Solids, Liquids, and gases dissolve to form solutions.
What is a solution?
A mixture of 2 or more substances that are uniformly mixed
with each other.
What does it mean to dissolve?
When one substance mixes into another and completely
disappears. For example, NaCl in H2O.
The Solution Process
In order for a solute to be
dissolved in a solvent, the solute
and solvent must be attracted to
each other. The solute and the
solvent molecules in a solution are
then expanded (spread out), and
able to mix with each other. In the
example below, NaCl is broken
apart because +/- water molecules
are attracted to the +Na and the Cl. The water breaks down the
crystal, dissolving the salt.
Three types of interactions in the solution process:
• solvent-solvent interaction
• solute-solute interaction
• solvent-solute interaction
Δ Hsoln = Δ H1 + Δ H2 + ΔH3
Energy Changes in Solution
To determine the enthalpy
change, we divide the
process into 3 steps.
1. Separation of solute
particles.
2. Separation of solvent
particles to make ‘holes’.
3. Formation of new
interactions between solute
and solvent.
Enthalpy Changes in Solution
The enthalpy change of
the overall process
depends on H for each of
these steps.
Start
End
Start
End
Enthalpy changes during dissolution
ΔHsoln = ΔH1 + ΔH2 + ΔH3
The enthalpy of
solution, ΔHsoln, can
be either positive or
negative.
ΔHsoln (MgSO4)= -91.2 kJ/mol --> exothermic
ΔHsoln (NH4NO3)= 26.4 kJ/mol --> endothermic
The term solubility refers to the maximum amount of material that
will dissolve in a given amount of solvent at a given temperature to
produce a stable solution.
Ack! What does this mean? It means how much solute (stuff) can
dissolve in a solvent.
It is dependant on temperature, how much solvent you have, how
much solute you have, and whether or not you mix the solution.
Nature of the solute and solvent
A solute is soluble when it dissolves completely in a solvent.
A solute is insoluble when it cannot dissolve in a solvent.
Some solutes are partially soluble, meaning only a bit dissolves
while the rest remains solid.
When two liquids totally mix they are said to be miscible.
An example of this would be alcohol and water.
When to liquids do not mix they are said to be immiscible.
An example would be oil and water:
“like dissolves like”
•
Two substances with similar intermolecular forces are
likely to be soluble in each other.
non-polar molecules are soluble in non-polar solvents
CCl4 in C6H6
•
polar molecules are soluble in polar solvents
C2H5OH in H2O
•
ionic compounds are more soluble in polar solvents
NaCl in H2O or NH3 (l)
polar dissolves polar
like dissolves like
Nonpolar dissolves nonpolar
SOLUBILITY
The solubility of a solute in a solvent at a particular temperature is the
number of grams of the solute necessary to saturate 100 gm of the
solvent at that temperature.
FACTORS AFFECTING SOLUBILITY
There are six main factors that control solubility of a solute.
(1) Temperature **
(2) Nature of solute or solvent
(3) Pressure
(4) Concentration **
(5) Time
(6) Mixing **
Effect of Temperature on Solubility
• Solubility increases when temperature goes up (most of the time).
• More solute dissolves in the solvent at higher temperatures.
• Solubility decreases when temperature goes down (most of the time).
•Less solute dissolves in the solvent at lower temperatures.
 In endothermic reactions solubility increases with the increase in temperature
and vice versa.
For example: solubility of potassium nitrate increases with the increase in
temperature.
 In exothermic reactions solubility decrease with the increase in temperature.
For example: solubility of calcium oxide decreases with the increase in
temperature.
 Gases are more soluble in cold solvent than in hot solvent.
Temperature and Solutions
Solubility: the maximum solute can dissolve in a given amount of
solvent
(in a given T).
T
Solubility 
T
Crystal is formed,
Solubility 
gas in liquid: T ↑
Solubility ↓
By looking at the plot of solubilities below, you can see that most
solids increase in solubility with an increase in temperature.
Solubility Curve
The y-axis (dependant) is
solubility: how much solute
dissolves in the solvent.
The x-axis (independent) is
temperature.
How much solute can
dissolve is dependant on the
temperature.
Gases, however, decrease in solubility with an increase in temperature.
Learning Check
A. Why would a bottle of carbonated drink possibly
burst (explode) when it is left out in the hot sun ?
B. Why would fish die in water that gets too warm?
Effect of Pressure on Solubility
Henry's Law states that "The amount of any given gas that will
dissolve in a liquid at a given temperature is a function of the partial
pressure of that gas in contact with the liquid..." What this means for
divers is that gas molecules will dissolve into the blood in proportion
to the partial pressure of that gas in the lungs (as "warm-blooded"
creatures, our core body temperature remains relatively constant).
Henry’s Law is used in breathalizers to
determine how much alcohol a person
has drunk.
Pressure and Solutions
Henry’s law
P
Solubility  (gas in liquid)
Pressure and Solubility of Gases
The solubility of a gas in a liquid is proportional to the
pressure of the gas over the solution (Henry’s law).
c is the concentration (M) of the dissolved gas
c = kP
P is the pressure of the gas over the solution
k is a constant (mol/L•atm) that depends only
on temperature
low P
high P
low c
high c
12.5
Effect of Concentration: Degrees of Saturation
• When referring to solutions, there are three degrees of saturation — unsaturated, saturated, and
supersaturated.
• If a solution is unsaturated, the solvent is capable of dissolving more solute.
• When the solution is saturated, the solvent has dissolved the maximum amount of solute
that it can at the given temperature.
• At this point we say that the solution is in a state of dynamic equilibrium—the
processes of dissolving and precipitation are happening at the same rate.
• A supersaturated solution is
one in which the solvent contains
more solute than it can
theoretically hold at a given
temperature.
• Supersaturated solutions
are often formed by heating
a solution and dissolving
more solute, then cooling
the solution down slowly.
These solutions are unstable
and crystallize readily.
Supersaturated solution
Seeding
A surface on which to
being crystallizing.
When a solution reaches saturation, some of the solid precipitates
out and forms a solid on the bottom of the beaker.
Sometimes a reaction happens when a solution is made, but one of
the products is insoluble in the solvent and precipitates out.
A precipitate
is a solid that
forms from a
solution,
typically
because the
solid is either
insoluble in
the solvent,
or is no
longer
soluble.
Concentration Terms
Solutions are often referred to as being concentrated or dilute. These two
terms are very general. While concentrated indicates that there is a lot of
solute dissolved in the solvent (perhaps the solution is near to being
saturated) and dilute indicates that a small amount of solute is dissolved in
the solvent, we often need to be exact with quantities in chemistry.
Effect of Time: Diffusion & Dissolving
As time goes on, due to
the random (Brownian)
movement of the
molecules, the solute
will completely dissolve
in the solvent.
Effect of Mixing
You have done several experiments in
which you were instructed to add a
solute to a solvent and stir. What was
the reason behind the stirring?
When you stir a solute in a solution,
you increase the movement of the
molecules. When the molecules of the
solute move more and faster, they
come in higher contact with the
molecules of the solution. The
solution is able to attack the solute and
pull it apart, dissolving the solution.
Mixing causes the solute to dissolve in the solvent faster than if not
mixed.
Molarity (M)
The molarity of a solution is a measure of the number of moles of
solute per liter of solution. This is the most common concentration
unit used in chemistry. Let’s now run through how you calculate the
molarity of a solution.
Example
Calculate the molarity of a solution prepared by dissolving 20.0 g
of solid NaOH in enough water to make 100 mL of solution.
Explanation
Convert grams to moles:
Then convert mL to liters:
Then divide:
Molality (m)
The molality of a solution is a measure of the number of moles of solute
per kilogram of solvent. The molality is dependent on the mass of the
solvent in the solution. Try an example:
A solution is prepared by mixing 80.0 g of sodium hydroxide (NaOH) with
500.0 g of water. Calculate the molality of this solution.
Explanation
Convert grams of solute to moles:
Convert grams of solvent to kg:
Divide:
.
What is the molality of a 5.86 M ethanol (C2H5OH) solution
whose density is 0.927 g/mL?
m =
moles of solute
moles of solute
M =
mass of solvent (kg)
liters of solution
Assume 1 L of solution:
5.86 moles ethanol = 270 g ethanol
927 g of solution (1000 mL x 0.927 g/mL)
mass of solvent = mass of solution – mass of solute
= 927 g – 270 g = 657 g = 0.657 kg
m =
moles of solute
=
mass of solvent (kg)
5.86 moles C2H5OH
0.657 kg solvent
= 8.92 m
Changing Molarity to Molality
If we know the
density of the
solution, we can
calculate the
molality from
the molarity,
and vice versa.
Mass Percent (Weight Percent)
The mass percent of a solution is another way of expressing its
concentration.
Mass percent is found by dividing the mass of the solute by
the mass of the solution and multiplying by 100.
A solution of NaOH that is 28% NaOH by mass contains 28 g of NaOH for
each 100 g of solution. Here’s the equation:
Now try a problem involving the equation:
A solution is prepared by mixing 5.00 g ethanol (C2H5OH) with 100.0 g
water. Calculate the mass percent of ethanol in this solution.
Explanation
Plugging the values we were given into the mass percent equation, we get:
Convert % mass to Molarity
• What is the Molarity of a 95% acetic acid
solution? (density = 1.049 g/mL)
If you assume 1 L, that amount of solution = 1049 g
95% of the solution is acetic acid
1049 g solution x 0.95 = 997 g solute
997 g X 1 mol/60.05 g = 16.6 mol solute
Since we assumed 1 L, that’s 16.6 mol / 1 L or 16.6 M
Percent concentration:
Weight solute
× 100
Weight / volume (W / V)% =
Volume of solution (mL)
Weight / Weight (W / W)% =
Weight solute
× 100
Weight of solution
Volume solute (mL)
× 100
Volume / volume (V / V)% =
Volume of solution (mL)
Solubility
The maximum amount of solute that can
dissolve in a specific amount of solvent
usually 100 g.
g of solute
100 g water
Learning Check
At 40C, the solubility of KBr is 80 g/100 g H2O.
Indicate if the following solutions are
(S) saturated or (U) unsaturated
A. ___60 g KBr in 100 g of water at 40C
B. ___200 g KBr in 200 g of water at 40C
C. ___25 KBr in 50 g of water at 40C
Concentration
Parts per Million (ppm):
ppm =
g solute
× 106
g solvent
Parts per billion (ppb):
ppb =
g solute
× 109
g solvent
Dilution is the process of taking a more concentrated solution and adding water
to make it less concentrated. The more concentrated solution before the dilution is
performed is known as the stock solution. You can relate the concentration of the
stock solution to the concentration of the diluted solution using the equation
below:
C1V1 = C2V2
where C is concentration (molarity) and V is the volume, in liters, of the solution.
Try the following example using this equation.
What volume of 6.0 M sulfuric acid (H2SO4) must be used
to prepare 2.0 L of a 0.10 M H2SO4 solution?
Explanation
Just plug the numbers into the formula! Be careful to read closely.
C1V1 = C2V2
(6.0 M) (V1) = (0.10 M) (2.0 L)
V1 = 0.033 L or 33 mL should be measured out and then diluted by adding
enough water to make 2.00 L total volume.
This should be a review!!!
Dilution is the procedure for preparing a less concentrated
solution from a more concentrated solution.
Dilution
Add Solvent
Moles of solute
before dilution (i)
=
Moles of solute
after dilution (f)
MiVi
=
MfVf
How would you prepare 60.0 mL of 0.2 M
HNO3 from a stock solution of 4.00 M HNO3?
MiVi = MfVf
Mi = 4.00
Vi =
Mf = 0.200
MfVf
Mi
Vf = 0.06 L
Vi = ? L
0.200 x 0.06
=
= 0.003 L = 3 mL
4.00
3 mL of acid + 57 mL of water = 60 mL of solution
What is the mole Fraction of Gas A in
mixture I?
What is the mole fraction of Gas B in
mixture II?
Partial Pressure in terms of mole fraction:
XAPtotal = PA
(XA = mole fraction of A)
Example: If there are 3 moles of gas A, 4 moles of gas B and 5 moles
of gas C in a mixture of gases and the pressure of A is found to be 2.5 atm, what is the
total pressure of the sample of gases?
Fractional crystallization is the separation of a mixture of substances
into pure components on the basis of their differing solubilities.
Suppose you have 90 g
KNO3 contaminated with 10
g NaCl.
Fractional crystallization:
1. Dissolve sample in 100
mL of water at 600C
2. Cool solution to 00C
3. All NaCl will stay in
solution (s = 34.2g/100g)
4. 78 g of PURE KNO3 will
precipitate (s = 12 g/100g).
90 g – 12 g = 78 g
Fractional Distillation Apparatus
Gravimetric Analysis
1. Dissolve unknown substance in water
2. React unknown with known substance to form a precipitate
3. Filter and dry precipitate
4. Weigh precipitate
5. Use chemical formula and mass of precipitate to determine
amount of unknown ion
Titrations
In a titration a solution of accurately known concentration is added
gradually added to another solution of unknown concentration until
the chemical reaction between the two solutions is complete.
Equivalence point – the point at which the reaction is complete
Indicator – substance that changes color at (or near) the
equivalence point
Slowly add base
to unknown acid
UNTIL
the indicator
changes color
What volume of a 1.420 M NaOH solution is
Required to titrate 25.00 mL of a 4.50 M H2SO4
solution?
WRITE THE CHEMICAL EQUATION!
H2SO4 + 2NaOH
volume acid
25.00 mL x
M
moles acid
rx
2H2O + Na2SO4
moles base
acid
coef.
4.50 mol H2SO4
2 mol NaOH
1000 mL soln
x
1 mol H2SO4
M
volume base
base
x
1000 ml soln
1.420 mol NaOH
= 158 mL
Electrolytes
bulb
+
Na
Clelectrolyte +
-
Electrolyte: conduct an electric current.
Ionization
NaCl → Na+ + Cl-
strong electrolytes: molecules dissociate completely to ions (NaCl).
weak electrolytes: molecules dissociate partially to ions (CH3COOH).
nonelectrolytes: molecules do not dissociate to ions (DI water).
An electrolyte is a substance that, when dissolved in water,
results in a solution that can conduct electricity.
A nonelectrolyte is a substance that, when dissolved, results
in a solution that does not conduct electricity.
nonelectrolyte
weak electrolyte
strong electrolyte
Osmotic Pressure
Semipermeable
membrane
osmotic
pressure
Higher concentration → Higher osmotic pressure
Osmotic Pressure (p)
Osmosis is the selective passage of solvent molecules through a porous
membrane from a dilute solution to a more concentrated one.
A semipermeable membrane allows the passage of solvent molecules
but blocks the passage of solute molecules.
Osmotic pressure (p) is the pressure required to stop osmosis.
dilute
more
concentrated
Osmotic Pressure (p)
High
P
Low
P
p = MRT
Osmolarity (osmol) = M × i
M is the molarity of the solution
R is the gas constant
T is the temperature (in K)
M: molarity
i: number of particles
Osmolarity ↑ → Osmotic pressure ↑
Isotonic solution
Hypotonic solution
Crenation
Hypertonic solution
Hemolysis
A cell in an:
isotonic
solution
hypotonic
solution
hypertonic
solution
Dialysis
Dilute solution
Chemistry In Action:
Desalination
Freezing Point Depression
• The freezing point of a substance is defined as the temperature at which the
vapor pressure of the solid and the liquid states of that substance are equal.
• If the vapor pressure of the liquid is lowered, the freezing point decreases.
• Why is a solution’s freezing point depressed below that of a pure solvent?
• The answer lies in the fact that molecules cluster in order to freeze. They
must be attracted to one another and have a spot in which to cluster; if they
act as a solvent, solute molecules get in the way and prevent them from
clustering tightly together. The more ions in solution, the greater the effect
on the freezing point.
We can calculate the effect of these solute particles by using the following
formula:
Tf = Kf msolute i
where
Tf = the change in freezing point
Kf = molar freezing point depression constant for the substance (for water =
1.86ºC/m)
m = molality of the solution
i = number of ions in solution (this is equal to 1 for covalent compounds and is
equal to the number of ions in solution for ionic compounds)
Freezing-Point Depression
ΔTf = T f0 – Tf
T 0f is the freezing point of
the pure solvent
T f is the freezing point of
the solution
T f0 > Tf
ΔTf > 0
ΔTf = Kf m
m is the molality of the solution
Kf is the molal freezing-point
depression constant (0C/m)
Change in Freezing Point
Common Applications
of Freezing Point
Depression
Propylene glycol
Ethylene
glycol –
deadly to
small
animals
Freezing Point Depression
At what temperature will a 5.4 molal
solution of NaCl freeze?
Solution
∆TFP = Kf • m • i
∆TFP = (1.86 oC/molal) • 5.4 m • 2
∆TFP = 20.1 oC
FP = 0 – 20.1 = -20.1 oC
Boiling Point Elevation
• The boiling point of a substance is the temperature at which the vapor
pressure equals atmospheric pressure.
• Because vapor pressure is lowered by the addition of a nonvolatile
solute, the boiling point is increased. Why?
• Since the solute particles get in the way of the solvent particles
trying to escape the substance as they move around faster, it will take
more energy for the vapor pressure to reach atmospheric pressure,
and thus the boiling point increases.
• We can calculate the change in boiling point in a way that’s similar
to how we calculate the change in freezing point:
Tb = Kbmsolutei
where
Kb = molar boiling point elevation constant (for water = 0.51˚C/m)
Boiling-Point Elevation
ΔTb = Tb – T b 0
T b0is the boiling point of
the pure solvent
T b is the boiling point of
the solution
Tb > T b 0
ΔTb > 0
ΔTb = Kb m
m is the molality of the solution
Kb is the molal boiling-point
elevation constant (0C/m)
Change in Boiling Point
Common Applications of
Boiling Point Elevation
Now try a problem that deals with freezing point depression and boiling point
elevation.
Calculate the freezing point and boiling point of a solution of 100 g of ethylene
glycol (C2H6O2) in 900 g of water.
Explanation:
Calculate molality:
Freezing point depression = Tf = (m)(Kf)(i)
Tf = (1.79)(1.86)(1) = 3.33ºC
Freezing point = 0ºC - 3.33ºC = -3.33ºC
Boiling point elevation = Tb = (m)(Kb)(i)
Tb = (1.79)(0.51)(1) = 0.91ºC
Boiling point = 100ºC + 0.91ºC = 100.91ºC
What is the freezing point of a solution containing 478 g of
ethylene glycol (antifreeze) in 3202 g of water? The molar
mass of ethylene glycol is 62.01 g.
ΔTf = Kf mi
Kf water = 1.86 0C/m
478 g x
1 mol
62.01 g
moles of solute
=
m =
mass of solvent (kg)
= 2.41 m
3.202 kg solvent
ΔTf = Kf mi = 1.86 0C/m x 2.41 m x 1 = 4.48 0C
ΔTf = T f0 – Tf
Tf = T f0 – ΔTf = 0.00 0C – 4.48 0C = -4.48 0C
Colligative Properties of Nonelectrolyte Solutions
Colligative properties are properties that depend only on the number of
solute particles in solution and not on the nature of the solute particles.
Vapor-Pressure Lowering (Raoult’s Law)
P1 = X1 P 1
0
Raoult’s law
P 10
= vapor pressure of pure solvent
X1 = mole fraction of the solvent
If the solution contains only one solute:
X1 = 1 – X2
P 10 - P1 = DP = X2 P 10
X2 = mole fraction of the solute
SAMPLE EXERCISE Calculation of Vapor-Pressure Lowering
Glycerin (C3H8O3) is a nonvolatile nonelectrolyte with a density of 1.26 g/mL at 25°C.
Calculate the vapor pressure at 25°C of a solution made by adding 50.0 mL of glycerin
to 500.0 mL of water. The vapor pressure of pure water at 25°C is 23.8 torr.
Solution
Analyze: Our goal is to calculate the vapor pressure of a solution, given the volumes of
solute and solvent and the density of the solute.
Plan: We can use Raoult’s law to calculate the vapor pressure of a solution. The mole
fraction of the solvent in the solution, XA, is the ratio of the number of moles of solvent
(H2O) to total solution (moles C3H8O3 + moles H2O).
Solve: To calculate the mole fraction of water in the solution, we must determine the
number of moles of C3H8O3 and H2O:
We now use Raoult’s law to calculate the vapor pressure of water for the solution:
The vapor pressure of the solution has been lowered by 0.6 torr relative to that of pure water.
Colligative Properties of Nonelectrolyte Solutions
Colligative properties are properties that depend only on the number of
solute particles in solution and not on the nature of the solute particles.
Vapor-Pressure Lowering
P1 = X1 P 1 0
Boiling-Point Elevation
DTb = Kb m
Freezing-Point Depression
DTf = Kf m
Osmotic Pressure (p)
p = MRT
Colligative Properties of Electrolyte Solutions
0.1 m Na+ ions & 0.1 m Cl- ions
0.1 m NaCl solution
Colligative properties are properties that depend only on the number of
solute particles in solution and not on the nature of the solute particles.
0.1 m NaCl solution
van’t Hoff factor (i) =
0.2 m ions in solution
actual number of particles in soln after dissociation
number of formula units initially dissolved in soln
i should be
nonelectrolytes
NaCl
CaCl2
1
2
3
Change in Freezing
Point
Which would you use for the streets
of New Glarus to lower the
freezing point of ice and why?
Would the temperature make any
difference in your decision?
a) sand, SiO2
b) Rock salt, NaCl
c) Ice Melt, CaCl2
Colligative Properties of Electrolyte Solutions
Boiling-Point Elevation
ΔTb = i Kb m
Freezing-Point Depression
ΔTf = i Kf m
Osmotic Pressure (p)
p = iMRT
Tyndall Effect
The Tyndall Effect is caused by reflection of light by very small particles in
suspension in a transparent medium. It is often seen from the dust in the air
when sunlight comes in through a window, or comes down through holes in
clouds. It is seen when headlight beams are visible on foggy nights, and in
most X-File episodes when Moulder and Sculley check out some dark
place with flashlights.
Tyndall effect is easily seen using a laser pointer aimed at the the mist from
this ultrasonic humidifier's mist
In liquids the tyndall effect can be easily
seen by using a laser pointer. If you dilute
milk to where it is almost clear, or if you
have any type of sol, such as colloidal silver,
then the beam of the laser can be easily seen
as it travels through the liquid.
Tyndall effect is seen here using a laser
pointer. The glass on the left contains 5 ppm
of HVAC colloidal silver and the one on the
right is from the tap after the bubbles have
settled out.