Demonstration: Analyzing Mixtures FOR THE TEACHER Summary In this demonstration, the teacher will create a sequence of mixtures in a single test tube. Water, oil, food coloring and soap will be used in this demonstration to introduce the concepts of heterogeneous and homogeneous solutions as well as the idea of miscibility. Students will also have the opportunity to analyze the properties, such as surface tension of each component used. Grade Level Elementary, Middle or High School Objectives By the end of this demonstration, students (depending on grade level) should be able to Differentiate between heterogeneous and homogenous mixtures Define a solution Identify instances of surface tension Explain the properties of soap as an emulsifying agent Describe the ability of alike molecules to mix, and the inability of dislike molecules to mix Chemistry Topics This demonstration supports students’ understanding of Mixtures Solutions Physical Properties Time Teacher Preparation: 5 minutes Lesson: 10 minutes Materials ~10mL Oil (vegetable or canola) ~10mL of water 2 disposable pipets Food coloring Liquid soap/dish soap (colorless) Large test tube with stopper
Safety Students should wear proper safety gear during chemistry demonstrations. Safety goggles and lab apron are required.
American Association of Chemistry Teachers
|3-36
1
Teacher Notes This demonstration can be used at a variety of grade levels. Teachers should scale the content depending on the age level and ability. Elementary students focus on concepts: homogeneous mixtures, heterogeneous mixtures, solutions, surface tension Middle school students should focus on the elementary concepts and: physical properties of water, miscibility of solutions, properties of an emulsifying agent. High school students should focus on the elementary and the middle school concepts as well as: molecular structures of each solution and the impact on miscibility, as well as the molecular structure of an emulsifying agent (how does soap work?) As an alternative to a demonstration, teachers may want to give older students to opportunity to conduct this demonstration as lab activity, and have students complete the procedures individually. Demonstration Procedure: o Using a pipet, first fill the test tube approximately 1/3 full with water. o Next, using a new pipet add an equal amount of oil to the test tube so that it is 2/3 full. o The water and oil should be clearly separated in the test tube, and you will be able to see a line of surface tension dividing the two liquids. o Next, invert a bottle of food coloring so that it is directly above the test tube opening. o Add one drop to the test tube (2-3 drops will work also), attempting to aim the drop in the center so it does not touch the sides of the test tube.
o o o
o
o
The food coloring will form a sphere and will not mix with the oil. This is a good opportunity to talk about miscibility, molecular structure and heterogeneous solutions depending upon grade level. The food coloring will “sit” on the surface tension line that divides the oil and the vinegar. You will need to flick the test tube will your finger or gently tap the end of the test tube to break the barrier. Note: this can take multiple attempts! Again it will provide a good opportunity to discuss surface tension. The food color will break through the surface tension barrier and will mix quickly with the water, forming a colored, homogeneous, miscible solution. Students will love this! Next, add one drop of clear liquid soap to the test tube, cap it securely with a test tube and shake vigorously!
American Association of Chemistry Teachers
|3-36
2
The entire mixture of water, oil and food coloring will mix together. This would be a good time to discuss the meaning of emulsifying agent as well as its physical properties and molecular structure. Additional research might be valuable for students, such as investigating how soap works, if you do not discuss emulsifying agent. o
American Association of Chemistry Teachers
|3-36
3
