New Jersey Center for Teaching and Learning
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Progressive Science Initiative
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Solutions:
Mixtures, Solubility and
Concentration
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Solutions
The infamous saltwater crocodile cannot survive in freshwater. It
needs a mixture of water and many solutes.
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Mixtures vs. Pure Substances
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Mixtures contain two or more pure substances. Mixtures do not
obey the law of definite composition therefore the relative
amounts of each substance can vary depending on the sample.
Pure water
Salt water
contains
H2O
89% O, 11% H
chemical
H2O, Ca2+, Cl-, Na+.....
composition by mass
varies
separation method
physical
All mixtures can be separated using physical means while
most pure substances cannot with the exception of thermal
decomposition of certain pure substances such as metal
carbonates and metal chlorates.
What are some methods for physically separating mixtures?
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Mixtures
Mixtures are classified as suspensions, colloids, or solutions
based on particle size.
Suspension
Particle Size
Colloid
> 1000 nm 1000 nm <-->1 nm
Solution
< 1 nm
Settling
Yes
No
No
Homogenous
No
Yes
Yes
Tyndall Effect
(particles
scatter light)
Yes
Yes
No
Given the similarity between colloids and solutions, the
Tyndall effect is often key to distinguishing them apart.
Click here to see a short
animation of the Tyndall Effect
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Suspensions
Due to their large particle size, suspensions can often be
separated by filtration.
When precipitates form a mixture of aqueous solutions, a
suspension is created with the solid precipitate settling out in an
aqueous medium. The solid precipitate can be easily separated
by proper filtration.
filtration
precipitate
+
Other examples of suspensions include sand in water and
snow in air.
Colloids
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Due to their smaller particle size, colloids cannot be separated
by filtration. In addition, the particles neither settle nor dissolve
in the greater medium.
Fog is a classic example of a colloid, as the water droplets
neither dissolve in the surrounding gaseous medium nor do
they settle out.
The Tyndall Effect is easy to see when driving through fog as
the light from the headlights gets scattered by the particles as
visualized below by sunshine on a foggy morning.
Solutions
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Solutions contain the smallest solute particles that dissolve in a
medium called the solvent. Solutions are homogeneous mixtures
because regardless of sample size, the ratio of solute particles to
solvent remains the same.
Uniform mixture of
solute (purple) and
solvent (green)
If you took a 5mL sample or a 200mL sample of a solution
you would find the exact same ratio of solute to solvent.
Solutions
Solutions contain the smallest solute particles that dissolve in a
medium called the solvent. Solutions are homologous mixtures
because regardless of sample size, the ratio of solute particles to
solven remains the same.
Salt water is a classic example of a solution. The Na+ and Clions are dissolved in the solvent, creating a uniform material.
+
-
++
+
+
Cl-
+
+
Na+
+ +
+ +
Cl-
- Na+ - +
+ +
+
+
Na+
- + +
Cl+ +
Due to their small size and interactions with the solvent, the
solute particles cannot be filtered out.
How could you physically separate solutes from solvents in a
solution?
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1 Which of the following would be TRUE regarding
mixtures?
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A A sample of a mixture will never be uniform in
composition
B They can be typically separated using only
chemical methods
C They vary in composition from sample to sample.
D Only solutions are considered true mixtures
E None of these are true
2 A mixture cannot be separated by filtration and does
not demonstrate the Tyndall Effect. Which of the
following could this mixture be?
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A NaCl(s)
B NaCl(aq)
C Sand dissolved in water
D Fog in the air
E Pure water
3 Which of the following physical methods is often
employed to separate a suspension?
A Distillation
B Filtration
C Evaporation
D Chromatography
E Lithography
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4 Which of the following would NOT be TRUE of a
solution?
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A There are no interactions between the solute and
the solvent
B Solutions cannot be separated by filtration
C A sample of a solution will be uniform in
composition
D Solutions have smaller particles than do colloids
and suspensions
E Solutions do not demonstrate the Tyndall Effect
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Solubility
The solubility of a solute is defined as the amount of solute that
can dissolve in a certain quantity of solvent. The solubility of a
solute depends on its state and its affinity for the solvent.
Solubility is commonly expressed as g solute/100 g of solvent.
Substance
Solubility in water @23 C
CH3OH
infinite
CH3Cl
0.47 g/100 g water
CCl4
0.081 g/100 g water
Note: The more polar the molecule, the more affinity for water as it is
also polar. CCl4 is non-polar and therefore has an extremely small
solubility in water. The phrase "like dissolves like" is applicable here.
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Solubility
The solubility of a solute in a solvent is highly temperature
dependent. In general, solids and liquids dissolve better at higher
temperatures while gases are more soluble at lower temperatures.
Solubility of NaCl (g/100g water) at different temperatures.
0C
10 C
50 C
80 C
100 C
35.65
35.72
36.69
37.93
38.99
Solubility of NH3 gas (mL/100 mL water) at different temperatures.
0C
10 C
50 C
80 C
100 C
11.7
9.0
3.33
1.38
0.88
Note: The decrease in gas solubility with temperature can be explained by
remembering that if the gas molecules have a high kinetic energy, they are
likely to weaken any solute -solvent attraction and escape the solution.
5 Which of the following would likely be the LEAST
soluble in water?
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A CO2
B HCl
C PH3
D CHCl3
E CH3OH
6 Which of the following would be most likely to
dissolve in hexane (C6H14)?
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A CH3OH
B H2O
C Br2
D CH2Cl2
E NaCl
7 Which of the following is TRUE regarding solubility?
A O2 gas would be more soluble at 10 C than 20 C
B Polar substances are most miscible in non-polar
solvents
C In general, as the temperature increases, the
solubility of most solids decrease
D Solubility is not temperature dependent
E Solubility is not dependent on the polarity of the
solute or solvent
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8 Which of the following would NOT be a miscible pair of
solute and solvent?
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A KOH and H2O
B CCl4 and C6H6
C CH3OH and H2O
D CH3OH and CH3CH2OH
E CH3OH and CCl4
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Solubility
Saturated solutions contain the maximum dissolved solute at that
temperature. Unsaturated solutions contain less and
supersaturated solutions contain more.
Unsaturated
Saturated
Supersaturated
In a saturated solution, undissolved solute and dissolved
solute are in equilibrium.
In an unsaturated solution any new solute will dissolve
whereas in a supersaturated solution, the amount of
undissolved solute is growing.
Solubility Curves
A solubility curve shows how much solute can dissolve in a certain
amount of solvent at a given temperature.
Temperature in Celsius
The line represents the amount of solute necessary for the
dissolved and undissolved amounts to be in equilibrium - a
saturated solution. Below the line the solution is unsaturated and
above the line the solution is supersaturated.
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Solubility Curves
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Solubility, grams per 100mL H20
The solubility curve for a given salt is difficult to predict.
Solubility Curves
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The solubility curves for gases clearly show the inverse
relationship of gas solubility and temperature.
Note: Cooler streams have higher dissolved oxygen (DO)
levels than warmer streams therefore supporting a different
variety of life.
Solubility Curves
The solubility of a gas also depends on the pressure. The higher
the partial pressure of that gas above the liquid, the greater the
solubility. This is known as Henry's Law.
We can view the dissolved gas and undissolved gas
above a solution as an equilibrium situation.
Gas(dissolved) <--> Gas (undissolved)
If the partial pressure of the undissolved gas is
increased above a liquid, the equilibrium will shift left
and more gas will dissolve.
Gas(dissolved) <--> Gas (undissolved)
Note: This is how soft drink manufacturers carbonate your soda. They simply
crank up the partial pressure of CO2 above the liquid and that causes more CO2
to dissolve. They then smack a lid on top so the pressure is maintained.
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9 The curve to the right best represents the
solubility curve for
A KI
B C6H12O6
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Solubility (g/
100g H2O)
C CO2
Temperature
D NaCl
E None of these
10 How many moles of NaCl would dissolve in a
saturated solution containing 450 g of water @10 C.
Assume the solubility of NaCl is 35.72 g/100 g water
@10 C.
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Temperature in Celsius
11 Using the solubility curve below, how many grams of
solute could be recrystallized if the temperature is
dropped from 50 C to 20 C?
Temperature in Celsius
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12 Under which set of conditions will a gas dissolve
best?
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A High Temp, High Pressure
B Low Temp, High Pressure
C Low Temp, Low Pressure
D High Temp, Low Pressure
13 Which of the following explain why a soda will
become flat over time at a given temperature if left
open?
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A Once opened, the partial pressure has diminished
above the liquid thereby lowering the solubility
B The polarity of the gas changes as the pressure
changes
C The CO2 gas is replaced by other less bubbly gases
D CO2 is polar
14 As the temperature increases, what must be true
of the partial pressure of a gas above a liquid in
order to maintain the same solubility of that gas
in the liquid?
A It must be increased
B It must be decreased
C It has no influence, temperature plays the dominant
role
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Solution Concentration
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There are many ways to express the amount of solute
dissolved in a solvent.
Common Concentration Units
Molarity (M) = mol solute/L solution
Molality (m) = mol solute/kg solvent
Mole Fraction (X) = nsolute/ntotal
% by volume = Vsolute/Vsolution
% by mass = gsolute/gtotal
Molarity is temperature dependent unlike the others.
How would increasing the temperature of a solution affect its
Molarity (M)?
15 What is the molarity of an aqueous NaCl solution
@10 C assuming a solubility of 45.72 g NaCl/100 g
water?Assume a density of 1g/mL of water.
16 How many moles of OH- ion are present in a 100 mL
aqueous solution of 0.5 M Mg(OH)2?
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17 How many mL of water must have evaporated from a
200 mL 0.3 M HCl solution to produce a 0.5 M
solution of HCl?
18 A solution consisting of ethanol (CH3CH2OH) and
water is 25% ethanol by mass. What is the molality of
this solution?
19 An aqueous NaCl solution is heated. Assuming no
evaporation of the solvent, which of the following will
be TRUE?
A The molarity will increase
B The molarity will decrease
C The molality will decrease
D The molality will increase
E The mole fraction of solute will decrease
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20 What is the mole fraction of an aqueous 200 mL
solution that is 0.45 M CH3OH. Assume the density of
the solution is 1.03 g/mL.
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In the next section, we will examine the
dissolving process and how this
influences the many properties of a
solution....
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