Colloids and Suspensions
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Printed: January 22, 2015
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C HAPTER
Chapter 1. Colloids and Suspensions
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Colloids and Suspensions
Lesson Objectives
• Describe the properties of a suspension.
• Describe the properties of a colloid and distinguish from a solution or a suspension.
Lesson Vocabulary
•
•
•
•
colloid
emulsion
suspension
Tyndall effect
Check Your Understanding
Recalling Prior Knowledge
• What are the identifying characteristics of a heterogeneous mixture?
• What prevents oil and water from being miscible?
Solutions are homogeneous mixtures, meaning that a solution is completely uniform throughout. There are two
other types of mixtures that are related to a solution, but which are heterogeneous. In this lesson, you will learn the
distinguishing characteristics of suspensions and colloids.
Suspensions
Take a glass of water and throw in a handful of sand or dirt. Stir it and stir it and stir it. Have you made a solution?
Sand and dirt do not dissolve in water, and though it may look homogeneous for a few moments, the sand or dirt
gradually sinks to the bottom of the glass ( Figure 1.1).
A suspension is a heterogeneous mixture in which some of the particles settle out of the mixture upon standing.
The particles in a suspension are far larger than those of a solution, so gravity is able to pull them down out of
the dispersion medium (water). The diameter for the dispersed particles in a suspension, such as the sand in the
suspension described above, is typically at least 1000 times greater than those in a solution. Unlike a solution,
the dispersed particles can be separated from the dispersion medium by filtering. Suspensions are still considered
heterogeneous because the different substances in the mixture will not remain uniformly dispersed if they are not
actively being mixed.
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FIGURE 1.1
Colloids
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. Table 1.1 summarizes the properties and distinctions between solutions, colloids, and
suspensions.
TABLE 1.1: Properties of Solutions, Colloids, and Suspensions
Solutions
Homogeneous
Particle size: 0.01-1 nm; atoms,
ions, or molecules
Colloids
Heterogeneous
Particle size: 1-1000 nm, dispersed;
large molecules or aggregates
Do not separate on standing
Cannot be separated by filtration
Do not scatter light
Do not separate on standing
Cannot be separated by filtration
Scatter light (Tyndall effect)
Suspensions
Heterogeneous
Particle size: over 1000 nm, suspended; large particles or aggregates
Particles settle out
Can be separated by filtration
May either scatter light or be
opaque
Colloids are unlike solutions because their dispersed particles are much larger than those of a solution. The dispersed
particles of a colloid cannot be separated by filtration, but they scatter light, a phenomenon called the Tyndall effect.
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 1.2). The Tyndall 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. Suspensions may scatter
light, but if the number of suspended particles is sufficiently large, the suspension may simply be opaque and the
2light scattering will not occur.
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Chapter 1. Colloids and Suspensions
Examples of Colloids
Table 1.2 lists examples of colloidal systems, most of which are very familiar. Some of these are shown in Figure
1.3. The dispersed phase describes the particles, while the dispersion medium is the material in which the particles
are distributed.
TABLE 1.2: Classes of Colloids
Class of Colloid
Sol and gel
Dispersed Phase
solid
Dispersion Medium
liquid
Solid aerosol
Solid emulsion
Liquid emulsion
Liquid aerosol
solid
liquid
liquid
liquid
gas
solid
liquid
gas
Foam
Foam
gas
gas
solid
liquid
Examples
paint, jellies, blood,
gelatin, mud
smoke, dust in air
cheese, butter
milk, mayonnaise
fog, mist, clouds, aerosol
spray
marshmallow
whipped cream, shaving
cream
FIGURE 1.3
Some common colloids:
(A) gelatin
dessert (gel); (B) smoke (solid aerosol);
(C) butter (solid emulsion); (D) mayonnaise (liquid emulsion); (E) fog (liquid
aerosol); (F) marshmallows (foam); (G)
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. An emulsion 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
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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.
Lesson Summary
• Suspensions and colloids are heterogeneous mixtures. A suspension is identifiable because its particles are
large and settle out of the dispersing medium due to the effects of gravity.
• The dispersed particles of a colloid are intermediate in size between those of a solution and a suspension.
Colloids are distinguishable from solutions because of the light scattering phenomenon called the Tyndall
effect. Gels, aerosols, foams, and emulsions are some classes of colloids.
Lesson Review Questions
Reviewing Concepts
1.
2.
3.
4.
5.
6.
How can you distinguish between a suspension and a solution?
How big are the particles in a colloid compared to those of a suspension and a solution?
What is the Tyndall effect? Why don’t solutions demonstrate the Tyndall effect?
Explain the difference between the dispersed phase and the dispersing medium of a colloid.
Explain the difference between an emulsion and an emulsifying agent.
If you add a large spoonful of salt to a glass of standing water, the salt sinks to the bottom. Is this a suspension?
Explain.
Problems
7. Identify each of the following descriptions or examples as being representative of a solution, suspension, or
colloid. More than one answer may apply.
a.
b.
c.
d.
e.
f.
g.
h.
i.
dispersed particles can be filtered out
heterogeneous
particles are not visible to the unaided eye
paint
lemonade with no pulp
particle size larger than 1 nm
milk
particles do not settle upon standing
fog
Further Reading / Supplemental Links
• Types of Mixtures –Solutions, Suspensions, Colloids, (http://www.edinformatics.com/math_science/mixtur
es.htm )
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Chapter 1. Colloids and Suspensions
Points to Consider
Solutions have a wide array of properties and uses. While some solutes are very soluble in certain solvents, some
other solutes are not.
• How is solubility measured?
• What factors affect solubility?
References
1. Eric Hodel (Flickr: drbrain). http://www.flickr.com/photos/drbrain/99856909/ . CC-BY-NC-SA 2.0
2. (A) CK-12 Foundation - Joy Sheng; (B) Dave Stokes. (B) http://www.flickr.com/photos/33909700@N02/31
59696620/ . (A) CC-BY-NC-SA 3.0; (B) CC-BY 2.0
3. (A) Image copyright Ildi Papp, 2012; (B) Courtesy of US Geological Survey; (C) Steve Karg (Wikipedia:
Skarg); (D) little blue hen; (E) Mike Behnken (Flickr: MikeBehnken); (F) John Morgan (Flickr: JohnMorgan); (G) Steven Depolo (Flickr: stevendepolo). (A) http://www.shutterstock.com/; (B) http://commons
.wikimedia.org/wiki/File:Pyroclastic_flows_at_Mayon_Volcano.jpg; (C) http://commons.wikimedia.org/wiki/F
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.com/photos/mikebehnken/5007703932/; (F) http://www.flickr.com/photos/aidanmorgan/2256639109/; (G) h
ttp://www.flickr.com/photos/stevendepolo/5159788513/ . (A) Used under license from Shutterstock; (B)
Public Domain; (C) CC-BY 2.5; (D)-(G) CC-BY 2.0
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