The Foundations of Chemistry

Elevations of boiling points and depressions of freezing points, which will be discussed

later, are usually quite small for solutions of typical concentrations. They can be measured,

however, with specially constructed differential thermometers that measure small temper-

ature changes accurately to the nearest 0.001°C.

EXAMPLE 14-7 Boiling Point Elevation

Predict the boiling point of the 1.25 msucrose solution in Example 14-2.

Plan

We first find the increasein boiling point from the relationship TbKbm.The boiling point

is higherby this amount than the normal boiling point of pure water.

Solution

From Table 14-2, Kbfor H 2 O0.512°C/m,so

Tb(0.512°C/m)(1.25 m)0.640°C

The solution would boil at a temperature that is 0.640°C higher than pure water would boil.

The normal boiling point of pure water is exactly 100°C, so at 1.00 atm this solution is predicted

to boil at 100°C
0.640°C100.640°C.

You should now work Exercise 46.

FREEZING POINT DEPRESSION

Molecules of liquids move more slowly and approach one another more closely as the

temperature is lowered. The freezing point of a liquid is the temperature at which the

forces of attraction among molecules are just great enough to overcome their kinetic ener-

gies and thus cause a phase change from the liquid to the solid state. Strictly speaking,

the freezing (melting) point of a substance is the temperature at which the liquid and solid

phases are in equilibrium. When a dilute solution freezes, it is the solventthat begins to

solidify first, leaving the solute in a more concentrated solution. Solvent molecules in a

solution are somewhat more separated from one another (because of solute particles) than

they are in the pure solvent. Consequently, the temperature of a solution must be lowered

below the freezing point of the pure solvent to freeze it.

14-12

When a solution freezes, the solvent
solidifies as the pure substance. For
this photo a dye has been added. As
the solute freezes along the wall of
the test tube, the dye concentration
increases near the center.

564 CHAPTER 14: Solutions

TABLE 14-2 Some Properties of Common Solvents

Solvent bp (pure) Kb(°C/m) fp (pure) Kf(°C/m)

water 100* 0.512 0* 1.86

benzene 80.1 2.53 5.48 5.12

acetic acid 118.1 3.07 16.6 3.90

nitrobenzene 210.88 5.24 5.7 7.00

phenol 182 3.56 43 7.40

camphor 207.42 5.61 178.40 40.0

*Exact values.