
because it is insoluble in water. However, the soap stabilizes a grease-water mixture because
one end of a soap molecule is polar, and the other end is nonpolar. This allows the grease to be
removed from your hands or your clothing by washing with soapy water.
Colloidal Silver
Colloidal silver is a mineral. Despite promoters’ claims, silver has no known function in the body
and is not an essential mineral supplement. Colloidal silver products were once available as
over-the-counter drug products, but in 1999, the U.S. Food and Drug Administration (FDA) ruled
that these colloidal silver products were not considered safe or effective. Colloidal silver
products marketed for medical purposes or promoted for unproven uses are now considered
“misbranded” under the law without appropriate FDA approval as a new drug. There are
currently no FDA-approved over-the-counter or prescription drugs containing silver that are taken
by mouth. However, there are still colloidal silver products being sold as homeopathic remedies
and dietary supplement.
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