experimental values, especially considering the assumptions made in deriv-
ing equation 7.39.Example 7.11
Use equation 7.39 to justify the effect on solubility for a compound if the
temperature is increased. Assume that the temperature is lower than the melt-
ing point of the pure solute.SolutionIf (^) fusHis positive (and by definition, it is), then the term ( (^) fusH)/Ris neg-
ative. When the temperature is increased, 1/Tgets smaller, so the value of
[(1/T(1/TMP)] gets smaller. (When T TMP,1/T 1/TMP. So as 1/Tgets
smaller, the difference [(1/T) (1/TMP)] gets smaller.) Therefore, the prod-
uct [( (^) fusH)/R][1/T) (1/TMP)] becomes a smaller negative number as T
increases. The inverse logarithm of a smaller negative number is a larger dec-
imal number. So, as Tincreases,xdissolved soluteincreases. In other words, as the
temperature is increased, the solubility of the solute increases. This is consis-
tent with almost all solutes. (There are a few solutes that decrease in solubil-
ity with increase in temperature, but they are rare.)
7.7 Solid/Solid Solutions
Many solids are actually solutions of two or more solid components. Alloys are
solid solutions.Steelis an alloy of iron, and there are many kinds of steel whose
properties depend on the other components of the solution as well as their
mole fraction, as shown in Table 7.2.Amalgamsare alloys of mercury. Many
dental fillings are amalgams, which are alloys of mercury (although the per-
ceived danger—not the actual danger!—of mercury poisoning is making amal-
gam fillings less popular). Bronze (an alloy of copper and tin), brass (an alloy
of copper and zinc), solder, pewter, colored glass, doped silicon for semicon-
ductors—are all examples of solid solution.188 CHAPTER 7 Equilibria in Multiple-Component Systems
Table 7.2 Examples of solid/solid solutionsa
Name Composition Uses
Alnicob 12 Al,20 Ni, 5 Co, remainder Fe Permanent magnets
Monimax 47 Ni, 3 Mo, remainder Fe Wire for electromagnets
Wood’s metal 50 Bi, 25 Pb, 12.5 Sn, 12.5 Cd Fire sprinkler systems
Solder 25 Pb, 25 Sn, 50 Bi Low-melting solder
Stainless steel #304 18–20 Cr, 8–12 Ni, 1 Si, 2 Mn, A standard stainless steel
0.08 C, rest Fe
Stainless steel #440c 16–18 Cr, 1 Mn, 1 Si, 0.6–0.75 C, High-quality stainless steel
0.75 Mo, rest Fe
Babbitt metal 89 Sn, 7 Sb, 4 Cu Bearing friction reduction
Constantan 45 Ni, 55 Cu Thermocouples
Gunmetal 90 Cu, 10 Sn Guns
Sterling silver 92.5 Ag, 7.5 Cu (or other metal) Durable silver items
aAll numbers are in weight percent.
bThere are several different alnico compositions, some of which have other metallic components.
cThere are dozens of types of stainless steel, each with its own unique properties.