Science | Science Experiment of the Week 7 | 276 - White Foam

Why is the foam on a glass of carbonated beverage white or almost white, instead of being the color of the drink?

This Week's Experiment - #276 White Foam

These experiments are from Robert Krampf - The Happy Scientist

This week's experiment came from a question sent to me by the Hood family in Kentucky. They wanted to know why the foam on a glass of carbonated beverage is white or almost white, instead of being the color of the drink. To explore this, you will need:

a glass of water
soap
a drinking straw
some clear, plastic wrap

First, pour yourself a nice glass of some carbonated beverage. Use ice so it will be nice and cold, as well as making lots of foam. Taste the foam. It tastes the same as the drink, because that is what it is made of. You may want to drink some of the soda for a comparison. Feel free to refresh your memory with a swallow periodically during the experiment. Look closely at the foam. Bubbles! It is made of lots and lots of bubbles.

Now wait a minute. Aren't bubbles clear? Pour a glass of water and add a little soap. Stir it a bit and then use the straw to blow a bubble. What color is it? Clear, with swirls of color. Now blow a few more bubbles. Adding more bubbles makes it look more white instead of clear. Stir the water briskly and you will get some foam, similar to what you saw on the soda. Do not taste this foam, because it would taste like soap, not soda.

Another way to look at this idea is with a sheet of clear, plastic wrap. Tear off a piece about 6 inches long. Hold it up and notice that it is nice and clear. You can easily read this e-mail on your computer monitor through it. Now fold the plastic in half and smooth it flat. Pick it up and you can still see through it, although this writing will be fuzzy. Fold it in half again. Now you probably can't read through it at all, although you can still see shapes. Fold it again and things get much less clear. Another fold and you can hardly see through it at all. One last time and it now looks white instead of clear. Did the plastic suddenly become cloudy? Unfold it and you will find that the plastic is just as clear as it was when you started. Then why did it look white when it was folded?

With a single layer of plastic wrap, Most of the light passes through just fine, but a small amount of the light is bent and scattered in all directions. While a single layer only scatters the light a small amount, each layer that you add scatters it a bit more. As the light is scattered, all the colors are blended together, giving white light.

This also explains why snow and clouds are white. Snow is made up of ice, which is clear. Clouds are made of tiny drops of clear water. Both of them scatter light. As the light passes through lots of snow flakes or water drops, it is scattered enough to produce the white color.

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