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

AP Chemistry Curriculum Framework

This lesson supports the following learning objectives:

• Big Idea 2: Chemical and physical properties of materials can be explained by the structure and the arrangement of atoms, ions, or molecules and the forces between them.
  • 2.30 The student is able to explain a representation that connects properties of a covalent solid to its structural attributes and to the interactions present at the atomic level.
  • 2.32 The student is able to explain a representation that connects properties of a molecular solid to its structural attributes and to the interactions present at the atomic level.
• Big Idea 5: The laws of thermodynamics describe the essential role of energy and explain and predict the direction of changes in matter.
  • 2.8 The student can draw and/or interpret representations of solutions that show the interactions between the solute and solvent.
  • 5.9 The student is able to make claims and/or predictions regarding relative magnitudes of the forces acting within collections of interacting molecules based on the distribution of electrons within the molecules and the types of intermolecular forces through which the molecules interact.

NGSS Alignment

5-PS1-3: Make observations and measurements to identify materials based on their properties.

5-PS1-4: Conduct an investigation to determine whether the mixing of two or more substances results in new substances.

MS-PS1-2: Analyze and interpret data on the properties of substances before and after the substances interact to determine if a chemical reaction has occurred.

• HS-PS1-3: Plan and conduct an investigation to gather evidence to compare the structure of substances at the bulk scale to infer the strength of electrical forces between particles.

• Science & Engineering Practices: Planning and carrying out Investigations.
• Crosscutting Concepts:
  • Patterns: Different patterns may be observed at each of the scales at which a system is studied and can provide evidence for causality in explanations of phenomena.

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.

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 the opportunity to conduct this demonstration as lab activity, and have students complete the procedures individually.
• Additional research might be valuable for students, such as investigating how soap works, if you do not discuss emulsifying agent.

Demonstration Procedure:

1. Using a pipet, first fill the test tube approximately 1/3 full with water.
2. Next, using a new pipet, add an equal amount of oil to the test tube so that it is 2/3 full. 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.
3. Next, invert a bottle of food coloring so that it is directly above the test tube opening. 
4. Add one drop of food coloring 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.
   • The food coloring will form a sphere and will not mix with the oil. The food coloring will “sit” on the surface tension line that divides the oil and the water.
   • This is a good opportunity to talk about miscibility, molecular structure and heterogeneous solutions depending upon grade level.
5. Flick the test tube with 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, solution. Students will love this! You can point out that the food coloring was not soluble/miscible in the oil but is in the water.
6. Next, put the cap on the test tube and shake it vigorously. Then, hold the test tube vertically and allow the oil and colored water to separate.
7. Next, add one drop of clear liquid soap to the test tube, cap it securely with a test tube and shake vigorously!
• 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.