Soap

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 settle onto 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 not otherwise 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 fit inside 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 vinegar will 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 in the 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 the surface 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 stay at 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 length of 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 near each 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 reflected from the inner and outer surfaces of the film of a soap bubble can interfere with each other. Where the crests of the light waves reflected from the inner and outer surfaces of the film meet, the intensity of the light increases. If the crest of a wave reflected from the inner surface meets the valley of a wave from the outer surface, the intensity of the light will be diminished. Whether the crest of a wave meets another crest or a valley is determined by the length of the wave and by the thickness of the film. If the thickness of the film is a multiple of the wavelength of the light, the crests of waves reflected from the inner surface will meet the crests of waves reflected from the outer surfaces. If the thickness of the film is an odd multiple of half the wavelength, the crests of the waves reflected from the inner surface will meet the valleys of the waves reflected from the outer surface. Because the thickness of the film varies and the wavelength of the light determines its color, different areas of the bubble will have different colors. The colors of a film of oil on a wet parking lot are produced in the same way as the colors of a soap bubble.

If your soap bubbles remained floating on the carbon dioxide for more than a minute, you may have noticed that the bubbles were slowly becoming larger. You also may have noticed that the bubbles slowly sank into the container. Both the growth and the sinking of the bubbles is a result of the same process. When you blew the bubble, it was filled with air. When it settled into the container of carbon dioxide, the bubble was surrounded by this gas. The bubble grows because carbon dioxide moves into the bubble (through the soap film) faster than air moves out of the bubble. Carbon dioxide can move through the soap film more quickly than air, because it is more soluble in water than is air. (Water is the major component of the bubble-soap solution.) As the amount of carbon dioxide in the bubble increases, the bubble becomes heavier and sinks lower into the carbon dioxide in which it is floating.