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Chemistry 15.3
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15.3 Heterogeneous Aqueous Systems
In 1996, American astronaut Shannon Lucid shared a gelatin dessert with her Russian crewmates. Gelatin is a heterogeneous mixture called a colloid. You will learn more about the characteristics of colloids and a related mixture called a suspension.
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Heterogeneous Aqueous Systems
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Suspensions
Suspensions
What is the difference between a suspension and a solution?
15.3
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Heterogeneous Aqueous Systems
> Suspensions
A suspension is a mixture from which particles settle out upon standing.
A suspension differs from a solution because the particles of a suspension are much larger and do not stay suspended indefinitely.
15.3
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Heterogeneous Aqueous Systems
> Suspensions
Suspensions are heterogenous because at least two substances can be clearly identified.
15.3
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Heterogeneous Aqueous Systems
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Colloids
Colloids
What distinguishes a colloid from a suspension and a solution?
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Heterogeneous Aqueous Systems
> Colloids
A colloid is a heterogeneous mixture containing particles that range in size from 1 nm to 1000 nm. The particles in a colloid are spread throughout the dispersion medium.
15.3
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Heterogeneous Aqueous Systems
> Colloids
Colloids have particles smaller than those in suspensions and larger than those in solutions.
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Heterogeneous Aqueous Systems
> Colloids15.3
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Heterogeneous Aqueous Systems
> Colloids
The Tyndall Effect
The scattering of visible light by colloidal particles is called the Tyndall effect.
15.3
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Heterogeneous Aqueous Systems
> Colloids
Particles in colloids and suspensions reflect or scatter light in all directions. Solutions do not scatter light.
15.3
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Heterogeneous Aqueous Systems
> Colloids
Brownian Motion
• The chaotic movement of colloidal particles, which was first observed by the Scottish botanist Robert Brown (1773–1858), is called Brownian motion.
• Brownian motion is caused by collisions of the molecules of the dispersion medium with the small, dispersed colloidal particles.
15.3
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Heterogeneous Aqueous Systems
> Colloids
Coagulation
• A colloidal system can be destroyed, or coagulated, by the addition of ions having a charge opposite to that of the colloidal particles.
• The added ions neutralize the charged colloidal particles. The particles can clump together to form heavier aggregates and precipitate from the dispersion.
15.3
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Heterogeneous Aqueous Systems
> Colloids
Emulsions
An emulsion is a colloidal dispersion of a liquid in a liquid. An emulsifying agent is essential for the formation of an emulsion and for maintaining the emulsion’s stability.
15.3
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Heterogeneous Aqueous Systems
> Colloids
Mayonnaise is heterogeneous mixture of oil and vinegar. Such a mixture would quickly separate without the presence of egg yolk, which is the emulsifying agent.
15.3
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Heterogeneous Aqueous Systems
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Colloids15.3
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Section Quiz
-or-Continue to: Launch:
Assess students’ understanding of the concepts in Section
15.3 Section Quiz.
15.3.
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1. One characteristic that is unique to a suspension is that the particles in it
a. settle due to gravity.
b. are too small to be seen.
c. cannot be filtered.
d. produce the Tyndall effect.
15.3 Section Quiz.
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2. Homogenized milk contains finely divided particles that do not settle out rapidly and are not readily filtered. Thus, homogenized milk is a
a. colloid.
b. substance.
c. suspension.
d. solution.
15.3 Section Quiz.
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3. The scattering of light known as the Tyndall effect can be seen in
a. emulsions and solutions.
b. colloids and suspensions.
c. solutions and suspensions.
d. colloids and solutions.
15.3 Section Quiz.
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Heterogeneous Aqueous Systems
> Concept Map 15
Concept Map 15 Solve the Concept Map with the help of an interactive guided tutorial.
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