Return to Chemistry IndexSoap Bubble ChemistryTheresa Colby Montessori Elem School Al Oldenburg Lindblom HS Al Tobecksen Fenger HSObjectives: 1. Students will understand the chemistry of soap bubble films. 2. Students will build their own model for making large soap bubbles. 3. Students will investigate with prepared geometric wire models to see the maximum number of planes, the maximum number of lines and the sizes of the angles that are produced when the planes and lines intersect.Materials: 1. Pop-it beads strung into a long chain and in a large jar 2. Straws 3. String 4. Prepared wire and string models 5. Two strings of suckers 6. Prepared soap bubble solution 7. Buckets and trays 8. ProtractorsSuggested Strategy: For an attention-getter, let the pop-it beads pull themselves out of the jar in which they are contained. The last pop-it bead is pushed into a small hole drilled into a racquetball. Starting from the racquetball, count off 18 sections of pop-it beads and separate that from the chain. Ask the students what does this small piece of chain represent (ans. - a soap molecule). Review soap molecules and how they arrange themselves in water. See diagram that follows. /\/\/\/\/\/\/\/\/\O (H_{2}O) O/\/\/\/\/\/\/\/\/\ /\/\/\/\/\/\/\/\/\O (H_{2}O) O/\/\/\/\/\/\/\/\/\ /\/\/\/\/\/\/\/\/\O (H_{2}O) O/\/\/\/\/\/\/\/\/\ Present three questions: 1) What is the shortest possible way to connect two points? (Ans. - a straight line.) 2) What is the shortest possible way to connect three points? (Most people would say a triangle, but that is wrong - see diagram 1 below.) 3) What is the shortest possible way to connect four points? (Most people would say a square, but that is wrong - see diagram 2 below.) Using two plexiglass plates and small rubber suction cups (first two suction cups, then three, then four) and an overhead projector, let the soap bubbles show the answers. Some students may guess that planes of soap bubbles meet at 120 degrees since it will be very clear on the screen; some students may surmise that only a maximum of three planes will ever intersect - and both guesses are correct! Present another question: what is the maximum number of lines that can intersect a single vertex in a soap bubble model and what angle(s) do these lines form? (Ans. - four lines maximum and the angle is 109.23 degrees - it is very unlikely anyone would know it or guess it.) Bring out the models, give each group a protractor and tell them to go outside to find out. (Soap bubbles are very sloppy.) Before you turn the students loose, show them how to make a large bubble maker. Take two meters of string, double it up so it is only one meter long, run it through two straws and tie the ends of the string together. Slide the straws so they are opposite each other, dip it into the solution, wave it in the air and you get really big bubbles. Back in the classroom - follow up! Why do the soap bubble films assume the shapes that they do? The answer is that soap film has the property that its surface area has a minimum value when it has reached equilibrium. What forces are involved? Answer - gravitational potential energy (GPE), surface tension, and the compressional energy of trapped air.Preparation of Bubble Solution: 85% water 10% liquid detergent 5% glycerinDiagrams: (See "Suggested Strategy") Diagram 1 Diagram 2 | \ / 120 | 120 degrees 120 \____/120 degrees / \ /120 \ / \ / \ 120 Shortest length connecting Shortest length connecting three points. four points.