Transcript 15.3 Heterogeneous Aqueous Systems

15.3 Heterogeneous Aqueous Systems >
Chapter 15
Water and Aqueous Systems
15.1 Water and Its Properties
15.2 Homogeneous Aqueous
Systems
15.3 Heterogeneous Aqueous
Systems
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15.3 Heterogeneous Aqueous Systems >
CHEMISTRY
& YOU
Why are some sunsets red?
The atmosphere
contains particles of
water and dust. As
sunlight passes
through the
particles, it is
scattered. However,
not all wavelengths
are scattered to the
same extent.
2
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15.3 Heterogeneous Aqueous Systems > Suspensions
Suspensions
What is the difference between a
suspension and a solution?
3
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15.3 Heterogeneous Aqueous Systems > Suspensions
Suspensions
What is the difference between a
suspension and a solution?
• A suspension is a mixture from which
particles settle out upon standing.
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15.3 Heterogeneous Aqueous Systems > Suspensions
A suspension differs from a solution
because the particles of a suspension
are much larger and do not stay
suspended indefinitely.
5
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15.3 Heterogeneous Aqueous Systems > Suspensions
A suspension differs from a solution
because the particles of a suspension
are much larger and do not stay
suspended indefinitely.
• The particles in a typical suspension have
an average diameter greater than 1000 nm.
• By contrast, the particle size in a solution is
usually about 1 nm.
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15.3 Heterogeneous Aqueous Systems > Suspensions
• A solution is a homogeneous mixture.
• Suspensions are heterogeneous
because at least two substances can be
clearly identified.
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15.3 Heterogeneous Aqueous Systems > Suspensions
The difference between a solution and suspension
is easily seen when the type of mixture is filtered.
The small size of the
solute particles in a
solution allows them to
pass through filter paper.
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The particles of a
suspension can be
removed by filtration.
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15.3 Heterogeneous Aqueous Systems >
Explain why a mixture of sand and
water can be separated by filtration,
but a mixture of salt and water cannot.
9
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15.3 Heterogeneous Aqueous Systems >
Explain why a mixture of sand and
water can be separated by filtration,
but a mixture of salt and water cannot.
A mixture of sand and water is a suspension,
and a mixture of salt and water is a solution.
The particles in the sand mixture are much
larger than the ions in the salt mixture. The
sand particles are too large to pass through
filter paper; the ions are not.
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15.3 Heterogeneous Aqueous Systems > Colloids
Colloids
What distinguishes a colloid from
a suspension and a solution?
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15.3 Heterogeneous Aqueous Systems > Colloids
A colloid is a heterogeneous mixture
containing particles that range in size
from 1 nm to 1000 nm.
• The particles are spread, or dispersed,
throughout the dispersion medium, which can
be a solid, liquid, or gas.
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15.3 Heterogeneous Aqueous Systems > Colloids
The first substances to be identified as
colloids were glues.
Some Colloidal Systems
System
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Dispersed
phase
Dispersion
medium
Type
Example
Gas
Liquid
Foam
Whipped cream
Gas
Solid
Foam
Marshmallow
Liquid
Liquid
Emulsion
Milk, mayonnaise
Liquid
Gas
Aerosol
Fog, aerosol
Solid
Gas
Smoke
Dust in air
Solid
Liquid
Sols, gels
Egg white, jelly, paint, blood,
starch in water, gelatin
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15.3 Heterogeneous Aqueous Systems > Colloids
Colloids have particles smaller
than those in suspensions and
larger than those in solutions.
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15.3 Heterogeneous Aqueous Systems > Colloids
Colloids have particles smaller
than those in suspensions and
larger than those in solutions.
• These intermediate-sized particles cannot
be retained by filter paper as are the
larger particles of a suspension.
• They do not settle out with time.
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15.3 Heterogeneous Aqueous Systems > Colloids
The Tyndall Effect
You cannot see a beam of sunlight
unless the light passes through particles
of water (mist) or dust in the air.
• These particles scatter the sunlight.
• Similarly, a beam of light is visible as it
passes through a colloid.
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15.3 Heterogeneous Aqueous Systems > Colloids
The Tyndall Effect
The scattering of visible light by colloidal
particles is called the Tyndall effect.
Flashlight
Solution
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Colloid
Suspension
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15.3 Heterogeneous Aqueous Systems > Colloids
• Suspensions also exhibit the Tyndall
effect.
• The particles in solutions are too small
to scatter light.
Flashlight
Solution
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Colloid
Suspension
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15.3 Heterogeneous Aqueous Systems >
CHEMISTRY
& YOU
What would be the ideal conditions to
see a red sunset?
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15.3 Heterogeneous Aqueous Systems >
CHEMISTRY
& YOU
What would be the ideal conditions to
see a red sunset?
A misty or foggy evening would be ideal
for seeing a red sunset. There would be
a large number of particles to scatter the
sunlight.
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15.3 Heterogeneous Aqueous Systems > Colloids
Brownian Motion
Flashes of light, or scintillations, are
seen when colloids are studied under a
microscope.
• Colloids scintillate because the particles
reflecting and scattering the light move
erratically.
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15.3 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.
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15.3 Heterogeneous Aqueous Systems > Colloids
Brownian Motion
Brownian motion is caused by collisions
of the molecules of the dispersion
medium with the small, dispersed
colloidal particles.
• These collisions help prevent the colloidal
particles from setting.
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15.3 Heterogeneous Aqueous Systems > Colloids
Coagulation
Colloidal particles also tend to stay
suspended because they become
charged by adsorbing ions from the
dispersing medium onto their surface.
• Adsorption means to adhere to a surface.
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15.3 Heterogeneous Aqueous Systems > Colloids
Coagulation
All the colloidal particles in a particular
colloidal system will have the same
charge, although the colloidal system is
neutral.
• The repulsion between the like-charged
particles prevents the particles from forming
heavier aggregates that would have a
greater tendency to settle out.
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15.3 Heterogeneous Aqueous Systems > Colloids
Coagulation
A colloidal system can be destroyed or
coagulated by the addition of
electrolytes.
• The added ions neutralize the charged
colloidal particles.
• The particles can clump together to form
heavier aggregates and settle out from the
dispersion.
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15.3 Heterogeneous Aqueous Systems > Colloids
Emulsions
An emulsion is a colloidal dispersion of
a liquid in a liquid.
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15.3 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.
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15.3 Heterogeneous Aqueous Systems > Colloids
Emulsions
• Oils and greases are not soluble in
water.
• However, oils and greases readily form
a colloidal dispersion if soap or
detergent is added to the water.
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15.3 Heterogeneous Aqueous Systems > Colloids
Emulsions
• One end of a large soap or detergent
molecule is polar and is attracted to water
molecules.
• The other end of the soap or detergent
molecule is nonpolar and is soluble in oil or
grease.
• Soaps and other emulsifying agents thus
allow the formation of colloidal dispersions
between liquids that do not ordinarily mix.
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15.3 Heterogeneous Aqueous Systems > Colloids
This table summarizes the properties of solutions,
colloids, and suspensions.
Properties of Solutions, Colloids, and Suspensions
System
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Property
Solution
Colloid
Particle type
Ions, atoms,
Large molecules or
small molecules particles
Large particles or
aggregates
Particle size
0.1–1 nm
1–1000 nm
1000 nm and larger
Effect of light
No scattering
Exhibits Tyndall effect
Exhibits Tyndall effect
Effect of
gravity
Stable, does
not separate
Stable, does not
separate
Unstable, sediment
forms
Filtration
Particles not
Particles not retained
retained on filter on filter
Particles retained on
filter
Uniformity
Homogeneous
Heterogeneous
Heterogeneous
Suspension
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15.3 Heterogeneous Aqueous Systems >
Which of the following is a colloidal
system?
A. mud
B. gasoline
C. blood
D. a mixture of sugar and water
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15.3 Heterogeneous Aqueous Systems >
Which of the following is a colloidal
system?
A. mud
B. gasoline
C. blood
D. a mixture of sugar and water
33
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15.3 Heterogeneous Aqueous Systems > Key Concepts
A suspension differs from a solution
because the particles of a suspension
are much larger and do not stay
suspended indefinitely.
Colloids have particles smaller than
those in suspensions and larger than
those in solutions.
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15.3 Heterogeneous Aqueous Systems > Glossary Terms
• suspension: a mixture from which
some of the particles settle out slowly
upon standing
• colloid: a mixture whose particles are
intermediate in size between those of a
suspension and a solute solution
• Tyndall effect: scattering of light by
particles in a colloid or suspension,
which causes a beam of light to become
visible
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15.3 Heterogeneous Aqueous Systems > Glossary Terms
• Brownian motion: the chaotic
movement of colloidal particles, caused
by collision with particles of the solvent
in which they are dispersed
• emulsion: the colloidal dispersion of
one liquid in another
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15.3 Heterogeneous Aqueous Systems >
END OF 15.3
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