Colloid, Emulsion, Mixture Worksheet - PowerPoint

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					        Chapter 15
Water and Aqueous Systems
                     Water

Water makes plant and animal life on Earth
possible
• Water is present on the earth’s surface.
• Water reserves are deep underground.
• Ice and snow dominate the polar regions.
• Water vapor from evaporation of surface water
  and steam from geysers and volcanoes is
  always present in the atmosphere.
           Water’s Properties

H2O – the oxygen atom forms a covalent bond to
each of the hydrogen atoms
Because of its greater electronegativity, oxygen
 attracts the electron pair of the covalent O – H
 bond to a greater extend than hydrogen.
As a result, the Oxygen atom acquires a partial
 negative charge (δ-)
The less electronegative hydrogen atoms acquire
 partial positive charges (δ+)
           Water’s Properties

The O – H bonds are highly polar.
Polar bond – a covalent bond between atoms in
 which the electrons are shared unequally.
How do the polarities of the two O – H bonds
 affect the polarity of the molecule?
The shape of the molecule is the determining
 factor.
           Water’s Properties

The bond angle of water is approximately 105
 which give it a bent shape.
Polar molecule – a molecule in which one side
 of the molecule is slightly negative and the
 opposite side is slightly positive.
The water molecule as a whole is polar.
Polarity – refers to the net molecular dipole
 resulting from electronegativity differences
 between covalently bonded atoms
           Water’s Properties
In general, polar molecules are attracted to one
 another by dipole interactions.
Dipole interactions – intermolecular forces
 resulting from the attraction of oppositely
 charged regions of polar molecules.
The negative end of one molecule attracts the
 positive end of another molecule
             Water’s Properties
The intermolecular attractions among water
molecules result in the formation of hydrogen
bonds.
Hydrogen bonds – attractive forces in which a
 hydrogen covalently bonded to a very
 electronegative atom is also weakly bonded to an
 unshared electron pair of another electronegative
 atom.
Many unique and important properties of water,
 including its high surface tension and low vapor
 pressure, result from hydrogen bonding.
               Surface Tension
The surface of water acts like a skin.
The skinlike property of water’s
surface is explained by water’s
ability to form hydrogen bonds.

The molecules within the body of the liquid form
 hydrogen bonds with other molecules that surround
 them on all sides.
The attractive forces on each of these molecules are
 balanced.
               Surface Tension
 Water molecules at the surface of the
liquid experience an unbalanced
attraction.
Water molecules are hydrogen-bonded
on only one side of the drop.
As a result, water molecules at the surface tend to
 be drawn inward.

Surface tension – the inward force, or pull that
 tends to minimize the surface area of a liquid
    Spherical Shape of Water Drops
All liquids have surface tension, but water’s is higher
than most.
The surface tension of water tends
to hold a drop of liquid in a spherical
shape.
The drop is not a perfect sphere, because the force
 of gravity tends to pull it down, causing it to flatten.

This is why, on some surfaces, water tends to bead
 up rather than spread out.
         Why A Spherical Shape?
Nature tends to find the path of least resistance
Moving molecules takes work
A spherical shape provides the
minimum surface area for a given
volume.
Molecules expend the least energy possible to move
 into a spherical arrangement while maximizing their
 interactions with one another.
To adopt any other shape, more work would have to
 be done. The worked saved is the surface tension.
                   Surfactants
It is possible to decrease the surface tension of
 water by adding a surfactant.
Surfactant – any substance that interferes with the
 hydrogen bonding between water molecules and
 thereby reduces the surface tension.
Examples of surfactants are soaps and detergents.

Adding a detergent to beads of water on a greasy
 surface reduces the surface tension causing the
 beads of water to collapse and spread out.
                Vapor Pressure
Hydrogen bonding also explains water’s unusually
low vapor pressure.
Vapor pressure is the result of molecules escaping
 the surface of the liquid & entering the vapor phase.
Hydrogen bonds hold water molecules to one
 another. The tendency to escape is low, thus
 evaporation is slow.
It is a good thing because all the lakes and oceans
  would tend to evaporate.
           Water in the Solid State
Water is the solid state exhibits unique properties
the same as a liquid.
Ice cubes float in your glass of iced tea because
 solid water has a lower density than liquid water.
As a typical liquid cools, it begins to contract and its
 density increases gradually.
Increasing density means the molecules of the liquid
 move closer together so that a given volume of a
 liquid contains more molecules and thus more
 mass.
           Water in the Solid State
Eventually the liquid solidifies and it continues to
cool, having a greater density than the liquid
Because the density of a typical solid is greater than
 that of its liquid, the solid sinks in its own liquid.
As water begins to cool, it behaves initially like a
 typical liquid.
It contracts slightly and its density gradually
  increases.
         Water in the Solid State
When the temperature of water falls below 4º C, the
density of water actually starts to decrease.
Below 4º C, water no longer behaves like a typical
 liquid.
Hydrogen bonds hold the water molecules in place in
 the solid phase.

The structure of ice is a regular open framework of
 water molecules arranges like a honeycomb.
         Water in the Solid State
 Extensive hydrogen bonding in
 ice holds the water molecules
farther apart in a more ordered
arrangement than in liquid water.
When ice melts, the framework collapses and the
 water molecules pack closer together, making liquid
 water more dense than ice.
          Water in the Solid State

The fact that ice floats has important consequences
for organisms.
A layer of ice on the top of a pond acts as an
 insulator for the water beneath, preventing it from
 freezing solid except in extreme conditions.
The liquid water at the bottom is warmer than 0ºC,
 fish and other aquatic life are better able to survive.
         Water in the Solid State

Ice melts at 0ºC. This is a high melting
 temperature for a molecule with such a low molar
 mass.
A considerable amount of energy is required to
 return water in the solid state to the liquid state.
The heat absorbed when 1g of water changes
 from a solid to a liquid is 334 J.
                   Question
What causes the high surface tension and low
vapor pressure of water?
Water molecules are hydrogen-bonded to each
 other, but not to air molecules. Net attraction is
 inward, minimizing the water surface area.
 Hydrogen bonding makes it more difficult for
 water molecules to escape from the liquid phase
 to the vapor phase.
                 Questions

How would you describe the structure of ice?
Honeycomb-like structure of hydrogen-bonded
 water molecules.
What effect does a surfactant have on the surface
 tension of water?

Surfactants lower the surface tension by
 interfering with hydrogen bonding.
                    Questions
What are two factors that determine how spherical a
drop of liquid will be?
Surface tension of a liquid tend to hold a drop of liquid
 in a spherical shape; gravity tends to flatten the drop.
The molecules water (H2O) and methane (CH4) have
 similar masses, but methane changes from a gas to a
 liquid at -161 C. Water becomes a gas at 100ºC.
 What could account for the difference?
Water has intermolecular hydrogen bonding between
 its molecules; methane does not.
                   Questions
Why is the surface tension of water so high compared
to that of other liquids?
Water molecules form a large number of hydrogen
 bonds, in addition to dipole-dipole forces between
 molecules.
Why does water form a meniscus in a narrow tube?
Water molecules have a greater attraction to the
 molecules on the surface of the glass than they do to
 each other.
End of Section 15.1
           Solvents and Solutes
Water dissolves so many of the substances that it
comes in contact with that you won’t find chemically
pure water in nature.
Even the tap water you drink is a solution that contains
 varying amounts of dissolved minerals and gases.
Aqueous solution – water that contains dissolved
 substances.
Solvent – the dissolving medium
Solute – the dissolved particles
            Solvents and Solutes
Solutions are homogeneous mixtures. They are also
stable mixtures.
Example: salt (NaCl) does not settle out of the solution
 when allowed to stand. (provided other conditions,
 like temperature remain constant)
Solute particles can be atoms, ions,
or molecules and their average
diameter are usually less than 1nm.
If you filter a solution through filter paper, both the
  solute and the solvent pass through the filter.
           Solvents and Solutes
Ionic compounds and polar covalent molecules
 dissolve most readily in water.
Ionic compounds – composed of a positive and
 negative ion (ex: metal and non metal)
Polar covalent molecules – electrons are shared
 equally between atoms (covalent) and one side of the
 molecule is slightly negative and the opposite side is
 slightly positive.
Nonpolar covalent molecules, such as methane and
 compounds found in oil, grease & gasoline, do not
 dissolve in water.
           The Solution Process

Water molecules are in constant motion because of
their kinetic energy.
When a crystal of NaCl is place in water, the water
 molecules collide with it.
Since the water molecule is polar, the partial positive
 charge on the H+ attracts the negative solute ion Cl-
The partial negative charge on the O2- attracts the
 positive solute ion Na+
                   Solvation

As individual solute ions break away from the crystal,
the negatively (Cl-) and positively (Na+) charged ions
become surrounded by solvent molecules and the
ionic crystal dissolves.
Solvation – the process by
which the positive and
negative ions on an ionic
solid become surrounded
by solvent molecules.
       Insoluble Ionic Compounds

In some ionic compounds, the attractions among the
 ions in the crystals are stronger than the attractions
 exerted by water.
These compounds cannot be solvated to any
 significant extent and are therefore nearly insoluble.
Barium sulfate (BaSO4) and calcium carbonate
 (CaCO3) – nearly insoluble ionic compounds
           The Solution Process

As a rule, polar solvents such as water dissolve ionic
compounds and polar compounds.
Nonpolar solvents such as gasoline dissolve
 nonpolar compounds.
                Like dissolves like
                   Questions

What must happen for an ionic solid to dissolve?

The molecules of the solvent must be able to
 overcome the attractive forces between ions that
 hold the solid together.
What part of a water molecules is attracted to a
 negatively charges solute ion?

The hydrogen atoms
                   Reminders
Solutions are homogeneous mixtures containing a
solvent and one or more solutes.

Usually the solvent is defined as the component in
 the system that is present in the greatest amount.
A water-soluble solute can be a solid, liquid, or a gas.

Anion is a negatively charged atom or group of atoms

Cation is a positively charges atom or group of
 atoms.
      Electrolytes & Nonelectrolytes
Electrolyte – compound that conducts electric current
when it is in an aqueous solution or in the molten state.
All ionic compounds are electrolytes because they
 dissociate into ions.
                 NaCl        Na+ + Cl-
Nonelectrolyte – compound that does not conduct
 electric current in aqueous solutions or in the molten
 state
Many molecular compounds are nonelectrolyes
 because they are not composed of ions.
   Electrolytes & Nonelectrolytes
Some polar molecular compounds are
nonelectrolytes in the pure state, but become
electrolytes when they dissolve in water.
This occurs because they ionize in solution.
Ex: neither ammonia or hydrogen chloride is an
 electrolyte in the pure state.
           NH3 + H2O        NH4+ + OH-
            HCl + H2O       H3O+ + Cl-
Both conduct electricity in aqueous solutions
 because ions form.
            Strong Electrolytes
Not all electrolytes conduct an electric current to
the same degree.
Strong Electrolyte – a solution that is a good
 conductor of electricity because a large portion of
 the solute exists as ions.
Strong Acids
HCl, HBr, HI, HNO3, HClO3, HClO4, and H2SO4
Strong Bases
NaOH, KOH, LiOH, Ba(OH)2, and Ca(OH)2
Salts            NaCl, KBr, MgCl2 …
   Electrolytes & Nonelectrolytes
Weak electrolyte – solution that conducts
electricity poorly because only a fraction of the
solute exists as ions.

Weak Acids
HF, HC2H3O2 (acetic acid), H2CO3 (carbonic acid),
 H3PO4 (phosphoric acid) …..
Weak Bases
NH3 (ammonia), C5H5N (pyridine), and several
 more, all containing "N"
    Electrolytes & Nonelectrolytes

     A solution conducts electricity if it
                contain ions.

Electrolytes are excreted through the skin via
 sweat, and they must be replenished or cramps
 and heat stroke may occur.

Sports drinks are a good source of electrolytes;
 they contain Na+, K+ and Ca+
                   Hydrates

When an aqueous solution of copper(II) sulfate
 (CuSO4) is allowed to evaporate, deep blue crystals
 of copper(II) sulfate pentahydrate are deposited.

The chemical formula for this compound is
 CuSO4 · 5H2O

Water of Hydration or Water of Crystallization –
 the water contained in a crystal.
                     Hydrates

Hydrate – a compound that contains water of hydration
When writing the formula of a hydrate, use a dot to
 connect the formula of the compound and the number
 of water molecules per formula unit.

                    CuSO4 · 5H2O
Crystals of copper(II) sulfate pentahydrate always
 contain five molecules of water for each copper and
 sulfate ion pair.
                    Hydrates

When CuSO4 · 5H2O is heated above 100ºC, the
 crystals lose their water of hydration and crumble to a
 white anhydrous powder of CuSO4.
If anhydrous CuSO4 is treated with water, the blue
  CuSO4 · 5H2O is regenerated.
                        + heat
    CuSO4 · 5H2O                 CuSO4(s) + 5 H2O (g)
                        - heat

Each hydrate contains a fixed quantity of water and
 has a definite composition.
            Efflorescent Hydrates
 The forces holding the water molecules in hydrates
 are not very strong, so the water is easily lost and
 regained.
Because the water molecules are held by weak
 forces, it is often possible to estimate the vapor
 pressure of the hydrates.

If a hydrate has a vapor pressure higher than the
  pressure of water vapor in the air, the hydrate will
  lose its water of hydration – effloresce.
          Hygroscopic Hydrates
 Hydrated salts that have a low vapor pressure
 remove water from moist air to form higher
 hydrates.
These hydrates and other compounds that remove
 moisture from air are called hygroscopic.
         CaCl2 · H2O         CaCl2 · 2H2O
Calcium chloride monohydrate spontaneously
 absorbs a second molecule of water when exposed
 to moist air.
          Hygroscopic Hydrates
 CaCl2 · H2O is used a a desiccant in the laboratory.
Desiccant – a substance used to absorb moisture
 from the air and create a dry atmosphere.
Desiccants can be added to a sealed container to
 keep substances inside the container dry.
Desiccants can be added to liquid solvents to keep
 them dry.
When a desiccant has absorbed all the water it can
 hold, it can be returned to its anhydrous state by
 heating.
                Percent Water
 Suppose you are measuring a mass of Na2CO3 for
  a chemical reaction and you want to use the
  hydrate form of the compound because it is less
  expensive than the anhydrous compound.
To determine what percent of the hydrate is water.
1. Determine the mass of the number of moles of
   water in one mole of hydrate.
2. Determine the total mass of the hydrate.
% Water = mass of water / mass of hydrate x 100%
                   Question
 What is the percent by mass of water in
 Na2CO3 · 10H2O?
Mass of 10 moles of H2O = 18 x 10 = 180 g
Mass of Na2CO3 · 10H2O =
46 + 12 + 48 + 180 = 286g
% Water = mass of water / mass of hydrate x 100%

% Water = 180 / 286 = 62.9 %
                   Question
 What is the percent by mass of water in
 CuSO4 · 5H2O?
Mass of 5 moles of H2O = 18 x 5 = 90 g.
Mass of CuSO4 · 5H2O =
 63.5 + 32.1 + 64 + 90 = 249.6
% Water = mass of water / mass of hydrate x 100%

% Water = 90 / 249.6 = 36.1 %
                   Question
 What is the percent by mass of water in
 CaCl2 · 6H2O?
Mass of 6 moles of H2O = 18 x 6 = 108 g
Mass of CaCl2 · 6H2O =
40.1 + 71 + 108 = 219.1g
% Water = mass of water / mass of hydrate x 100%

% Water = 108 / 219.1 = 49.3 %
                   Questions
 In the formation of a solution, how does the solvent
  differ from the solute?
The dissolving medium is the solvent, and the
  dissolved particles are the solute
Describe what happens to the solute and the solvent
 when an ionic compound dissolves in water.
As individual solute ions break away from the crystal,
  the negatively and positively charged ions become
  surrounded by solvent molecules and the ionic
  crystal dissolves.
                   Questions
Why are all ionic compounds electrolytes?
Because they dissociate into ions.
How do you write the formula for a hydrate?
Use a dot to connect the formula of the compound
 with the number of water molecules per formula
 unit.

Why are all ionic compounds electrolytes?
Because they dissociate into ions.
                   Questions

Which of the following substances dissolve to a
 significant extent in water? Explain your answer.
             CH4 – insoluble, nonpolar
               KCl – soluble, ionic
             He – insoluble, nonpolar
              MgSO4 – soluble, ionic
             Sucrose – soluble, polar
             NaHCO3 – soluble, ionic
                    Questions
Distinguish between efflorescent and hygroscopic
  substances.
Efflorescent compounds lose water to the air.
  Hygroscopic compounds remove moisture from air,
  sometimes forming hydrates.

Identify the solvent and the solute in vinegar, a dilute
  aqueous solution of acetic acid.
Water is the solvent, acetic acid is the solute
                    Questions
What types of substances will dissolve in water to
 form aqueous solutions?
Ionic compounds and polar covalent molecules.

What type of electrolyte is nickel(II) chloride (NiCl2)?
 Explain.
NiCl2 is an ionic compound and is soluble in water,
  therefore, it is a strong electrolyte.
                Question

Get into groups of 2 and find the percent by
 mass of water in the compounds in Table
 15.2 on page 455
End of Section 15.2
  Heterogeneous Aqueous Systems
Heterogeneous mixtures are not solutions.
If you shake a piece of clay with water, the clay
   breaks into fine particles.
The water becomes cloudy because the clay
  particles are suspended in the water.
If you stop shaking, the particles begin to settle out.
Suspension – a mixture from which particles settle
 out upon standing.
                  Suspensions
A suspension differs from a solution because the
  particles of a suspension are much larger and do
  not stay suspended indefinitely.
The larger size of suspended particles means that
  gravity plays a larger role in causing them to settle
  out of the mixture.
Cooks use suspensions of flour or cornstarch in
 water to thicken sauces and gravies.
                 Suspensions
Suspensions are heterogeneous because at least
  two substances can be clearly identified.
You can clearly see the dispersed phase (clay) in the
  dispersion medium (water).
If muddy water is filtered, the filter traps the
   suspended clay particles and clear water passes
   through.
                   Suspensions
How does the filtration of a suspension compare with
 the filtration of a solution?
Filtration of a solution – both the solvent and the
   solute will pass through the filter. (small size of the
   solute particles)
Filtration of a suspension – the filter will trap the
   suspended particles and the liquid will pass
   through (large size of the suspended particles)
                      Colloids
Gelatin is referred to as a heterogeneous mixture
 called a colloid.

What does that tell you about
the composition of gelatin?
It is not uniform throughout.
Would you expect a colloid to
have smaller or larger particles
than a solution?
Larger
                   Colloids
Colloid – a heterogeneous mixture containing
 particles that range in size from 1nm to 1000 nm.
The particles are spread throughout the dispersion
  medium, which can be a solid, liquid or gas.
    glues        gelatin     paint       milk
      smog       smoke      cream    asphalt
  Ink                       sea foam   aerosols
                      Colloids
A colloid is a type of mixture that appears to be a
  solution but it is actually a mechanical mixture.
A colloidal system consists of two separate phases: a
  dispersed phase (internal phase) and a continuous
  phase (dispersion medium).

In a colloid, the dispersed phase is made of tiny
  particles or droplets that are distributed evenly
  throughout the continuous phase.
                The Tyndall Effect
Ordinarily you can’t see a beam of sunlight unless
  the light passes through particles of water or dust
  in the air.
A beam of light is visible as it
passes through a colloid.
Tyndall effect – the scattering
of visible light by colloidal particles
Suspensions also exhibit the Tyndall effect, but
  solutions do not. (particles are too small to scatter light)
               Brownian Motion
Brownian Motion – The chaotic movement of
  colloidal particles (first observed by Robert Brown
  1773 – 1858)
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      Digital video microscopy

settling.
                    Coagulation
Colloidal particles also tend to stay suspended
 because they become charged by adsorbing ions
 from the dispersing medium onto their surface.
Adsorption is a process that occurs when a gas or
 liquid solute accumulates on the surface of a solid
 or a liquid (adsorbent), forming a molecular or
 atomic film (the adsorbate).

It is different from absorption, in which a substance
                                                a solution
    diffuses into a liquid or solid to form video microscopy
                                        Digital
                   Coagulation
Some colloidal particles become positively charged
  by adsorbing positively charged ions
Other colloidal particles become negatively charged
  by adsorbing negatively charged ions.

All the particles in a particular colloidal system will
  have the same charge.


                                    Digital video microscopy
                 Coagulation
The repulsion between the like-charged particles
  prevents the particles from forming heavier
  aggregates that would have a greater tendency to
  settle out.
Thus 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.
          Coagulation Example

          Milk is a colloid of protein and fat.
The curdling of milk occurs when bacteria produce
  enough lactic acid to cause dispersed protein and
  fat particles to coagulate into larger particles, which
  separate from the rest of the mixture.
         Comparable to the clotting of blood


                                  Digital video microscopy
                 Quick Review

Main difference between solutions, suspensions, and
 colloids is particle size.

Solution particles – typically less than 1 nm diameter

Colloid particles – between 1 nm and 1000 nm

Suspension particles - typically larger than 1000nm

                                  Digital video microscopy
                 Quick Review

Smaller particles are less susceptible to the effects of
 gravity and are influenced more by the effects of
 Brownian movement.

The collisions of molecules with extremely small
  colloidal particles are sufficiently energetic to move
  colloidal particles in a random fashion that prevents
  their settling to the bottom.

                                   Digital video microscopy
                    Emulsions

Emulsion – 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.
Ex. Oils and greases are not soluble in water.
However, the readily form a colloidal dispersion if
 soap or detergent is added to the water.
                                  Digital video microscopy
                   Emulsions

Soaps and detergents are emulsifying agent
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.
Emulsifying agents thus allow the formation of
 colloidal dispersions between liquids that do not
 ordinarily mix.
                                 Digital video microscopy
                   Emulsions

An example of an emulsion is
mayonnaise
Mayonnaise is a heterogeneous mixture of oil and
 vinegar, which would quickly separate without the
 presence of egg yolk (the emulsifying agent.)
Milk, margarine and butter are also emulsions.
Cosmetics, shampoos, and lotions are formulated
 with emulsifiers to maintain consistent quality.
                                 Digital video microscopy
                    Questions

In what way are colloids similar to solutions?
In both, dispersed particles are small enough to pass
  through standard filter paper and to withstand the
  pull of gravity.
In what way are colloids similar to suspensions?
Both types of mixtures produce the Tyndall effect.


                                  Digital video microscopy
                Review
        Properties of Solutions
Solutions
• Particle type – ions, atoms,
  small molecules
• Particle size – 0.1 – 1 nm
• Effect of light – no scattering
• Effect of gravity – stable, does
  not separate
• Filtration – particles not
  retained on filter             Digital video microscopy
• Uniformity - homogeneous
              Review
        Properties of Colloids
Colloids
• Particle type – large molecules or particles
• Particle size – 1 – 1000 nm
• Effect of light – exhibits Tyndall effect
• Effect of gravity – stable, does not separate
• Filtration – particles not
  retained on filter
• Uniformity - borderline
                                Digital video microscopy
             Review
    Properties of Suspensions
Suspension
• Particle type – large particles or aggregates
• Particle size – 1000nm and larger
• Effect of light – exhibits Tyndall effect
• Effect of gravity - unstable, sediment forms
• Filtration – particles retained
   on filter
• Uniformity – heterogeneous
                                Digital video microscopy
               Questions

How does a suspension differ from a solution?

Particles of a suspension are much larger and do
  not stay suspended indefinitely.

What distinguishes a colloid from a suspension
 and a solution?
Colloids have particles smaller than those in
 suspensions and larger than those in solutions.
                Questions

How can you determine through observation that
 a mixture is a suspension?

The particles in a suspension will settle out over
  time.

Could you separate a colloid by filtering? Explain

Particles in a colloid such as gelatin are smaller
  than the holes in filter paper and cannot be
  removed by filtering.
                 Question

How can the Tyndall effect be used to distinguish
 between a colloid and a solution?

A beam of light is visible as it passes through a
  colloid; it is invisible as it passes through a
  solution.
                      Question
What causes Brownian motion?
Collisions of the molecules of the dispersion medium
 with the small, dispersed colloidal particles.
Can the presence of Brownian motion distinguish
 between a solution and a colloid? Explain.
Flashes of light (scintillations) are seen when colloids
  are studied under a microscope.
Colloids scintillate because the particles reflecting and
 scattering the light move erratically.
Particles in a solution are too small to be seen under a
  microscope and do not cause scintillations.
End of chapter 15

				
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