Chapter 15: Solutions - Weironline.net

Freezing Point Depression
The freezing point depression of a solution is another
colligative property of solutions. At a solvent’s freezing
point temperature, the particles no longer have
sufficient kinetic energy to overcome the interparticle
attractive forces; the particles form into a more
organized structure in the solid state. In a solution,
the solute particles interfere with the attractive forces
among the solvent particles. This prevents the solvent
from entering the solid state at its normal freezing
point. The freezing point of a solution is always
lower than that of a pure solvent. Figure 15-18
shows the differences in boiling and melting points
of pure water and an aqueous solution. By comparing
the solid and dashed lines, you can see that the
temperature range over which the aqueous solution
exists as a liquid is greater than that of pure water.
You can observe the effect of freezing point depression
by doing the miniLAB on this page.
A solution’s freezing point depression, �T f , is the difference
in temperature between its freezing point and the freezing point
of its pure solvent. For nonelectrolytes, the value of the freezing
point depression, which is symbolized as �T f , is directly proportional
to the solution’s molality.
miniLAB
�T f = K f m
Freezing Point Depression
Measuring The colligative property of freezing
point depression can be observed in a simple laboratory
investigation. You will measure the temperatures
of two beakers and their contents.
Materials 400-mL beakers (2), crushed ice, rock
salt (NaCl), water, stirring rods (2), graduated
cylinder, thermometers (2), balance
Procedure
1. Fill two 400-mL beakers with crushed ice. Add
50 mL of cold tap water to each beaker.
2. Stir the contents of each beaker with a stirring
rod until both beakers are at a constant temperature,
approximately one minute.
3. Measure the temperature of each beaker using
a thermometer and record the readings.
4. Add 75 g of rock salt to one of the beakers.
Continue stirring both beakers. Some of the
salt will dissolve.
1 atm
Increasing Pressure
Solution
SOLID
Freezing point
of solution
Pure solvent
LIQUID
�P
GAS
�Tf �Tb
Increasing Temperature
Normal freezing
point of water
Figure 15-18
Normal boiling
point of water
Boiling point
of solution
This phase diagram shows how temperature
and pressure affect the solid, liquid,
and gas phases of a pure solvent (solid
lines) and a solution (dashed lines). The
difference between the solid and dashed
lines corresponds to vapor pressure lowering
(�P), boiling point elevation (�T b ),
and freezing point depression (�T f).
5. When the temperature in each beaker is constant,
record the readings.
6. To clean up, flush the contents of each beaker
down the drain with excess water.
Analysis
1. Compare your readings taken for the ice water
and the salt water. How do you explain the
observed temperature change?
2. Why was salt only added to one of the beakers?
3. Salt is a strong electrolyte that produces two
ions, Na � and Cl � , when it dissociates in water.
Why is this important to consider when calculating
the colligative property of freezing
point depression?
4. Predict if it would it be better to use coarse
rock salt or fine table salt when making homemade
ice cream. Explain.
15.3 Colligative Properties of Solutions 473