Nearly everyone has enjoyed playing with soap bubbles. These fragile spheres of soap film filled with air are both beautiful and captivating. However, few people have observed them closely or at length, because soap bubbles are fragile and very light. When you blow soap bubbles out of doors, the slightest breeze carries them away. If you blow them indoors in still air, the bubbles soon settleonto a surface and break. However, because they are very light, soap bubbles will float on a gas that is only slightly more dense than the air that fills them. Such a gas is carbon dioxide. When soap bubbles settle into a container of carbon dioxide, the bubbles float on the carbon dioxide and can be examined closely. Under this close examination, soap bubbles reveal many properties that are nototherwise easily seen.
To float soap bubbles, you will need the following materials:
▪ soap bubble solution
▪ a wand for blowing soap bubbles
▪ a large transparent container with an open top (an empty 38-liter [10-gallon] aquarium works nicely)
▪ 125 milliliters (½ cup) of baking soda (sodium bicarbonate)
▪ 250 milliliters (1 cup) vinegar
▪ shallow glass dish to fitinside large container (such as a glass baking dish)
Set the large container on a table away from drafts and where you can easily look through its sides. Place the glass dish inside on the bottom of the large transparent container. Put 125 milliliters (½ cup) of baking soda in the glass dish. Pour 250 milliliters (1 cup) of vinegar into the dish with the baking soda. The mixture of soda and vinegarwill immediately start to fizz as they react and form carbon dioxide gas. Carbon dioxide is more dense than air and so it will be held in the large container as long as it is not disturbed by drafts of air over the container. Because carbon dioxide is colorless, you cannot see it inside the container. However, you will soon be able to detect its presence with soap bubbles.
After the fizzing inthe dish has subsided (about a minute), gently blow several soap bubbles over the opening of the large container, so that they settle into the container. This may take a bit of practice. (Do not blow directly into the container, you will blow the carbon dioxide out of it.) When a soap bubble settles into the container it will not sink to the bottom, as it would in air. Instead, it will float on thesurface of the invisible carbon dioxide in the container.
While the bubble is floating on the carbon dioxide in the container, you can observe the soap bubble closely. Note what the bubble looks like. What color is the bubble? Can you see more than one color on the bubble? Do the colors change? Notice the size of the bubble. Does its size change? Observe the position of the bubble. Does it stayat the same level in the container? Does it rise or sink?
When you have finished observing the bubbles, dispose of the mixture in the glass dish by rinsing it down the drain with water.
The colors of a soap bubble come from reflections of the white light that falls on the bubble. White light, such as from the sun or from a light bulb, contains light of all colors. Light has waves, and the lengthof the wave, from crest to crest, determines the color of the light. When light reflects from a bubble, some of each wave reflects at the outside surface of the soap film. Some light travels through the soap film, and reflects from the inside surface of the film.
Interference between waves occurs whenever waves travel through the same space. Interference occurs when two rocks are tossed neareach other into a lake. Circular waves on the surface of the water spread out from where each rock entered the water. Where the crests of two waves meet, interference between the waves causes the motion of the surface of the water to increase. Where a crest and a valley meet, interference reduces the motion of the water's surface. Similar interference can occur in waves of light.
Waves of light...
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