Unit 7.3 Solutions: Solute - Solvent
Is snow an aqueous solution? In the winter, the
temperature often gets well below the freezing point of
water. This condition can create problem in car
radiators. If the water freezes, water hoses will break,
the engine block can crack, and significant damage can be
done to the car.
Solute and Solvent
When one substance dissolves into another, a solution is formed. A solution is a homogeneous
mixture consisting of a solute dissolved into a solvent. The solute is the substance that is being
dissolved, while the solvent is the dissolving medium. Solutions can be formed with many different types
and forms of solutes and solvents.
Types of Solutions
Type
Solvent
Solute
Example
gas/gas
nitrogen
oxygen
air
liquid/gas
water
carbon dioxide
soda pop
liquid/liquid
water
ethylene glycol
antifreeze
liquid/solid
water
salts
seawater
solid/solid
iron
carbon and chromium
stainless steel (alloys)
Gas Solutions
Our air is a homogeneous mixture of many different gases and therefore qualifies as a solution.
Approximately 78% of the atmosphere is nitrogen, making it the solvent for this solution. The next
major constituent is oxygen (about 21%), followed by the inert gas argon (0.9%), carbon dioxide (0.04%)
and trace amounts of neon, methane, helium, and other gases.
Solid-Solid Solutions
Solid-solid solutions such as brass, bronze, and sterling silver are called alloys. Bronze (composed mainly
of copper with added tin) was widely used in making weapons in times past dating back to at least 2400
B.C. This metal alloy was hard and tough, but was eventually replaced by iron.
A turkey timer is an example of an alloy in use. The timer has a plastic
casing, B, along with a plastic plunger, A. There is a spring, C, that will
push the plunger up, when the turkey reaches the appropriate
temperature. In the tip, is a solid solution alloy, D. The alloy holds the
plunger down until the turkey reaches the “done” temperature,
approximately 165 o F. At that temperature, the alloy melts allowing
the spring to push the plunger up., signaling that the turkey is fully
cooked.
Liquid-Solid Solutions
Perhaps the most familiar liquid-solid solution is dental amalgam, used to fill teeth when there is a cavity.
Approximately 50% of the amalgam material is liquid mercury to which a powdered alloy of silver, tin and
copper is added. Mercury is used because it binds well with the solid metal alloy. However, the use of
mercury-based dental amalgam has gone under question in recent years because of concerns regarding
the toxicity of mercury. Many filling are now out of a composite resin, which do not contain mercury.
We want to focus on solutions where the solvent is water. An aqueous solution is water that contains
one or more dissolved substances. The dissolved substances in an aqueous solution may be solids, gases,
or other liquids. Examples include vinegar (acetic acid in water), alcoholic beverages (ethanol in water),
and liquid cough medicines (various drugs in water).
In order to be a true solution, a mixture must be stable. When
sugar is fully dissolved into water, it can stand for an indefinite
amount of time and the sugar will not settle out of the solution.
Further, if the sugar-water solution is passed through a filter it
will be unchanged. The dissolved sugar particles will pass through
the filter along with the water. This is because the dissolved
particles in a solution are very small, usually less than 1 nm in
diameter. Solute particles can be atoms, ions, or molecules,
depending on the type of substance that has been dissolved.
How do you make sure a compound is pure? Solutions are
often used to purify compounds. When compounds are
synthesized, they often have contaminating materials
mixed in with them. The process of recrystallization can
be used to remove these impurities. The crystals are
dissolved in a hot solvent, forming a solution. When the
solvent is cooled the compound is no longer as soluble and
will precipitate out of solution, leaving other materials
still dissolved.
Saturated and Unsaturated Solutions
Table salt (NaCl) readily dissolves in water. Suppose that you have a beaker of water to which you add
some salt, stirring until it dissolves. So you add more and that dissolves. You keep adding more and more
salt, eventually reaching a point that no more of the salt will dissolve no matter how long or how
vigorously you stir it. Why? On the molecular level, we know that action of the water causes the
individual ions to break apart from the salt crystal and enter the solution, where they remain hydrated
(surrounded) by water molecules. What also happens is that some of the dissolved ions collide back again
with the crystal and remain there. Recrystallization is the process of dissolved solute returning to the
solid state. At some point the rate at which the solid salt is dissolving becomes equal to the rate at
which the dissolved solute is recrystallizing. When that point is reached, the total amount of dissolved
salt remains unchanged. Solution equilibrium is the physical state described by the opposing processes of
dissolution and recrystallization occurring at the same rate. The solution equilibrium for the dissolving of
sodium chloride can be represented by one of two equations.
NaCl (s)

NaCl (aq)
While this shows the change of state back and forth between solid and aqueous solution, the preferred
equation also shows the dissociation that occurs as an ionic solid dissolves.
NaCl (s)

Na+1 (aq)
+
Cl-1 (aq)
When the solution equilibrium point is reached and no more solute will dissolve, the solution is said to be
saturated. A saturated solution is a solution that contains the maximum amount of solute that is
capable of being dissolved. At 20°C, the maximum amount of NaCl that will dissolve in 100 mL of water
is 36.0 g of NaCl. If any more NaCl is added past that point, it will not dissolve because the solution is
saturated. What if more water is added to the solution instead? Now more NaCl would be capable of
dissolving in the additional
solvent. An unsaturated
solution is a solution that
contains less than the
maximum amount of solute
that is capable of being
dissolved. When 30.0 grams of
NaCl is added to 100 mL of
water, it all dissolves, forming
an unsaturated solution. When
40.0 grams is added, 36.0
grams dissolves and 4.0 grams
remains undissolved, forming a
saturated solution.
How can you tell if a solution is saturated or unsaturated? If more solute is added and it does not
dissolve, then the original solution was saturated. If the added solute dissolves, then the original solution
was unsaturated. A solution that has been allowed to reach equilibrium but which has extra undissolved
solute at the bottom of the container must be saturated.
How does a hot or a cold pack work? In the picture, a
thermal pack is applied to the back. Small packs can be used
either for heating or cooling, depending on the material
used. A heat pack contains a supersaturated solution of
material such as sodium acetate. The solution is clear until a
small metal trigger is activated. The sodium acetate then
crystallizes out of solution and generates heat in the
process.
Supersaturated Solutions
Some solutes, such as sodium acetate, do not recrystallize easily. Suppose an exactly saturated solution
of sodium acetate is prepared at 50°C. As it cools back to room temperature, no crystals appear in the
solution, even though the solubility of sodium acetate is lower at room temperature. A supersaturated
solution is a solution that contains more than the maximum amount of solute that is capable of
being dissolved at a given temperature. The recrystallization of the excess dissolved solute in a
supersaturated solution can be initiated by the addition of a tiny crystal of solute, called a seed crystal.
The seed crystal provides a nucleation site on which the excess dissolved crystals can begin to grow.
Recrystallization from a supersaturated solution is typically very fast. Rock candy is often made this way.
Summary

A solution is a homogeneous mixture of a solute in a solvent.

A solute is the material present in the smaller amount in the solution. It is being dissolved.

A solvent is the material present in the larger amount in the solution. It is doing the dissolving.

Saturated solutions contain the maximum amount of solute that can be dissolved.

Unsaturated solutions contain less than the maximum amount of solute that can be dissolved.

Supersaturated solutions contain more than the maximum amount of solute that can be dissolved.
This is done by heating the solution.

A supersaturated solution can recyrstallize when a seed crystal is added to the solution.
Review
1.
What is a solution?
2.
What is the solute?
3.
What is the solvent?
4.
Why is nitrogen the solvent in air?
5.
Why is the preferred equation for solution equilibrium of NaCl an equilibrium between solid NaCl
and ions?
6.
What will happen if a saturated solution of NaCl that also contains solid NaCl, has more water
added?
7.
How do we know a solution is supersaturated?
Answers
1.
A solution is a homogeneous mixture consisting of a solute dissolved into a solvent.
2.
The substance in the smaller amount in the solution. It is the substance that is dissolved. There
can be more than one solute.
3.
The substance in the greatest amount in the solution. It is the substance that does the
dissolving.
4.
Nitrogen is in the greatest amount in the air. It is 78% of the atmosphere.
5.
The equation showing solid NaCl and the ions, Na+1 and Cl-1 is a more accurate representation.
6.
If more water is added to a saturated solution of NaCl that also has solid NaCl in the bottom of
the container, more of the NaCl will dissolve. (If more NaCl is added, the NaCl will sink to the
bottom of the container. No more will dissolve.)
7.
Add one seed crystal of the solute. If it is supersaturated, the salt will recrystallize rapidly.