Chemistry Reference
In-Depth Information
one another and the solute particles for one another in order for
significant dissolving to occur.
❒
Like dissolves like.
A. What type of solvent should be chosen to try to dissolve naphthalene,
a type of moth repellent?
C
10
H
8
,
15.2
Saturated, Unsaturated, and Supersaturated Solutions
Most solutes will dissolve only to a certain extent in a given solvent at a cer-
tain temperature. For example, only 6.35 g of boric acid, (used to bathe
infected eyes), will dissolve in 100 g of water at If we add more than
6.35 g of to the water at that temperature, the excess quantity
will not dissolve. We state that the
solubility
of
H
3
BO
3
30°C.
H
3
BO
3
H
3
BO
3
H
3
BO
3
in water at
30°C
is
6.35 g per 100 g of water.
A solution containing as much solute as it can stably hold at a given tem-
perature is said to be a
saturated solution.
If the solution contains less solute,
it is said to be an
unsaturated solution.
Thus, a solution containing 2.00 g of
in 100 g of water at is unsaturated.
A change in temperature affects the solubility of a solute in a given solvent
(Figure 15.2). Most solid solutes get more soluble in liquid solvents as the tem-
perature is raised, but gases dissolved in liquids get less soluble. If we have a
saturated solution of such a solid solute at a high temperature and we lower the
solution's temperature, we expect that the excess solute will crystallize from the
solution at the lower temperature. For example, at the solubility of
in water is 27.6 g per 100 g of water. If a solution containing 27.6 g of
in 100 g of water at is cooled to only 6.35 g of the will con-
tinue to be soluble, and 21.2
5
g of will crystallize from the solution. (This
type of process, called
recrystallization,
is often used to purify solid substances.)
When we cool saturated solutions of certain solutes, such as sodium acetate,
the solute often has difficulty forming the first crystal, resulting in an unstable
solution. More solute is in the solvent than is stable at the lower temperature.
This type of solution is called a
supersaturated solution.
If a supersaturated
solution is shaken or the inner surface of the container is scratched with a glass
rod, the excess solute may crystallize out. A more certain way to get a super-
saturated solution to crystallize is to add to it a tiny crystal of the solid solute,
on which the excess solute can crystallize (Figure 15.3).
H
3
BO
3
30°C
100°C,
H
3
BO
3
H
3
BO
3
100°C
30°C,
H
3
BO
3
H
3
BO
3
240
220
200
C
12
H
22
O
11
180
160
140
120
100
80
60
40
20
0
KNO
3
KBr
EXAMPLE 15.2
KCl
Na
2
SO
4
The number of grams of sodium acetate that will dissolve in 100 g of water is
119 g at and 170 g at
(a) If 170 g of sodium acetate is placed in 100 g of water at
NaCl
0°C
100°C.
Na
2
SO
4
•
10H
2
O
0°C,
how much
0
20
40
60
80
will dissolve?
(b) Is the resulting system homogeneous or heterogeneous?
(c) Ignoring any excess solid solute that may be present, is the solution
saturated, unsaturated, or supersaturated?
(d) If the system is then raised to
Temperature (°C)
Figure 15.2
Variation
of Solubility with Temperature
of Various Solutes in Water
100°C,
is it homogeneous or heterogeneous?
