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The Colligative Properties of Solutions

Description of Activity

Title: The Colligative Properties of Solutions.

Overview: In this series of activities, small groups (2-4) of students determine the densities of water, antifreeze, and a variety of water antifreeze solutions. The students then graph density versus percent solution antifreeze. Next, students determine and plot the boiling points and freezing points of the various water-antifreeze solutions.

Materials: Skill Development Activity:

Graduated Cylinder
Centigram Balance
Ten 15-mL test tubes with stoppers
Test tube rack

Commercial Antifreeze: (It is recommended that environmentally safe antifreeze be used. For another safe alternative substance with the same density as toxic antifreeze, add 2.5 mL Karo syrup to 7.5 mL propylene glycol.)

Inquiry Lab Activity: Materials used in the Skill Development Lab. Solutions of various percent antifreeze prepared in the skill development lab. Thermometer or temperature probe. Boiling Chips (to prevent super-heating).

Background

Colligative properties are properties of solutions that depend on the amount of solute particles in the solution (concentration) and are independent of the nature of the solute. Freezing point depression, boiling point elevation, vapor pressure lowering, and osmotic pressure are all colligative properties.

When water is the solvent, the boiling point of water will increase 0.512C for each 76 grams of propylene glycol (antifreeze) added to 1000 grams of water. The freezing point will decrease 1.86C for each of 76 grams of propylene glycol added to 1000 grams of water.

Teaching and Learning Strategies

Students determine the boiling and freezing points of the solutions. They notice that density is a good indicator of boiling point. When students determine the freezing point of various solutions, they notice that the freezing point steadily decreases then begins to increase. This discrepant event helps students to realize that density is correlated to, but not the cause of, the changes in the boiling and freezing points.

This laboratory exercise gives students an opportunity to notice that as antifreeze is added:

a. The density of the solution increases,
b. As the density of the solution increases, the boiling point of the solution also increases, and
c. As the density of the solution increases, the freezing point, however, decreases initially, but then begins to increase again.

Prerequisite instruction:

A discussion of the calculation of density and the proper use of a balance are necessary prior to starting the activities. Additionally students may need instruction about boiling point and freezing point determination depending on previous lab experience.

Boiling points of water-antifreeze solutions are easy to determine in a laboratory and require minimal equipment, although students need to be extremely careful. Pour 2 to 3 mL of the solution to be tested in a clean test tube and add a boiling chip to prevent super-heating. Measure the boiling point by recording the temperature at which the liquid first starts to boil. Remember that the boiling point changes upon prolonged boiling of a solution. Therefore, record the boiling point as early as possible.

Determining the freezing points of water-antifreeze solutions is much more difficult due to the extreme cold required. However, since substances melt and freeze at the same temperature, the freezing points of water-antifreeze solutions are equal to the melting points of ice-antifreeze solutions, which are easily determined. Students measure 90 grams of ice and place it in a Styrofoam cup. Next, they place a thermometer in the ice and stir carefully. Reading the temperature every 30 seconds until the temperature remains constant. This is the freezing point of ice. (The freezing point of water equals the melting point of ice.) Students then add 10 grams of antifreeze to the 90 grams of ice in the cup and measure the freezing point of the 10-percent antifreeze solution by recording the lowest temperature reached. Record the freezing point in data tables. You may wish to assign various lab stations the following ice-antifreeze mixtures: 80 grams ice--20 grams antifreeze, 70 grams ice--30 grams antifreeze, 60 grams ice--40 grams antifreeze, 50 grams ice--50 grams antifreeze, 40 grams ice--60 grams antifreeze, 30 grams ice--70 grams antifreeze, 20 grams ice--80 grams antifreeze, 10 grams ice--90 grams antifreeze. Record the results on the chalkboard or the overhead.

Safe Operating Procedures:

Safety glasses and aprons should be worn at all times in the chemistry laboratory.

Chemical Caution: Ethylene-glycol-based antifreeze is highly toxic. Biodegradable antifreeze is recommended as many of the disposal problems are eliminated. Another safe alternative substance with the same density as toxic antifreeze, add 2.5 mL Karo syrup to 7.5 mL propylene glycol.

The use of boiling chips will help prevent super-heating.

Dispose of all chemicals properly. See Flinn Chemical Catalog.

Skills Development Lab

Density of Anti-freeze water solutions

Density is a characteristic property of matter. Different substances exhibit different densities. Density is defined as the mass of a substance divided by its volume. In the lab, you will determine the densities of water, antifreeze, and various solutions of antifreeze and water.

Procedure:

1. Determine and record the mass of a clean, dry 10-mL graduated cylinder.
2. Carefully fill the graduated cylinder with 10.0 mL of tap water.
3. Determine and record the mass of the graduated cylinder plus the water.
4. Empty the water and dry the graduated cylinder.
5. Carefully fill the graduated cylinder with 10 mL of antifreeze.
6. Determine and record the mass of the graduated cylinder plus the antifreeze.
7. Pour the antifreeze into a labeled test tube and save it for the next experiment.
8. Measure 1.0 mL of antifreeze in the dry graduated cylinder and fill to 10.0 mL with water. Record the mass. Pour this solution into a test tube labeled "10% solution". Save the solution for the next experiment.
9. Measure 2.0 mL of antifreeze into the graduated cylinder and fill to 10.0 mL with water. Record the mass and then pour this solution into a test tube labeled "20% solution". Save the solution for the next experiment.
10. Make 10 mL each of a 30%, 40%, 50%, 60%, 70%, 80%, and 90% antifreeze solution. Record the mass of each solution, label according to its percentage and save for the next experiment.
11. Clean up appropriately.

Assessment of Skill Development Lab:

Find the density of each of the solutions that were prepared. Graph density versus percent solution of antifreeze. Let pure water be 0 percent antifreeze and pure antifreeze be 100 percent antifreeze.

1. Is density a good indicator of concentration? Explain your reasoning.
2. Describe the general shape and trends in the graph you constructed.

Invitation to Learn

Antifreeze is used in car radiators to help prevent it from freezing or boiling over. Now that you can make different percent concentrations, how does changing percent concentration of antifreeze affect the boiling and freezing points of the water/ antifreeze solutions? What percent solution is most effective in preventing freezing and boiling in your car's radiator?

Assessment of Learning:

Suggested Evaluation Tools

The formal written lab report is, of course, an acceptable evaluation tool.

Conclusions:

1. What happened to the density of the water/antifreeze solution as the percentage of antifreeze changed?
2. Predict how density is related to boiling point and freezing point.
3. What happened to the freezing point when you had at least 50% antifreeze? Why did this happen?

Alternative Evaluation Tools

Prepare a newspaper advertisement which describes how to prepare a solution for your radiator which offers the greatest summer (boiling point elevation) and winter (freezing point depressions) benefits.

1. If solution A contains 5 grams of NaCl in 100 grams of water and solution B contains 10 grams of NaCl in 100 grams of water, which of the following comparisons is true?

A. A has a higher boiling pint and a higher freezing point than B.
B. A has a lower boiling point and a lower freezing point than B.
C. A has a lower boiling point and a higher freezing point than B.
D. A has a higher boiling point and a lower freezing point than B.

2. Adding sodium chloride to water will cause the:

A. boiling point to rise and the freezing point to lower.
B. boiling point to lower and the freezing point to rise.
C. both boiling point and freezing point to rise.
D. both boiling point and freezing point to lower.

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Updated September 25 1997 by Michelle Dumas