Molecular Forces At Work: Creating Soap Bubbles

Laz, Jerome Everett School

Objective: To make chemistry interesting to students by showing them it is part of the real world, rather than being confined to reagent bottles and test tubes in the classroom laboratory. The student will be able to: 1. discover how the terms "surface tension," "cohesion," and "adhesion" are related to soap bubbles, 2. hypothesize the outcome of the experiments, 3. analyze experimental data and conditions, 4. relate the polarity of the water molecule to the behavior of soaps and detergents. Background: This project is based upon molecular force and the degree of surface tension, which depends on the amount of energy in the intermolecular forces. Liquids that produce strong intermolecular forces have a strong surface tension. Water molecules form hydrogen bonds between each other and have strong intermolecular force; as a result, a strong surface tension is created. Soap bubbles are made up of soap molecules and water molecules. A soap bubble has a polar end and a nonpolar end. Water is a polar molecule. The polar end of the soap molecules are attracted to each other. The nonpolar ends of the soap molecules stick out from the water and help hold bubbles together. Apparatus needed: overhead projector silk thread glycerin Petri dishes detergent distilled water razor blades cheesecloth needles screening alcohol 2 small glass plates pepper soap bubble solution aluminum foil pipe cleaners or bubble wands beakers pans for bubble solution Recommended Strategies: 1. Float a razor blade or needle on water in a Petri dish on the overhead projector. Surface tension supports the object. 2. Make a loop of 5" of silk thread. Float in a low, wide container of water. Touch a bit of wet soap to the water inside of the silk loop and notice that it becomes a circle. 3. Sprinkle black pepper over the surface of water in a Petri dish. Add a drop of detergent to the center and watch the pepper spread out. 4. A hollow "boat" made of a 2" by 1" piece of aluminum foil, partly filled with a few drops of alcohol and with a 1/2 " length of lightweight cotton wick (such as is used in a cigarette lighter) over the stern, will skim over water. 5. Fill a wide mouthed jar with water, cover mouth with cheesecloth, and fasten securely. Now invert the jar quickly. If you punch a small hole in the cheesecloth with a pointed pencil, water runs out for an instant, but surface tension with the aid of atmospheric pressure "seals" the hole. 6. Support a piece of window screen over a pan. Slowly pour water onto the screen. Plain water will bead up. Detergent water will fall through the openings. 7. Recipe for making soap bubbles: 85% distilled water 10% detergent (Joy recommended) 5% glycerin Students make bubbles and observe irridesent color, relative thickness of the top and bottom of the bubble, movement of water within the bubble, and longevity of bubbles.
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