Aqueous Systems and Solutions
Solutions
Solutions
• An aqueous solution is water that
contains dissolved substances.
Solvents and Solutes
•In a solution, the dissolving medium is the solvent.
•The dissolved particles in a solution are the solute.
Solutions
• A solvent dissolves the solute.
• The solute becomes dispersed in the solvent.
• Solvents and solutes may be gases, liquids, or
solids.
• Solutions are homogeneous mixtures.
• Solute particles can be atoms, ions, or molecules.
• If you filter a solution through filter paper, both the
solute and solvent pass through the filter.
Solutions
What types of substances dissolve most
readily in water?
Substances that dissolve most readily
in water include ionic compounds and
polar covalent compounds.
• Nonpolar covalent compounds do not dissolve
in water. Examples of nonpolar covalent
compounds include methane (CH4), and
compounds found in oil, grease, and gasoline.
• However, oil and grease will dissolve in
gasoline.
Solutions
The Solution Process
• A water molecule is polar, with a partial
negative charge on the oxygen atom and
partial positive charges on the hydrogen
atoms.
• 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.
Solutions
The Solution Process
Solvated ions
•The process by which
the positive and
negative ions of an
ionic solid become
surrounded by solvent
molecules is called
solvation.
Surface of ionic solid
Solutions
The Solution Process
• Polar solvents such as
water dissolve ionic
compounds and polar
compounds.
• Nonpolar solvents such
as gasoline dissolve
nonpolar compounds.
• This relationship can be
summed up in the
expression “like dissolves
like.”
•Which of these compounds should not
dissolve in water?
A. HCl
B. C4H10
C. KI
D. NH3
•Which of these compounds should not
dissolve in water?
A. HCl
B. C4H10
C. KI
D. NH3
Electrolytes and Nonelectrolytes
electrolyte - a compound that conducts an
electric current when it dissolves in water or is
melted.
All ionic compounds are electrolytes
because they dissociate into ions.
nonelectrolyte - a compound that does not conduct
an electric current ever.
nonelectrolytes do not dissociate in water
Electrolytes and Nonelectrolytes
•Some polar molecular compounds are
nonelectrolytes in the pure state but
become electrolytes when they dissolve in
water. (ex. Ammonia NH3)
• For example, ammonia (NH3(g)) is not an
electrolyte in the pure state.
• Yet an aqueous solution of ammonia conducts an
electric current because ammonium ions (NH4+)
and hydroxide ions (OH–) form when ammonia
dissolves in water.
NH3(g) + H2O(l)  NH4+(aq) + OH–(aq)
Electrolytes and Nonelectrolytes
•Not all electrolytes conduct electric current
to the same degree.
• In a solution that contains a strong
electrolyte, all or nearly all of the solute
exists as ions.
• A weak electrolyte conducts an electric
current poorly because only a fraction of the
solute in the solution exists as ions.
Electrolytes and Nonelectrolytes
•Your cells use electrolytes, such as sodium and
potassium ions, to carry electrical impulses across
themselves and to other cells.
• An electrolyte imbalance
can occur if you become
dehydrated.
• When you exercise,
you can lose water
and electrolytes from
your body through
perspiration.
Hydrates
•The water contained in a crystal is called the
water of hydration or water of crystallization.
• hydrate – a compound that contains water
of hydration
• anhydrous - A substance that does not contain water.
Hydrates
CuSO45H2O(s)
Heating of a
sample of
blue
CuSO45H2O
begins.
+ heat
– heat
CuSO4(s) + 5H2O(g)
After a time,
much of the
blue hydrate
has been
converted to
white
anhydrous
CuSO4.
Hydrates
Each hydrate contains a fixed quantity of
water and has a definite composition.
Some Common Hydrates
Formula
Chemical name
Common name
MgSO47H2O
Magnesium sulfate heptahydrate
Epsom salt
Ba(OH)28H2O
Barium hydroxide octahydrate
CaCl22H2O
Calcium chloride dihydrate
CuSO45H2O
Copper(II) sulfate pentahydrate
Blue vitriol
Na2SO410H2O
Sodium sulfate decahydrate
Glauber’s salt
KAl(SO4)212H2O
Potassium aluminum sulfate
dodecahydrate
Alum
Na2B4O710H2O
Sodium tetraborate decahydrate
Borax
FeSO47H2O
Iron(II) sulfate heptahydrate
Green vitriol
H2SO4H2O
Sulfuric acid hydrate (mp 8.6oC)
Hydrates
Efflorescent Hydrates
The water molecules in hydrates are held
by weak forces, so hydrates often have
an appreciable vapor pressure.
• 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,
or effloresce.
Hydrates
Hygroscopic Hydrates
Hydrated ionic compounds that have low
vapor pressure remove water from moist
air to form higher hydrates.
• Hydrates and other compounds that
remove moisture from air are hygroscopic.
Hydrates
Hygroscopic Hydrates
Calcium chloride monohydrate
spontaneously absorbs a second molecule
of water when exposed to moist air.
• Calcium chloride is used as a
desiccant in the laboratory.
desiccant - a substance used
to absorb moisture from the
air and create a dry
atmosphere.
Hydrates
Deliquescent Compounds
Some compounds are so hygroscopic that
they become wet when exposed to
normally moist air.
• Deliquescent - compound that
removes sufficient water from the air
to dissolve completely and form
solutions.
Hydrates
Pellets of sodium hydroxide are deliquescent.
For this reason,
containers of NaOH
should always be
tightly stoppered.
The solution formed by a
deliquescent substance has
a lower vapor pressure than
that of the water in the air.
Suspensions
suspension - 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.
• 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.
Suspensions
• A solution is a homogeneous mixture.
• Suspensions are heterogeneous because at
least two substances can be clearly
identified.
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.
The particles of a
suspension can be
removed by filtration.
•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.
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, or
dispersed, throughout the dispersion
medium, which can be a solid, liquid, or
gas.
Colloids
The first substances to be identified as
colloids were glues.
Some Colloidal Systems
System
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
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.
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.
Colloids
The Tyndall Effect
•The scattering of visible light by colloidal
particles is called the Tyndall effect.
Flashlight
Solution
Colloid
Suspension
Colloids
• Suspensions also exhibit the Tyndall effect.
• The particles in solutions are too small to
scatter light.
Flashlight
Solution
Colloid
Suspension
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.
Colloids
Brownian Motion
•Flashes of light, or scintillations, are seen
when colloids are studied under a
microscope.
• This happens because the particles
reflecting and scattering the light move
erratically.
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.
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.
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.
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.
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.
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 allow
the formation of colloidal dispersions
between liquids that do not ordinarily mix.
Colloids
This table summarizes the properties of solutions,
colloids, and suspensions.
Properties of Solutions, Colloids, and Suspensions
System
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
Hydrates
•Calculate the percent by mass of water in
washing soda, sodium carbonate decahydrate
(Na2CO310H2O).
•Calculate the percent by mass of water in
epsom salt, magnesium sulfate heptahydrate
(MgSO47H2O).
Solution Formation
•Granulated sugar dissolves faster than sugar
cubes, and both granulated sugar and sugar
cubes dissolve faster in hot tea or when you
stir the tea.
Solution Formation
Composition of solvent and solute
determine whether or not a substance
will dissolve.
•Factors that affect how fast a substance dissolves
include:
• Agitation (stirring or shaking)
• Temperature
• Particle size of the solute
Solution Formation
• Agitation
If the contents of the glass are stirred, the
crystals dissolve more quickly.
• The dissolving process
occurs at the surface of the
sugar crystals.
• Stirring speeds up
dissolving because fresh
solvent is continually
brought in contact with the
surface of the solute.
Solution Formation
• Agitation
Agitation (stirring or shaking) affects only
the rate at which a solid solute dissolves.
• It does not influence the
amount of solute that will
dissolve.
• An insoluble substance
remains undissolved
regardless of how
vigorously or for how long
the solvent/solute system
is agitated.
Temperature
Solution Formation
•Temperature also influences the rate at
which a solute dissolves.
• Sugar dissolves much more
rapidly in hot tea than in
iced tea.
• Most solids dissolve faster
at higher temperatures.
Solution Formation
Temperature
•At higher temperatures, the kinetic energy of
water molecules is greater than at lower
temperatures, so the
•molecules move faster.
• At higher temperatures, solvent
molecules move faster, resulting
in more frequent collisions
between solute and solvent
molecules, resulting in faster
dissolving.
Solution Formation
• Particle Size of the Solute
The rate at which a solute dissolves also
depends upon the size of the solute
particles.
• The smaller particles in
granulated sugar expose a
much greater surface area
to the colliding water
molecules.
Solution Formation
• Particle Size of the Solute
The dissolving process is a surface
phenomenon.
• Smaller particles dissolve
faster because more
surface area of the solute is
exposed, speeding up the
rate of dissolving.
•Which of the following will not speed up
the rate at which a solid solute dissolves?
A.
B.
C.
D.
Increasing the temperature
Stirring the mixture
Crushing the solute
Decreasing the temperature
•Which of the following will not speed up
the rate at which a solid solute dissolves?
A.
B.
C.
D.
Increasing the temperature
Stirring the mixture
Crushing the solute
Decreasing the temperature
Solubility
•What is
happening in this
figure?
• Particles move from
the solid into the
solution.
• Some dissolved particles move from the solution back
to the solid.
• When two processes occur at the same rate, no net
change occurs in the overall system (equilibrium).
Solubility
Such a solution
is said to be
saturated.
• saturated solution contains the maximum
amount of solute for a given quantity of solvent at
a constant temperature and pressure.
Solubility
•In a saturated solution, a state of
dynamic equilibrium exists between the
solution and any undissolved solute,
provided that the temperature remains
constant.
Solubility
solubility of a substance is the amount of
solute that dissolves in a given quantity of
a solvent at a specified temperature and
pressure to produce a saturated solution.
• Solubility is usually expressed as
• grams of solute per 100 g of solvent
•
(g solute/100 g H2O).
• Sometimes the solubility of a gas is expressed in
grams per liter of solution (g/L).
Solubility
unsaturated solution contains less
solute than a saturated solution at a given
temperature and pressure
• If additional solute is added to an unsaturated
solution, the solute will dissolve until the
solution is saturated.
Solubility
miscible liquids dissolve in each other in all proportions.
• Ex: water and ethanol
immiscible - Liquids that are insoluble in each other
• Ex: oil and water
Interpret Graphs
Factors Affecting Solubility
• For a few
substances,
solubility decreases
with temperature.
Solubility (g/100g H2O)
Solubility for most solid substances increases
as the temperature of the solvent increases.
Temperature (°C)
Interpret Data
Solubilities of Substances in Water at Various Temperatures
Solubility (g/100 g H2O)
Substance
Formula
0°C
Barium hydroxide
Ba(OH)2
1.67
Barium sulfate
BaSO4
0.00019
0.00025
Calcium hydroxide
Ca(OH)2
0.189
0.173
Potassium chlorate
KClO3
4.0
7.4
19.3
56.0
Potassium chloride
KCl
27.6
34.0
42.6
57.6
Sodium chloride
NaCl
35.7
36.0
37.0
39.2
Sodium nitrate
NaNO3
74
88.0
114.0
Aluminum chloride
AlCl3
30.84
31.03
Silver nitrate
AgNO3
122
222.0
455.0
733
Sucrose (table sugar)
C12H22O11 179
230.9
260.4
487
Hydrogen
H2
0.00019
0.00016
0.00013
0.0
Oxygen
O2
0.0070
0.0043
0.0026
0.0
Carbon dioxide
CO2
0.335
0.169
0.076
0.0
20°C
31.89
50°C
100°C
—
—
0.00034
—
31.60
—
0.07
182
33.32
Factors Affecting
Solubility
supersaturated solution contains more
solute than it can theoretically hold at a
given temperature.
• The crystallization of a supersaturated solution
can be initiated if a very small crystal, called a
seed crystal, of the solute is added.
Factors Affecting
Solubility
•The rate at which excess solute deposits upon the
surface of a seed crystal can be very rapid.
The solution
is clear before
a seed crystal
is added.
Crystals begin to
form immediately
after the addition
of a seed crystal.
Excess solute
crystallizes rapidly.
Factors Affecting
Solubility
Temperature
The effect of temperature on the
solubility of gases in liquid solvents is
opposite that of solids.
• Solubility for most gases is greater in cold
water than in hot.
Pressure
Factors Affecting
Solubility
Changes in pressure have little effect on
the solubility of solids and liquids, but
pressure strongly influences the solubility
of gases.
• Gas solubility increases as the partial pressure
of the gas above the solution increases.
Factors Affecting
Solubility
Pressure
Carbonated beverages are a good example.
• These drinks contain
large amounts of
carbon dioxide (CO2)
dissolved in water.
• Dissolved CO2 makes
the liquid fizz
Pressure
• The drinks are bottled
under a high pressure
of CO2 gas, which
forces larger amounts
of the gas into
solution.
Factors Affecting
Solubility
Factors Affecting
Solubility
Pressure
• When the container is
opened, the partial
pressure of CO2 above
the liquid decreases.
• Immediately, bubbles
of CO2 form in the
liquid and escape
from the open bottle.
Factors Affecting
Solubility
Pressure
•How is the partial pressure of carbon
dioxide gas related to the solubility of CO2 in
a carbonated beverage?
• The relationship is described by Henry’s
law, which states that at a given
temperature, the solubility (S) of a gas in a
liquid is directly proportional to the pressure
(P) of the gas above the liquid.
Factors Affecting
Solubility
Pressure
• As the pressure of the gas above the liquid
increases, the solubility of the gas
increases.
• As the pressure of the gas decreases, the
solubility of the gas decreases.
•Explain why an opened container of a
carbonated beverage is more likely to go flat
sitting on the counter than in the
refrigerator.
The solubility of a gas in a liquid
increases with decreasing temperature.
More carbon dioxide will remain in
solution at the colder temperature found
in the refrigerator.
Factors Affecting
Solubility
Factors Affecting Solubility
•What factors affect the solubility of a
substance?
Temperature affects the solubility of
solid, liquid, and gaseous solutes in a
solvent; both temperature and pressure
affect the solubility of gaseous solutes.
Key Concepts
•Factors that determine how fast a substance
dissolves are stirring, temperature, and surface
area.
In a saturated solution, a state of dynamic
equilibrium exists between the solution
and any undissolved solute, provided that
the temperature remains constant.
Temperature affects the solubility of solid,
liquid, and gaseous solutes in a solvent;
both temperature and pressure affect the
solubility of gaseous solutes.