Solutions
Solutions
Like Dissolves Like
Hydrophilic and
Hydrophobic Molecules
Soaps, Detergents, and DryCleaning Agents
Units of Concentration
Like Dissolves Like
By convention, we assume that one or more solutes dissolve in a solvent to form a mixture
known as the solution. The photographs that accompany this section illustrate what happens
when we add a pair of solutes to a pair of solvents.
Solutes:
I2 and KMnO4
Solvents: H2O and CCl4
The solutes have two things in common. They are both solids, and they both have a deep violet or
purple color. The solvents are both colorless liquids, which do not mix.
The difference between the solutes is easy to understand. Iodine consists of individual I2 molecules
held together by relatively weak intermolecular bonds. Potassium permanganate consists of K+
and MnO4- ions held together by the strong force of attraction between ions of opposite charge. It
is therefore much easier to separate the I2 molecules in iodine than it is to separate the ions in
KMnO4.
There is also a significant difference between the solvents: CCl4 and H2O. The difference between
the electronegativities of the carbon and chlorine atoms in CCl4 is so small (EN = 0.56) that there
is relatively little ionic character in the C
Cl bonds.
Even if there was some separation of charge in these bonds, the CCl4 molecule wouldn't be polar,
because it has a symmetrical shape in which the four chlorine atoms point toward the corners of a
tetrahedron, as shown in the figure below. CCl4 is therefore best described as a nonpolar
solvent.
The difference between the electronegativities of the hydrogen and oxygen atoms in water is much
larger (EN = 1.24), and the H
O bonds in this molecule are therefore polar. If the H2O molecule
was linear, the polarity of the two O
H bonds would cancel, and the molecule would have no net
dipole moment. Water molecules are not linear, however, they have a bent, or angular shape. As a
result, water molecules have distinct positive and negative poles, and water is a polar molecule, as
shown in the figure below. Water is therefore classified as a polar solvent.
Because water molecules are bent, or angular, they have
distinct negative and positive poles. H2O is therefore an
example of a polar solvent
Because the solvents do not mix, when water and carbon tetrachloride are added to a separatory
funnel, two separate liquid phases are clearly visible. We can use the relative densities of CCl4
(1.594 g/cm3) and H2O (1.0 g/cm3) to decide which phase is water and which is carbon
tetrachloride. The denser CCl4 settles to the bottom of the funnel.
When a few crystals of iodine are added to the separatory funnel and the contents of the funnel
are shaken, the I2 dissolves in the CCl4 layer to form a violet-colored solution. The water layer
stays essentially colorless, which suggests that little if any I2 dissolves in water.
When this experiment is repeated with potassium permanganate, the water layer picks up the
characteristic purple color of the MnO4- ion, and the CCl4 layer remains colorless. This suggests
that KMnO4 dissolves in water but not in carbon tetrachloride. The results of this experiment are
summarized in the table below.
Solubilities of I2 and KMnO4 in CCl4 and Water
I2
H2O
CCl4
insoluble
very soluble
KMnO4 very soluble insoluble
This table raises two important questions. Why does KMnO4 dissolve in water, but not carbon
tetrachloride? Why does I2 dissolve in carbon tetrachloride, but not water?
It takes a lot of energy to separate the K+ and MnO4- ions in potassium permanganate. But these
ions can form weak bonds with neighboring water molecules, as shown in the figure below.
KMnO4 dissolves in water because the energy released when
bonds form between the K+ ion and the negative end of the
neighboring water molecules and between the MnO4- ion and
the positive end of the solvent molecules compensates for
the energy it takes to separate the K+ and MnO4- ions.
The energy released when these bonds form compensates for the energy that has to be invested
to rip apart the KMnO4 crystal. No such bonds can form between the K+ or MnO4- ions and the
nonpolar CCl4 molecules. As a result, KMnO4 can't dissolve in CCl4.
The I2 molecules in iodine and the CCl4 molecules in carbon tetrachloride are both held together by
weak intermolecular bonds. Similar intermolecular bonds can form between I2 and CCl4 molecules
in a solution of I2 in CCL4. I2 therefore readily dissolves in CCl4. The molecules in water are held
together by hydrogen bonds that are stronger than most intermolecular bonds. No interaction
between I2 and H2O molecules is strong enough to compensate for the hydrogen bonds that have
to be broken to dissolve iodine in water, so relatively little I2 dissolves in H2O.
We can summarize the results of this experiment by noting that nonpolar solutes (such as I2)
dissolve in nonpolar solvents (such as CCl4), whereas polar solutes (such as KMnO4) dissolve in
polar solvents (such as H2O). As a general rule, we can conclude that like dissolves like.
Practice Problem 1:
Elemental phosphorus is often stored under water because it doesn't dissolve in water.
Elemental phosphorus is very soluble in carbon disulfide, however. Explain why P4 is soluble
in CS2 but not in water.
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Practice Problem 2:
The iodide ion reacts with iodine in aqueous solution to form the I3-, or triiodide, ion.
I-(aq) + I2(aq)
I3-(aq)
What would happen if CCl4 was added to an aqueous solution that contained a mixture of KI,
I2, and KI3?
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Hydrophilic and Hydrophobic Molecules
The family of compounds known as the hydrocarbons contain only carbon and hydrogen. Because
the difference between the electronegativities of carbon and hydrogen is small (EN = 0.40),
hydrocarbons are nonpolar. As a result, they do not dissolve in polar solvents such as water.
Hydrocarbons are therefore described as immiscible (literally, "not mixable") in water.
When one of the hydrogen atoms in a hydrocarbon is replaced with an -OH group, the compound is
known as an alcohol, as shown in the figure below. As might be expected, alcohols have
properties between the extremes of hydrocarbons and water. When the hydrocarbon chain is short,
the alcohol is soluble in water. Methanol (CH3OH) and ethanol (CH3CH2OH) are infinitely soluble in
water, for example. There is no limit on the amount of these alcohols that can dissolve in a given
quantity of water. The alcohol in beer, wine, and hard liquors is ethanol, and mixtures of ethanol
and water can have any concentration between the extremes of pure alcohol (200 proof) and pure
water (0 proof).
The structure of the alcohol known as ethanol.
As the hydrocarbon chain becomes longer, the alcohol becomes less soluble in water, as shown in
the table below.
Solubilities of Alcohols in Water
Formula
Name
Solubility in Water (g/100 g)
CH3OH
methanol infinitely soluble
CH3CH2OH
ethanol
infinitely soluble
CH3(CH2)2OH propanol
infinitely soluble
CH3(CH2)3OH butanol
9
CH3(CH2)4OH pentanol
2.7
CH3(CH2)5OH hexanol
0.6
CH3(CH2)6OH heptanol
0.18
CH3(CH2)7OH octanol
0.054
CH3(CH2)9OH decanol
insoluble in water
One end of the alcohol molecules has so much nonpolar character it is called hydrophobic
(literally, "water-hating"), as shown in the figure below. The other end contains an -OH group that
can form hydrogen bonds to neighboring water molecules and is therefore said to be hydrophilic
(literally, "water-loving"). As the hydrocarbon chain becomes longer, the hydrophobic character of
the molecule increases, and the solubility of the alcohol in water gradually decreases until it
becomes essentially insoluble in water.
One end of this alcohol molecule is nonpolar, and therefore
hydrophobic. The other end is polar, and therefore
hydrophilic.
People encountering the terms hydrophilic and hydrophobic for the first time sometimes have
difficulty remembering which stands for water-hating and which stands for water-loving. If you can
remember that Hamlet's girlfriend was named Ophelia (not Ophobia), you might be able to
remember that the prefix philo- is commonly used to describe love
for example, in
philanthropist, philharmonic, philosopher, and so on.
The data in the table above show one consequence of the general rule that like dissolves like. As
molecules become more nonpolar, they become less soluble in water. The table below shows
another example of this rule. NaCl is relatively soluble in water. As the solvent becomes more
nonpolar, the solubility of this polar solute decreases.
Solubility of Sodium Chloride in Water and in Alcohols
Formula of Solvent Solvent Name Solubility of NaCl (g/100 g solvent)
H2O
water
35.92
CH3OH
methanol
1.40
CH3CH2OH
ethanol
0.065
CH3(CH2)2OH
propanol
0.012
CH3(CH2)3OH
butanol
0.005
CH3(CH2)4OH
pentanol
0.0018
Soaps, Detergents, and Dry-Cleaning Agents
The chemistry behind the manufacture of soap hasn't changed since it was made from animal fat
and the ash from wood fires almost 5000 years ago. Solid animal fats (such as the tallow obtained
during the butchering of sheep or cattle) and liquid plant oils (such as palm oil and coconut oil) are
still heated in the presence of a strong base to form a soft, waxy material that enhances the ability
of water to wash away the grease and oil that forms on our bodies and our clothes.
Animal fats and plant oils contain compounds known as fatty acids. Fatty acids, such as stearic
acid (see figure below), have small, polar, hydrophilic heads attached to long, nonpolar,
hydrophobic tails.
Fatty acids are seldom found by themselves in nature. They are usually bound to molecules of
glycerol (HOCH2CHOHCH2OH) to form triglycerides, such as the triglyceride known as trimyristin,
which can be isolated in high yield from nutmeg, shown in the figure below.
These triglycerides break down in the presence of a strong base to form the Na+ or K+ salt of the
fatty acid, as shown in the figure below. This reaction is called saponification, which literally means
"the making of soap."
The saponification of the trimyristin extracted from nutmeg.
Part of the cleaning action of soap results from the fact that soap molecules are surfactants
they tend to concentrate on the surface of water. They cling to the surface because they try to
orient their polar CO2- heads toward water molecules and their nonpolar CH3CH2CH2... tails away
from neighboring water molecules.
Water can't wash the soil out of clothes by itself because the soil particles that cling to textile fibers
are covered by a layer of nonpolar grease or oil molecules, which repels water. The nonpolar tails
of the soap molecules on the surface of water dissolve in the grease or oil that surrounds a soil
particle, as shown in the figure below. The soap molecules therefore disperse, or emulsify, the soil
particles, which makes it possible to wash these particles out of the clothes.
Most soaps are more dense than water. They can be made to float, however, by incorporating air
into the soap during its manufacture. Most soaps are also opaque; they absorb rather than
transmit light. Translucent soaps can be made by adding alcohol, sugar, and glycerol, which slow
down the growth of soap crystals while the soap solidifies. Liquid soaps are made by replacing the
sodium salts of the fatty acids with the more soluble K+ or NH4+ salts.
Forty years ago, more than 90% of the cleaning agents sold in the United States were soaps.
Today soap represents less than 20% of the market for cleaning agents. The primary reason for
the decline in the popularity of soap is the reaction between soap and "hard" water. The most
abundant positive ions in tap water are Na+, Ca2+, and Mg2+ ions. Water that is particularly rich in
Ca2+, Mg2+, or Fe3+ ions is said to be hard. Hard water interferes with the action of soap because
these ions combine with soap molecules to form insoluble precipitates that have no cleaning power.
These salts not only decrease the concentration of the soap molecules in solution, they actually
bind soil particles to clothing, leaving a dull, gray film.
One way around this problem is to "soften" the water by replacing the Ca2+ and Mg2+ ions with
Na+ ions. Many water softeners are filled with a resin that contains -SO3- ions attached to a
polymer, as shown in the figure below. The resin is treated with NaCl until each -SO3- ion picks up
an Na+ ion. When hard water flows over this resin, Ca2+ and Mg2+ ions bind to the -SO3- ions on
the polymer chain and Na+ ions are released into solution. Periodically, the resin becomes
saturated with Ca2+ and Mg2+ ions. When this happens, it has to be regenerated by being washed
with a concentrated solution of NaCl.
When a water softener is "charged," it is washed with a
concentrated NaCl solution until all of the -SO3- ions pick up
an Na+ ion. The softener then picks up Ca2+ and Mg2+ ions
from hard water, replacing these with Na+ ions.
There is another way around the problem of hard water. Instead of removing Ca2+ and Mg2+ ions
from water, we can find a cleaning agent that doesn't form insoluble salts with these ions.
Synthetic detergents are examples of such cleaning agents. Detergent molecules consist of long,
hydrophobic hydrocarbon tails attached to polar, hydrophilic -SO3- or -OSO3- heads, as shown in
the figure below.
The structure of one of the components of a synthetic
detergent.
By themselves, detergents don't have the cleaning power of soap. "Builders" are therefore added
to synthetic detergents to increase their strength. These builders are often salts of highly charged
ions, such as the triphosphate (P3O105-) ion.
Cloth fibers swell when they are washed in water. This leads to changes in the dimensions of the
cloth that can cause wrinkles -- which are local distortions in the structure of the fiber
or even
more serious damage, such as shrinking. These problems can be avoided by "dry cleaning," which
uses a nonpolar solvent that does not adhere to, or wet, the cloth fibers. The nonpolar solvents
used in dry cleaning dissolve the nonpolar grease or oil layer that coats soil particles, freeing the
soil particles to be removed by detergents added to the solvent, or by the tumbling action inside
the machine. Dry cleaning has the added advantage that it can remove oily soil at lower
temperatures than soap or detergent dissolved in water, so it is safer for delicate fabrics.
When dry cleaning was first introduced in the United States between 1910 and 1920, the solvent
was a mixture of hydrocarbons isolated from petroleum when gasoline was refined. Over the years,
these flammable hydrocarbon solvents have been replaced by halogenated hydrocarbons, such as
trichloroethane (Cl3C-CH3), trichloroethylene (Cl2C=CHCl), and perchloroethylene (Cl2C=CCl2).
Units of Concentration
The concentration of a solution is defined as the amount of solute dissolved in a given amount of
solvent or solution.
Concentration =
amount of solute
amount of solvent or solution
There are many ways in which the concentration of a solution can be described.
The molarity (M) of a solution is defined as the ratio of the number of moles of solute in the
solution divided by the volume of the solution in liters.
M = moles of solute
liters of solution
Practice Problem 3:
At 25oC, a saturated solution of chlorine in water can be prepared by dissolving 5.77 grams
of Cl2 gas in enough water to give a liter of solution. Calculate the molarity of this solution.
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Mass percent is literally the percentage of the total mass of a solution that is due to the solute.
Mass percent = mass of solute
x 100%
mass of solution
A 3.5% solution of hydrochloric acid, for example, has 3.5 grams of HCl in every 100 grams of
solution. The concentration of a solution in units of moles per liter can be calculated from the mass
percent and density of the solution.
It is also possible to describe the concentration of a solution in terms of the volume percent. This
unit is used to describe solutions of one liquid dissolved in another or mixtures of gases. Wine
labels, for example, describe the alcoholic content as 12% by volume, because 12% of the total
volume is alcohol.
Volume percent = volume of solute
x 100%
volume of solution
Molarity is the concentration unit most commonly used by chemists. It has one disadvantage. It
tells us how much solute we need to make a solution, and it gives us the volume of the solution
produced, but it doesn't tell us how much solvent will be required to prepare the solution. We can
make a 0.100 M solution of CuSO4, for example, by dissolving 0.100 mole of CuSO4 5 H2O in
enough water to give one liter of solution. But how much water is enough? Because the CuSO4 5
H2O crystals occupy some volume, it takes less than a liter of water, but we have no idea how
much less.
When it is important to know how much solute and solvent are present in a solution, chemists use
two other concentration units: molality and mole fraction.
The molality (m) of a solution is defined as the number of moles of solute in the solution divided
by the mass in kilograms of the solvent used to make the solution.
Molality (m) = moles of solute
kilograms of solvent
A 0.100 m solution of CuSO4, for example, can be prepared by dissolving 0.100 mole of CuSO4 in
1 kilogram of water. Because the density of water is about 1 g/cm3, or 1 g/mL, the volume of
water used to prepare this solution will be approximately one liter. The total volume of the solution,
however, will be larger than 1 liter because the CuSO4 5 H2O crystals will undoubtedly occupy
some volume. As a result, a 0.100 m solution is slightly more dilute than a 0.100 M solution of the
same solute.
Practice Problem 4:
A saturated solution of hydrogen sulfide in water can be prepared by bubbling H2S gas into
water until no more dissolves. Calculate the molality of this solution if 0.385 grams of H2S
gas dissolve in 100 grams of water at 20oC and 1 atm.
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Molality has an important advantage over molarity. The molarity of an aqueous solution changes
with temperature, because the density of water is sensitive to temperature. Because molality is
defined in terms of the mass of the solvent, not its volume, the molality of a solution does not
change with temperature.
The ratio of solute to solvent in a solution can also be described in terms of the mole fraction of the
solute or the solvent in a solution. By definition, the mole fraction of any component of a solution
is the fraction of the total number of moles of solute and solvent that come from that component.
The symbol for mole fraction is a Greek capital letter chi:
C. The mole fraction of the solute is
defined as the number of moles of solute divided by the total number of moles of solute and
solvent.
Csolute =
moles of solute
moles of solute + moles of solvent
Conversely, the mole fraction of the solvent is the number of moles of solvent divided by the total
number of moles of solute and solvent.
Csolvent =
moles of solvent
moles of solute + moles of solvent
In a solution that contains a single solute dissolved in a solvent, the sum of the mole fraction of
the solute and the solvent must be equal to 1.
Csolute + Csolvent = 1
Practice Problem 5:
Calculate the mole fractions of both the solute and the solvent in a saturated solution of
hydrogen sulfide in water at 20oC and 1 atm.
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