Unit 7 Solutions
A. The characteristics of a Solution.
1. A solution is a homogeneous mixture in which one substance
disperses uniformly throughout another.
 Solutions may exist in any of the three states of matter:
gas, liquid, or solid.
a. Air is a gaseous solution, made up of nitrogen, oxygen,
carbon dioxide
b. Water is a liquid solution with dissolved gaseous oxygen
c. Brass is a solid solution made up of copper and zinc.
d. Steel is a solution of carbon and iron.
2. Solution consists of two parts:
a. Solvent- dissolving medium, usually the majority
substance, can be any phase.
b. Solute- that which is dissolved in a solvent (usually
minority)
3. The formation of a solution- most solutions of interest to a
chemist, biologist or geologist are water solutions. The water
molecule has unique particulate-level properties that make it a
good solvent.
a. Steps in solution formation.
1. Forces of attraction must be broken
 solvent-solvent bonds are broken
 solute-solute bonds are broken
 energy is absorbed (endothermic)
2. New forces of attraction are formed
 solvent-solute bonds are formed
 energy is released (exothermic)
b. Water is a polar molecule meaning that it has a positive and
a negative pole.
c. When a soluble ionic substance is placed in water, the
negatively charged ions at the crystal’s surface are attracted
to the positive region of the water molecule. A “tug-ofwar” for the negative ion begins.
d. Water molecules tend to pull the negative ions from the
crystal while neighboring positive ions tend to hold them in
the crystal.
e. Once released, the ions are surrounded by the water
molecules.
This process is called “SOLVATION”
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f.
The dissolving process is reversible.
1. the dissolved solute particles move randomly
throughout the solution, they come into contact with
undissolved solute particles and recrystallize,
meaning return to the solid state.
2. NaCl(s)  Na+ (aq) + Cl- (aq)
g. The rate that the dissolving process occurs depends upon
the temperature. The rate of crystallization depends upon
the concentration of the solute at the crystal surface.
1. When the rates of the dissolving and crystallization
become the same, the solution is saturated at that
temperature. A dynamic equilibrium is reached.
NaCl(s)  Na+ (aq) + Cl- (aq)
4. Factors that affect the rate at which a solute dissolves.
a. Because the dissolving process occurs at the surface of the
solid being dissolved, the greater the amount of surface area
exposed to the solvent, the faster the dissolving will occur.
Therefore; decrease particle size by crushing or grinding.
b. Dissolving process is also increased by stirring the solution.
Stirring removes newly dissolved particles from the solid
surface and continually exposes the surface to fresh solvent.
c. Finally, dissolving occurs more rapidly at higher
temperatures. Particle movement is more rapid.
1. Vigorous collisions facilitate the disruption of
particle-to-particle (intermolecular) attractions.
5. Factors that determine solubility- the extent to which a
particular solute dissolves in a given solvent depends on several
factors:
a. Intermolecular Forces- solubility depends on attractions
between solute particles and attractions between solvent
particles.
1. If forces between molecules of A are about the same
as the forces between molecules of B, A and B will
probably dissolve in each other. This is paraphrased
as “like dissolves like”.
2. Water and oil are not soluble in each other. Oil is
nonpolar and water is polar. The attraction between
the two is very weak.
3. Water and alcohol are soluble because they are both
polar substances.
4. Consult a solubility table if unsure.
Type of solute vs. solubility of solvent
SOLUTE
POLAR
SOLVENT
NONPOLAR
SOLVENT
Polar
Nonpolar
Ionic
soluble
insoluble
soluble
insoluble
soluble
insoluble
b. Temperature- increases the ability of a solute to dissolve in
a liquid.
1. There is a notable exception: gases dissolved in
liquids are generally lower at increased temperatures.
2. Gas solubility generally decreases with increased
temperature because solution depends on solutesolvent bonds, which weaken as temperature
increases.
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c. Pressure - The solubility of gases increases when the
pressure above the gas is increased.
1. greater the pressure, more the gas dissolves.
2. This can again be explained by Le Châtelier's
Principle.
a. Carbonated beverages contain carbon dioxide
which is dissolved under pressure. Our
equilibrium system is: CO2(g)
CO2(aq) + heat
b. increase pressure, equilibrium shifts to the side
with fewer gas molecules, the right side.
Therefore; increasing the pressure forces gas
particles into solution.
3. Henry’s Law- at a given temperature, solubility (S)
of a gas in a liquid is directly proportional to pressure
(P).
S1 S2
=
P1 P2
6. Solution terminology- in discussing solutions, we us a
language closely related and sometimes overlapping terms
a. concentrated- a solution with a relatively large quantity of
solute per solvent
b. dilute- a solution with a relatively small quantity of solute
per solvent.
c. solubility- a measure of how much solute will dissolve in a
given amount of solvent at a given temperature.
d. saturated- a solution whose concentration is at the
solubility limit for a given temperature. Can’t dissolve any
more solute in the solvent.
e. unsaturated- the concentration of a solute that is less than
the solubility limit. Can hold more solute.
f. supersaturated- a solution in which the concentration of
the solute is greater than the normal solubility limit. More
solute dissolved than the solution should be able to hold.
g. miscible- when two liquids dissolve in each other in all
proportions. For example alcohol and water.
h. immiscible- liquids that are insoluble in each other. For
example oil and water.
i. nonelectrolyte- solution that does not conduct electric
current. Substances formed from covalent bonding do not
dissolve into ions upon entering the water.
j. strong electrolyte- solution that conducts electric current
strongly due to free moving ions.
k. weak electrolyte- solution that only weakly conducts
electric current.
l. complete dissociation- ionic solid that completely breaks
apart and dissolves in solution.
m. partial dissociation- ionic solid that only partial breaks
apart and dissolves.
Degree’s of saturation
Conductivity
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B. Solution Concentration- the amount of solute present per
given amount of solution.
1. Mass percent- one way to describe a solution’s concentration
is mass percent, which expresses the mass of solute present in
a given mass of solution.
Mass% =
=
mass solute
´100%
mass solution
grams solute
´100%
grams solute + grams solvent
2. Molarity- in working with liquids, volume is easier to measure
than mass. Therefore, a solution concentration based on
volume is more convenient.
a.
Molarity =
mole solute
liter solution
b. units are mol/L, symbol M
3. Percent by volume- =
vol. solute
´100%
vol. solution
4. Parts Per Million (PPM)
A unit of concentration often used when measuring levels of
pollutants in air, water, body fluids, etc.
a. One ppm is 1 part solute in 1,000,000 parts solution.
The common unit mg/liter is equal to ppm.
b. A solution with a concentration of 1 ppm has 1 gram
of substance for every million grams of solution.
c. Four drops of ink in a 55-gallon barrel of water
would produce an "ink concentration" of 1 ppm.
d.
ppm =
grams solute
´1,000,000
grams solution
5. Mole Fraction (X)- ratio of the number of moles of solute in a
solution to moles of solvent and solute together. n = moles
X=
n solute
nsolute + n solvent
6. Molality- volume of a solution changes with changes in
temprature; therefore, molarity is not always used. Mass of
solvent doesn’t change with temperature.
=
moles solute
kg solvent
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B. Dilution of concentrated solutions- to save time and space in
the laboratory, solutions that are routinely used are prepared in
concentrated form, called stock solutions. Water is then added
to achieve the molarity desired. Process of adding more solvent
to a solution is called dilution.
1. The following equation will be used: Vc × M c = Vd × Md
V= volume, M= molarity, c= concentrated solution, d=
dilute solution
a. With any problem involving dilution, three of the four
terms will be given.
C. Colligative Properties1. On adding a solute to a solvent, the properties of the solvent
are modified.
a. Vapor pressure lowering- addition of solute particles
reduces vapor pressure.
b. Freezing point decreases- solute particles interfere
with the intermolecular forces thus preventing the
solvent from entering the solid state.
•
•
Kf is the freezing point constant, for water it
is 1.86 ºC/m
m is the molality of the solution.
c. Boiling point increases- addition of solute particles
lowers vapor pressure and therefore increases boiling
point.
•
•
Kb is the boiling point constant, for water it
is 0.512 ºC/m
m is the molality of the solution.
d. Osmosis- adding additional solute particles to a
solution increases the osmotic pressure.
2. These changes are called COLLIGATIVE PROPERTIES.
a. They depend only on the NUMBER of solute
particles relative to solvent particles, not on the
KIND of solute particles.
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