The Foundations of Chemistry

EFFECT OF TEMPERATURE ON SOLUBILITY

In Section 13-6 we introduced LeChatelier’s Principle, which states that when a stress is

applied to a system at equilibrium, the system responds in a way that best relieves the stress.Recall

that exothermic processes release heat and endothermic processes absorb heat.

Exothermic: reactants88nproducts
heat

Endothermic: reactants
heat88nproducts

Many ionic solids dissolve by endothermic processes. Their solubilities in water usually

increaseas heat is added and the temperature increases. For example, KCl dissolves

endothermically.

H 2 O

KCl(s)
17.2 kJ8888nK
(aq)
Cl(aq)

Figure 14-6 shows that the solubility of KCl increases as the temperature increases because

more heat is available to increase the dissolving process. Raising the temperature (adding

heat) causes a stress on the solubility equilibrium. This stress favors the process that

consumesheat. In this case, more KCl dissolves.

14-6

Solid iodine, I 2 , dissolves to a
limited extent in water to give an
orange solution. This aqueous
solution does not mix with nonpolar
carbon tetrachloride, CCl 4 (left).
Iodine is much more soluble in the
nonpolar carbon tetrachloride. After
the funnel is shaken and the liquids
are allowed to separate (right), the
upper aqueous phase is lighter
orange and the lower CCl 4 layer is
much more highly colored. This is
because iodine is much more soluble
in the nonpolar carbon tetrachloride
than in water; much of the iodine
dissolves preferentially in the lower
(CCl 4 ) phase. The design of the
separatory funnel allows the lower
(denser) layer to be drained off.
Fresh CCl 4 could be added and the
process repeated. This method of
separation is called extraction.It takes
advantage of the different solubilities
of a solute in two immiscible liquids.

552 CHAPTER 14: Solutions

See the Saunders Interactive
General Chemistry CD-ROM,
Screen 14.6, Factors Affecting Solubility
(2): Temperature and LeChatelier’s
Principle.

Figure 14-6 A graph that illustrates the effect of temperature on the solubilities of some

salts. Some compounds exist either as nonhydrated crystalline substances or as hydrated

crystals. Hydrated and nonhydrated crystal forms of the same compounds often have

different solubilities because of the different total forces of attraction in the solids. The

discontinuities in the solubility curves for CaCl 2 and Na 2 SO 4 are due to transitions between

hydrated and nonhydrated crystal forms.

200

180

160

140

120

100

80

60

40

20

0

0 102030405060708090100

Temperature in degrees Celsius

Solubility in grams per 100 g (water)

KNO 3

CaCl 2

Na 2 SO 4

K 2 SO 4

NaCl

KBr

KCl

KI

CaC

l 2 

6 H^2

O

CaC

l 2 

4 H

O 2

Na^2

SO

4 

10

H^2

O