Lesson 10:
Solutions
Concepts conveyed | Materials | Procedure | Benefits | Resource
Concepts conveyed:
The purpose of this series of classroom activities is to illustrate colligative properties of a homogeneous solution and immiscibility.
Materials:
(for a group of about 60 students)
- 1 ice cream maker
- 1 bag of rock salt (~ 10 lb.)
- ingredients to make whatever flavor of homemade ice cream you would like (see below for example)
- 1 metal can (should hold 2 cups)
- 1 thermometer
- 1 bag of ice (~ 8 lb.)
- 3 tablespoons table salt
- 60 plastic spoons and cups
- 1 quart glass jar with a lid
- 1 bottle of shampoo
- 1 cup of cooking oil
- 1 and 1/2 cup water
- blue food coloring
- a stopwatch or a watch with a second hand
Example ingredients for homemade ice cream:
- 6 eggs
- any fruit, flavored syrups, nuts, etc. (quantity is up to the maker)
- 2 cans of evaporated milk
- 2 tablespoons of vanilla
- 2 cups sugar
- enough 2% milk to fill the remainder of the container
Procedure:
Part 1: Colligative Properties
Before the class period, prepare enough homemade ice cream for everyone in the class to have a sample. Divide up the ice cream into cups and keep in a freezer until the appropriate moment. Prepare the ingredients for another container of ice cream so they are ready to be placed into the machine during class time. After students arrive, ask for two volunteers. First, demonstrate the effect that salt has on the temperature of water. Do this by asking the students to take the metal can, fill it with crushed ice and 1 cup of water, and wait 2-3 minutes until condensation of moisture in the air collects on the outside of the can. Have the students check the temperature of the water in the can with a thermometer. Then, instruct the students to add 3 tablespoons of table salt. They should stir the icy, salty water, and allow the can to sit for a few minutes until a thin layer of frost forms on the outside of the can. Make sure they check the temperature after they add the salt, as well. The salt lowers the temperature of the water, freezing the water molecules on the outside of the can. This illustrates how rock salt lowers the freezing point, and thus, allows the temperature of the ice/water bath to acquire a lower equilibrium temperature, and aids in the freezing of the ice cream.
Next, start making the homemade ice cream. Ask for four new volunteers. One student will make the ice cream, one will be the time keeper, one will record the data, and the other will read the thermometer. Take the temperature of the brine (i.e. rock salt and ice water mixture) solution every five minutes until the ice cream is finished. The time keeper will tell the student reading the thermometer when each five-minute period is over. The student making the ice cream will continue adding rock salt to maintain a constant temperature. Use part of the time the students are making the ice cream to begin discussing the definition of colligative properties and how they relate to solutions. (Note: The ice cream should take 20-30 minutes to make.) To reduce the time spent in class spooning the freshly made ice cream into cups and serving it, the pre-made ice cream, which is already divided up into cups, can be served instead.
Part 2: Immiscibility
An oil/water mixture.
Add one cup of cooking oil, one and one-half cups of water, and five drops of blue food coloring to the one-quart glass jar. (The water-soluble food-coloring will dissolve primarily in the water and not the oil, making it easier to see the difference between the water and oil phases.) Place the lid on the jar and shake. In a few minutes, the oil and water will separate into layers. This illustrates that oil and water cannot form a solution (i.e. homogeneous mixture), only an emulsion.
Effect of shampoo on oil/water emulsions.
The results from this demonstration can be compared to the effect that shampoo has on the oils on human skin and hair. Pour one-half cup shampoo into the jar, return the lid and shake. The emulsion should become a soapy solution. Here, you have the option to explain the chemistry that is taking place. For example, you can discuss the concept of like substances dissolve other like substances. Part of a soap molecule has oil-like properties and the other part has water-like properties. Thus, soap dissolves in both the oil and the water, creating way for water to “dissolve” in the oil, breaking up the oil/water emulsion.
Benefits:
- Instructors can use this activity as a means to check classroom attendance by requiring students to hand in paper cups from the ice cream with their signatures on them.
- This is a fun and tasty way to discuss and to remember solutions and colligative properties.