Hands-on Science Carnival 2010 Activity Stations: Chemistry

Soap Bubbles

[Shopping List: Yellow Joy dishsoap; Blue Dawn dishsoap; glycerin; cornstarch; baking powder; measuring cups/spoons; buckets; big spoon; kiddie pool; bubble making tools; non-bendy straws; cotton yarn/string; scissors; soda 6-pack plastic holder]

1. Make up bubble mixture according to one of the following ratios:
Recipe 1: ~20 cups water, 1 cup dishsoap, 1 cup cornstarch, 2 tablespoons baking powder
Recipe 2: ~20 cups water, 1 cup dishsoap, ~ 1 tablespoon glycerin


2. Let the mixture age before using – test it periodically to determine optimal bubble making power
3. Pour bubble mix into kiddie pool or large buckets. Depending on thickness, you might have to dilute it with water.
4. Use hula hoop to make big bubbles, and other bubble tools to make small ones. Some require you to blow into them, but many of the bigger ones can just be swung lightly through the air to make the bubble. Take a round bubble tool and lightly move it up and down/back and forth to make the bubble ‘wiggle’.
5. Run a piece of string through two straws, knot it, and store the knot inside one of the straws. You can use this tool to make bubbles of various shapes, pinching them off by starting with the square open and then softly pushing the two straws together to close the square opening. You can also use this as a ‘window’ by wetting your hands with bubble mix and then pushing your hand through the bubble without breaking it. Let the kids make this as their own bubble-making tool to take home!
6. Make bubbles with your hands – cup your hands together so that they form an enclosed space, and plunge them into the bubble mix. When you pull them out you can either blow through the hole to make the bubble or let the wind do the work for you!
7. Dip a soda 6-pack holder into the bubble mix to make 6 small bubbles.

What's Happening: Bubbles are actually air encased by a layer of water sandwiched between two layers of soap (like a balloon with 3 skins). Water has too much surface tension by itself to make bubbles – the water molecules are so attracted to each other that they cannot easily bend and expand into different shapes when air is blown into them. The soap disrupts the surface tension enough to make the solution pliable and capable of moving easily and making bubbles. This stabilizing impact of soap is called the Marangoni effect.

Bubbles pop when the soap film is stretched too thin, and the water is able to evaporate. The glycerin in recipe #2 extends the lifetime of a bubble by hydrogen bonding with the water thereby delaying evaporation (that's why it's often used in skin moisturizing lotions.) You can touch a bubble with a wet hand without breaking it, and even stick your whole hand through a bubble. Yet with a dry hand the bubble will instantly break, because the soap film will stick to the dry object, strain the bubble surface, and break it, letting the air out. Just like pricking a balloon with something

Bubbles, no matter what shape they start out as, always end up as spheres. Why? A sphere is the shape with the smallest surface area to volume ratio, meaning that rearranging into a sphere minimizes the energy of the bubble. In other words, the tension that always exists in the soap-water-soap film will force the bubble to become a shape with small surface (film) area because it is the most efficient shape.

The color in a soap bubble is described as iridescent: the colors appear as a rainbow that changes depending on the angle of light. This is due to reflected lightwaves: some light reflects off the outer surface, some light enters the film and reflects off the inner surface, and some light will bounce between the two layers of soap several times before being reflected out again. Depending on the thickness of the soap film different wavelengths of light will undergo constructive or destructive interference as they cross the film. This means that the color we see will vary with the thickness of the bubble wall – you will notice that the recipe with cornstarch is thicker and consequently produces more vivid colors.

Variations: Add food coloring to the bubble solution to see how it changes the bubbles. Does blue food coloring make a blue bubble? Where did the food coloring go?