Freezing Point
Depression
Purpose
To examine the effect of a solute on the freezing behavior of a solution (solvent + solute).
Background
If you live in an area that has cold winters, you know that salt is often applied to the roads when it
snows. The salt causes corrosion problems for the vehicles that use the salted roads, so what is the
reason for its use? From previous experience, you know that a pure liquid freezes to a solid at a
constant temperature unique to that substance. But what happens to the freezing point if a solute is
dissolved in the pure solvent (liquid phase)? Will the solution freeze at a higher or lower temperature
than the pure liquid? Will the temperature remain constant throughout the freezing process?
In this experiment, you will investigate the effect of a solute on the freezing behavior of a solution.
When you are done, you should understand WHY salt is used on roads in the winter.
Materials (per group)
Safety Goggles Bunsen burner
Balance (0.001 g) Timer w/ a second hand (phone works)
Test tube Wire screen
Ring stand and iron ring Solvent: BHT, C15H24O
Thermometer Solutes: Naphthalene, C10H8
Graph paper Para-Dichlorobenzene C 6H4Cl2
Safety First
In this lab, observe all precautions, especially the ones listed below.
Caution: Wear your safety googles. (All steps.)
Caution: BHT, Naphthalene, and para-Dichlorobenzene are irritants and mildly toxic. Keep off
skin and do NOT breathe fine dust.
Caution: Avoid inhaling vapors. Keep away from face.
Caution: Exercise care when working with an open flame. Tie back hair and loose clothing. Do
not use the burner near flammable materials.
Caution: Return or dispose of all materials according to the instructions of your teacher.
Procedure
As you perform the experiment, record your observations in Data Table 1.
Part A. Freezing Point of Pure Solvent – BHT
1. Measure 4.000 grams of BHT, to the nearest 0.001g. Place the BHT into a clean, dry, test tube.
2. Secure the test tube in a utility clamp. You may attach it to the ring stand in the water, but you
may find it easier to just hold it.
3. Heat the water and the test tube until the BHT starts to melt. Add the thermometer and stir
gently.
4. CAUTION: Do not heat the BHT above 90oC.
5. When the BHT reaches 85oC (or is completely liquid), discontinue heating by removing the test
tube from the water bath.
6. As the BHT cools, take the temperature readings every 20 seconds. Record all temperatures to
the nearest 0.5oC. Gently stir for as long as possible, but do not force the thermometer once
solids start to form. Collect data until the temperature drops to about 55 oC (or you get 5 of the
same temperature readings in a row).
Part B. The Freezing Point with 0.0050 Mole of Solute Added
7. Calculate the mass of 0.0050 mole of the solute assigned to you by your teacher. Record its
name in the data table. Measure out this calculated mass to the nearest 0.001 g and record the
measurement.
8. Carefully, transfer the solute to the test tube of BHT used in Part A. Reheat the mixture, adding
the thermometer and stirring as before. Heat the mixture to about 85 oC.
9. Remove the ‘solution’ from the hot water bath and repeat Step 6 for this mixture.
Part C. The Freezing Point with 0.010 Mole of Solute
10. Using the mixture from Part B, repeat Step 7 (in other words add another 0.0050 mole of
solute). The solution now contains 0.010 mole of solute.
11. Remelt the mixture again, heating it to 85 oC. When all of the solute has dissolved, turn off the
burner and repeat Step 6.
12. Follow your teacher’s instructions for the proper disposal of the materials.
Analyses and Conclusions
1. Plot a graph of temperature (oC) versus time (minutes) for all three freezing point
determinations. Plot and label all three curves on the same graph, connecting the points in a
smooth curve. If your curve shows a dip, it is likely due to super cooling. The true freezing point
of the solution can be found by disregarding the dip and taking any horizontal portion of the
graph to the vertical axis. If you do not have supercooling, and no horizontal portion of the
graph, have your teacher help you discern the freezing point.
2. From your graph determine the freezing point of the pure solvent (BHT), and of the two
solutions.
a. Pure BHT
b. BHT + 0.0050 mole solute
c. BHT + 0.010 mole solute
3. What was the magnitude of the freezing point depression, T1, caused by the addition of the
first 0.0050 mole of solute? (Hint: same as 2b since we are subtracting from 0 oC, water’s fp).
4. What was the magnitude of the freezing point depression, T2, caused by the addition of 0.010
mole of solute?
5. If collected, record the class averages for T1 and T2.
6. Compare T1 and T2 for the solute in your experiment. What general statement can you make
concerning the relationship between the number of moles of solute and the magnitude of the
freezing point depression?
7. Compare the freezing point depression caused by Naphthalene to that caused by para-
Dichlorobenzene. Does your data support the idea that the freezing point depression depends
only on the number of moles of particles in the solution? Explain?
8. The freezing point depression of potassium acetate in BHT is two times greater than that of the
other two solutes when an equal number of moles is used. Explain.
9. The freezing point of BHT appeared to remain relatively constant throughout the freezing
process. Was that the same for the freezing point of the solutions? Explain.
GOING FURTHER
Develop a Hypothesis
Based on the results of this lab, develop a hypothesis concerning how the boiling point
of a solution compares to that of the pure solvent.
