Physical Chemistry , 1st ed.

A begins to melt. This reduces the amount of A in the solid (indicated by the

dotted line above the solid line in Figure 7.22b). When enough A melts that

the solid has the eutectic composition, the solid melts evenly as if it were a

pure compound. After the solid melts evenly, the system is composed of a

single liquid phase.

As with azeotropes, eutectics may be ternary, quaternary, and so on, but

their phase diagrams get very complex very quickly. A few important eutectics

have an impact on ordinary life. Ordinary solder is a eutectic of tin and lead

(63% and 37%, respectively) that melts at 183°C, whereas the melting points

of tin and lead are 232°C and 207°C. Wood’s metal is an alloy of bismuth, lead,

tin, and cadmium (50 25 12.512.5) that melts at 70°C (lower than the boil-

ing point of water!) that can be used in overhead fire sprinkler systems. NaCl

and H 2 O make a eutectic that melts at 21°C, which should be of some in-

terest to communities that use salt on icy roads in the winter. (The composi-

tion of this eutectic is about 23 weight percent NaCl.) An unusual eutectic ex-

ists for cesium and potassium. In a 7723 ratio, this eutectic melts at 48°C!

This eutectic would be a liquid metal at most terrestrial temperatures (and be

very reactive toward water).

In many cases, the solid-liquid equilibria are much more complicated than

Figures 7.21 and 7.22 suggest. This is due to two factors. First, solids may not

be “soluble” in all proportions, so there may be regions of immiscibility in the

temperature-composition phase diagram. Second, two components may form

stoichiometric compoundsthat can act as pure components. For example, in the

phase diagram for Na and K solutions, a “compound” having the stoichiome-

try Na 2 K can form. The presence of this stoichiometric compound can further

complicate the phase diagram. Figure 7.23 shows this in a temperature-

composition phase diagram for a Na/K solid/liquid solution. Other phase dia-

grams can get much more complicated, as shown in Figure 7.24.

7.7 Solid/Solid Solutions 191

MPB

M

N

MPA

Temperature

xA

0 0.2 1

(b)

Liquid

Solid

Liquid + solid

Liquid

+ solid

MPB

M

N

MPA

Temperature

xA

0 0.2 1

(a)

Liquid

Solid

Liquid + solid

Liquid

+ solid

Figure 7.22 The phase diagram described in
Example 7.13.

0 °C

97.5°C

0

Mole fraction Na

0.29

62 °C

• 27 °C

7 °C

0.66 1

Two phases

(liquid/solid K)

Two phases

(solid Na 2 K/solid Na)

Two phases

(liquid/solid Na 2 K)

Two phases

(liquid/solid Na)

Liquid

Two phases

(solid K/solid Na 2 K)

Figure 7.23 A more complicated solid solu-
tion phase diagram. This is for the Na/K system.
This phase diagram shows the existence of a stoi-
chiometric compound, Na 2 K.Source:Adapted
from T. M. Duncan and J. A. Reimer,Chemical
Engineering Design and Analysis: An Introduction,
Cambridge University Press, 1998.