Chapter 13 & 14 Solutions
Let’s review the classification of matter.
Matter
Matter
Mixtures
Substances
Matter
Mixtures
Substances
Elements
Compounds
Matter
Mixtures
Heterogeneous
Substances
Homogeneous
Elements
Compounds
Matter
Mixtures
Heterogeneous
Colloids
Substances
Homogeneous
Elements
Suspensions
Solutions
Compounds
What aren’t solutions?
1)Solutions are not colloids or suspensions.
a)Colloids have solute particles that are
aggregates or large molecules between 1 nm
and 1000 nm.
The particles (called the dispersed phase)
cannot be separated from the solvent (called
the dispersing medium) by filtration or
settling. A colloid will scatter light (Tyndall
effect).
 Tyndall Effect
b)Suspensions have solute particles that are
aggregates or molecules that are so large
(>1000nm) that they settle and can be
separated by a filter.
Although a suspension may scatter light,
they are not transparent.
What are solutions?
1)Solutions are homogenous mixtures. This
means solutions contain two or more
substances, and are mixed enough to “look all
the same”. Solutions do NOT have to be
liquids, bronze is a solid solution and air is a
gaseous solution.
A)All solutions have two parts. The substance
doing the dissolving is the solvent and the
substances being dissolved are the solutes.
The solutes must have particles between 0.011nm. The solutes can be atoms, molecules,
and ions. The solutes do not settle, scatter
light, nor can be filtered.
If you don’t know which is which, the substance
you have the most of overall is the solvent.
B)The most common solvent is water. It is so
common and dissolves so many things that it
is often called “the universal solvent” (the
solute is hydrated.) This is not entirely true, as
only polar solutes will dissolve in water (we say
they are soluble or miscible). Nonpolar
substances will not dissolve in water (we say
they are insoluble or immiscible.) This is due
differences in the types of intermolecular
forces – H-bonding and dipole for polar, but
only London forces for nonpolar.
If alcohol is the solvent, then the solution is often
called a tincture.
In general it is said “like dissolves like” meaning
polar solvents only dissolve polar solutes and
nonpolar solvents only dissolve nonpolar
solutes. Polar substances include salts, acids,
bases, and nonsymmetrical molecules like
water and NH3. Nonpolar substances include
most carbohyrates, fats, oils, and other
symmetrical molecules. The more similar two
substances are the more miscible they will be.
(Similar intermolecular forces!)
2)Even when things are soluble in water, or any
other solvent, different solutes will dissolve to
different amount. The actual amount (in grams)
that will dissolve in 100 grams of a solvent is
called the solubility. When discussing the
solubility of solutes, solutions are classified
into 3 types: saturated, unsaturated, and
supersaturated.
A)A saturated solution has dissolved as much
solute as it can. If you add sugar to iced tea
and sugar is just sitting at the bottom of the
glass your iced tea is saturated for sugar.
(Even though it seems there is no change,
there is an equilibrium occurring between
dissolution and crystallization.)
B) An unsaturated solution can still dissolve
more solute.
C)A supersaturated solution has been “tricked”
into dissolving more solute than it should be
able to. If any more solute is added to a
supersaturated solution, all the extra solute will
undissolve (precipitate). This is how sugar
crystal candy (rock candy) is made.
D)Because the solubility of solutions is
dependent on temperature, chemists represent
the amounts that can be dissolved with a
solubility curve:
1)Temperature is the
independent variable
(the one you control).
This is shown on the
bottom of the graph.
You can warm up the
solution with a stove
or cool it down in a
freezer.
2)The amount (in grams)
of how much will
dissolve at a particular
temperature is the
dependent variable (it
depends on the
temperature). This is
shown on the left side
of the graph. Notice
this is for all the
solutes in 100 grams of
water.
A)If the temperature
and amount of
dissolved solute is
on the line shown for
that solute, then the
solution is saturated.
For example, 70
grams of NH3 in 100
grams of water will
be a saturated
solution at 10 °C.
B)If the temperature and
amount of dissolved solute
is above the line, then the
solution is supersaturated.
If the saturated solution of
70 grams of NH3 is very
carefully heated, it will still
have 70 grams of NH3 at 20
°C. As this amount is
above the line, this would
be a supersaturated
solution.
C)If the temperature
and amount of
dissolved solute is
below the line, then
the solution is
unsaturated. An
unsaturated solution
could still dissolve
more solute if it is
added.
3)Temperature is not the only factor that will
affect how much solute can dissolve. Particle
size, pressure, and agitation also have an
effect.
A)Dissolving happens as the solvent touches the
solute and starts to pull it into pieces. This
happens only on the outside of the particles of
the solute.
One big chunk of solute has less outside
surface area than a lot of small pieces of
solute. Thus, if the solute is crushed or ground
it will dissolve faster. This only works for
dissolving solids in liquids.
B)Gases are affected by pressure. With high
pressure a lot of gas can be squeezed into a
little bit of space. High pressures will help
gases dissolve in a liquid, and low pressure
will help remove the gas from a liquid. This
only works for dissolving a gas in a liquid, and
is why a carbonated beverage goes “flat” after
it is opened (called Effervescence).
Henry’s Law shows that the solubility of a gas in
a liquid is directly proportional to the partial
pressure of the gas above the surface of the
liquid.
P1
P2
=
S1
S2
C) When a solid dissolves in a liquid, it takes time
for the liquid molecules to grab pieces of the
solid and float away. If the liquid is stirred, it
speeds up the floating away processes, so
more liquid can grab more solid in the same
amount of time. Thus stirring or shaking will
help a solid dissolve faster in a liquid.
D)When a gas dissolves in a liquid, it is basically
trapped by the liquid molecules. If the liquid is
stirred, it removes the water molecules that are
trapping the gas, and the gas will form bubbles
and eventually leave the liquid. This is why
shaking a carbonated beverage makes so many
bubbles.
Method
Temperature
Particle Size
Pressure
Agitation
Solid dissolved in
liquid
Gas dissolved in
liquid
Method
Solid dissolved in
liquid
Temperature Increase temperature
Particle Size
Pressure
Agitation
Gas dissolved in
liquid
Method
Solid dissolved in
liquid
Temperature Increase temperature
Particle Size
Pressure
Agitation
Gas dissolved in
liquid
Decrease
temperature
Method
Solid dissolved in
liquid
Temperature Increase temperature
Particle Size Make smaller pieces
Pressure
Agitation
Gas dissolved in
liquid
Decrease
temperature
Method
Solid dissolved in
liquid
Gas dissolved in
liquid
Temperature Increase temperature
Decrease
temperature
Particle Size Make smaller pieces
No effect
Pressure
Agitation
Method
Solid dissolved in
liquid
Gas dissolved in
liquid
Temperature Increase temperature
Decrease
temperature
Particle Size Make smaller pieces
No effect
Pressure
Agitation
No effect
Method
Solid dissolved in
liquid
Gas dissolved in
liquid
Temperature Increase temperature
Decrease
temperature
Particle Size Make smaller pieces
No effect
Pressure
Agitation
No effect
Increase pressure
Method
Solid dissolved in
liquid
Gas dissolved in
liquid
Temperature Increase temperature
Decrease
temperature
Particle Size Make smaller pieces
No effect
Pressure
No effect
Agitation
Agitate
Increase pressure
Method
Solid dissolved in
liquid
Gas dissolved in
liquid
Temperature Increase temperature
Decrease
temperature
Particle Size Make smaller pieces
No effect
Pressure
No effect
Increase pressure
Agitation
Agitate
Do not agitate
Solution Concentration
1)Just knowing that a solution is unsaturated
does not indicate much about the solution. Is
there only a little bit of solute, or is there a lot
and it is almost saturated? The only way to tell
is to know the solutions concentration.
2)In general if there is a lot of solute in the
solvent, the solution is called concentrated,
and if there is only a little bit of solute in the
solvent it is called dilute. But chemists need a
more exact way to know how concentrated or
dilute the solution is.
Chemists have 5 different ways to determine
numbers for concentration. Each way is
different and useful in different situations.
A)Percent by mass = mass of solute divided by
the mass of solution × 100%
50 grams of Ca(OH)2 are dissolved in 300 grams
of water. What is the percent by mass for
Ca(OH)2?
B) Percent by volume = volume of solute divided
by the volume of solution × 100%
C) Molarity = moles of solute divided by the liters
of solution
mol
M=
L
What is the molarity of a 3.50 L of solution that
contains 90.0 g of sodium chloride?
Because
n=
g
m
then M × L × m = w
where
M = Molarity
L = liters of solution
m = molar mass of solute
g = grams of solute
D) Molality = moles of solute divided by the
kilograms of solvent
mol
m=
kg
What is the molality of a solution made by
dissolving 17.1 g of sucrose (C12H22O11) in 125
g of water?
Because
n=
g
m
then m × kg × m = w
where
m = Molality
kg = kilograms of solvent
m = molar mass of solute
g = grams of solute
E) Mole fraction = moles of solute divided by the
total moles of solute and solvent
50 grams of NaCl, 30 grams of KF, and 50 grams
of water are mixed. What is the mole fraction
of water?
Colligative Properties
1)Physical properties of solutions that are
affected by how much solute is dissolved are
called colligative properties. (Colligative
means “depending on the collection”.) There
are three colligative properties that we’ll be
concerned with: electrolytes, boiling point
elevation, and freezing point depression.
A) When an ionic solute dissolves it ionizes
(breaks apart into positive and negative parts),
and these parts will allow electricity to travel
through the solution. For example: pure water
will not conduct electricity, but if table salt is
added, then the sodium and chlorine that break
apart will conduct electricity.
NaCl → Na+ + Cl-
Solutions that conduct electricity are called
electrolytes. The more of an ionizing solute
that is dissolved, the better the conduction of
electricity will be. Typical electrolytes are
acids, bases, and salts.
On the other hand, molecular solutes (covalent
compounds) don’t ionize (like sugar) will not
conduct electricity in water. So sugar water
will not conduct electricity. Solutions that don’t
conduct electricity are called nonelectrolytes.
B)Boiling happens when the molecules of a liquid
get enough energy to break through the liquidgas barrier and escape as a gas. If a solute is
added, it gets in the way of the liquid trying to
escape, and thus it takes more energy (more
heat) to get the liquid to boil. This is called
boiling point elevation.
If pure water boils at 100 °C, and salt is added,
the solution will no longer boil at 100 °C, but
will boil at a higher temperature. Often the
directions for cooking pasta require the
addition of salt to the water. This causes the
water to boil at a hotter temperature and thus
the pasta cooks quicker (and the salt adds
some flavor to the pasta).
The more solute that is added, the higher the
boiling point will be. Thus the boiling point
elevation is directly proportional to the molality
of the solution.
C)Similar to boiling point elevation is freezing
point depression. Now the solute lowers the
temperature at which the solution freezes.
Pure water freezes at 0°C, but salt water freezes
below that temperature.
Just like with boiling point elevation, freezing
point depression is directly proportional to the
molality of the solution. This is why “antifreeze” is added to car engines and salt is
added to icy roads. This is also why salt is
added to the ice when making ice cream - some
of the ice melts to make the rest of the ice
colder - cold enough to freeze cream.
∆T = m × Kf
∆T = m × Kb
Where
m = molality
Kf = freezing constant
Kb = boiling constant
Hint: you may use m × kg × m = g
10 g of LiCl are added to 100 g of water. What will
be the new boiling and freezing points of the
water? Kb for water = 0.51 °C/m and Kf for
water = -1.86 °C/m.