November 2014 | Resource Feature
By Regis Goode
“Chemistry is Fun” is the slogan on a pin that I wore on my lab coat for the 30 years I taught chemistry. Unfortunately, chemistry is sometimes more daunting than fun for students when they begin to learn the subject. I truly believe that chemistry is difficult for many students because we can’t “see” what is happening to the tiny atoms and molecules, we can only observe the results on the macroscopic scale. I think it is important to help students describe macroscopic phenomena at the molecular level and teach them to draw diagrams, or models, of the molecular view. These particle diagrams help students understand various chemical concepts and give the teacher a formative assessment of what students may or may not understand.
I developed the Magic Bubble activity to introduce students to particle diagrams. The activity helps students understand what happens when two miscible liquids mix. The activity involves a discrepant event when two liquids are mixed: isopropyl alcohol and water. Students begin to comprehend the small size of atoms and molecules when they discover and explain the process of mixing.
The initial preparation for this activity may take an hour or more, but once it is prepared, it can easily be stored and prepped for students in 15 minutes. Both teachers and students have fun with this activity!
Teacher instructions and preparation
View the full lesson in the Classroom Resources library.
- Color the water with blue food coloring and the isopropyl alcohol with yellow food coloring. Add enough food color to make the liquids dark yellow and dark blue.
- Cut the Beral pipet tips to about 3 cm. Fill 12 Beral pipets with blue water and 12 with yellow alcohol. I do not label the Beral pipets and initially I do not identify the liquids; I refer to them as the blue liquid and the yellow liquid. I tell students that neither liquid will hurt them, but they should handle them cautiously as they would any chemical in a lab setting.
- At each lab station, place one small glass tube*, two corks, a Beral pipet with blue water, and a Beral pipet with yellow alcohol. Provide small beakers or cups of the colored water and colored alcohol on each lab bench so students can refill their Beral pipets as needed.
- Before students carry out the Magic Bubble activity, I introduce particle diagrams, which reinforce the particle nature of matter. Using a beaker of water as an example, I draw a simple particle diagram that represents water particles in the beaker on the board (see below). I then add one drop of food coloring to the beaker of uncolored water. As the food coloring falls through the water, I ask students to draw a particle diagram of the water and food coloring in their notes. I walk around and observe what they are drawing without making any comments. Finally, I stir the beaker of water and food coloring and ask them to now draw a particle diagram of the beaker’s contents. Students can be reluctant to draw these models at first because they do not like to make mistakes. I assure them that there is no penalty for what they draw, whether it’s wrong or right. Once I’m satisfied they’ve thought about what’s happening on the particle level long enough, I have them coach me to draw the diagrams on the board. The class discusses various versions of the drawings and what they represent. See the example below for an example of a particle diagram. Note in each drawing there is the same number of water particles, and in the last two drawings the same number of food coloring particles.
- Students are then given the Magic Bubble student activity sheet. Once students are reminded of safety precautions, including not to use an open flame when working with alcohol, they go to the laboratory to complete the activity. I remind them that they are to interpret the molecular-level process and draw particle diagrams to complete their explanations.
- Teachers, realize that students cannot complete this activity without forming a bubble, because as the liquids mix, the total volume decreases and creates a bubble. When alcohol dissolves in water, the volumes are not additive because alcohol molecules can fit between water molecules. Mixing 50 mL of isopropyl alcohol with 50 mL of water produces a total volume of about 96 mL. For that reason, it is easy to determine if students have “cheated” by adding more liquid to fill the tube after they mix the solutions. I also like students to show me the filled tubes before they begin mixing, to verify that the tubes are indeed full before mixing.
|Before mixing||After mixing||The bubble|
- While students are carrying out the lab seemingly unsuccessfully, I complain and act surprised that they aren’t getting expected results. I encourage them to repeat the lab until they perform it correctly, and after three tries they can “give up.” You can play this up as much as the students can tolerate. Some students get frustrated, some dig in their heels and are determined to do it, and others start to see it as some sort of trick.
- Finally, I have them clean up and return to their seats. I act discouraged because they weren’t able to complete the activity correctly. I then show them how to do it and I use a large glass tube so they all can see what is happening. As I pour the liquids into the large tube, I solicit students’ suggestions of proper technique. Then I stopper the tube and mix the liquids. Of course, I also get a bubble when the two liquids mix. The students are so excited that mine also has a bubble!
- But then I ask students to tell me about the bubble. I ask questions such as:
- Where does the bubble come from?
- What is the bubble made up of?
- Why does the bubble form?
- I take out a large culture tube. I fill half of it with aquarium gravel. The other half I fill with sand, so the tube is completely full. I ask students to formulate a hypothesis of what will happen. Without commenting on their hypothesis, I cap the tube and start mixing the sand and gravel. Before very long, I get a “bubble.” Without any further discussion, the students write explanations about their own magic bubble investigation, including particle diagrams of what happens on the molecular level.
Although this activity touches on the concept of dissolving, its greatest value is getting students to realize that matter is composed of particles and that there is empty space between particles. After students complete their final explanations and particle diagrams, you can culminate the activity by discussing what happened on the molecular level and why a bubble forms, regardless of whether two liquids or two solids were used.
Because this is an introductory particle modeling activity, I do not expect students to draw individual atoms that make up the substances (note that above, water particles and food coloring particles are represented as single circles). As students learn more about elements, atoms, molecules, compounds, and mixtures, they refine and draw more detailed particle diagrams.