The Lava Lamp Lab: A Guided-Inquiry Approach to Intermolecular Forces

Marc Seigel
Chemistry Teacher, Middletown HS South
Middletown, NJ

October 2015

Guiding questions and student-generated questions are essential for a successful guided-inquiry activity. I created the lava lamp lab as a starter activity for my honors and college-prep chemistry classes to introduce intermolecular forces.

Unit Essential Question

What would the world look like if water were non-polar?

Objective

1. Develop a list of rules for creating the perfect lava lamp.
2. Explain why certain substances will react with water, but not oil.

Procedure

Note: This procedure should be explained orally to keep the class together and allow the teacher to record all questions asked for later reference. The students should then be divided into groups of 3 or 4, with each student assigned a specific job.

1. Obtain one 400 mL or 600 mL beaker, 1 dropper of food coloring, and 2 antacid tablets (Alka-Seltzer®).
2. One member of the group should fill the container a quarter of the way with tap water.
3. A second member of the group should add to the beaker twice as much vegetable oil as there is water.
1. At this point, write down something you observe about the border between the oil and water. Is there anything surprising about what you see?
2. Which substance is heavier?
4. The third group member should add 4 to 5 large drops of food coloring. As the drops fall through the liquid, make careful observations about what you see.
1. Did the food coloring interact differently with the oil than it did with the water? Discuss what you saw with your group and develop a hypothesis to explain what you observed.
5. The fourth group member should break the first antacid tablet in half and drop it into the beaker. Observe.
1. Is the tablet dissolving or is there a chemical reaction occurring?
2. Did the tablet interact with the oil differently than the water?
6. Take the other half of the tablet and drop it into the beaker.
1. Were your observations the same?
7. Take out your cell phones, turn on the flashlight feature, and place the light under the beaker. Add the remaining tablet.
1. Does the light shine through different parts of the mixture differently?
2. Does the whole tablet react faster than the half tablet?
3. What is similar between real lava lamps and the ones you created here?

Post-Lab

1. Watch this flipped video and take notes. It provides an overview of intermolecular forces. Note: Given the inquiry nature of this lab, the instructional video appears after the investigation has taken place rather than before.
2. Use what you learn in the flipped video about IMFs and work with a partner to answer the following questions:
1. Based on your observations about how the food coloring interacted with the oil versus the water, what intermolecular forces might exist around its molecules?
2. Do IMFs alter a compound’s ability to react? Use observations in the lab to support your answer.
3. If oil and water have opposite IMFs, why do they seem to mix at the border between them?

Activity extensions

I ask the class the questions posed above, and we discuss them together after they’ve had the opportunity to complete the post-lab. Note that the beginning questions are designed to get students to recall facts learned in my Introduction to Chemistry and Chemical Reactions units from earlier in the year, and have nothing to do with intermolecular forces.

Students know that food coloring dyes objects, but they are often surprised to realize it will not color oil. The food coloring will fall through the oil in a perfect drop until it hits the water barrier where it will sometimes stop—until the accumulated drops become too heavy and pass through. This leads to a discussion question: Can we make dyes that are oil soluble? Answer: Yes. (Check out a great demo about it at the blog Science Matters.) It can also spark a discussion about the type of substance found in a lava lamp.

Intermolecular forces primarily involve the interaction of molecules. At no point in this activity do I mention a single IMF, discuss molecular structure, or deal with polarity. The students have little understanding of these concepts because this activity was designed to generate questions they will attempt to answer through their study of the chemical principles in the unit.

Unlike the traditional approach of teaching the material before conducting labs, guided inquiry gives students the opportunity to learn by doing and make meaningful discoveries as they grapple with concepts.