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# Density and Buoyancy: Experimenting with Club Soda

Media Type:
Video

Running Time: 2m 05s
Size: 6.3 MB

or

Source: ZOOM

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Collection Credits

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Watch as a raisin dances in a cup of club soda, rising and falling in the liquid medium, in this video segment adapted from ZOOM. Look closely, and you'll learn that the cause of this phenomenon is the tiny gas-filled bubbles that attach -- and then detach -- from the raisin, thereby affecting the buoyancy of the raisin.

## Supplemental Media Available:

Background Essay

If the overall density of a composite object (one made up of more than one type of substance) is lower than the density of the fluid surrounding it, buoyant forces pushing on the object from below will be strong enough to make the object float. To determine overall density, the density of each part must be factored into the equation. The overall density of a composite object equals the total mass (i.e., the combined weight) of the two or more substances divided by their total volume.

A submarine is an example of a composite object. Its overall density will vary depending on how much air it contains at any given time. As a result, a submarine can be made to float or sink by allowing water to enter its ballast chambers or by forcing water out, thus changing the amount of air in the submarine and its mass as well. Since its volume doesn't change, the density of the submarine changes, and so does its buoyancy.

On its own, a raisin dropped into carbonated water sinks. Because it has a higher density than water, the volume of water it displaces weighs less than the raisin itself. Gas (carbon dioxide)-filled bubbles, however, rise in the denser medium of the water. You can see this happening in a cup of club soda, or underwater, when you blow air out of your mouth. Rising gas bubbles, when attached to an object like a raisin, increase the overall volume of the composite object without changing its overall weight very much. This means that the overall density of the raisin-bubble tandem decreases, and when it becomes lower than that of water, the raisin rises to the surface. As the raisin rises, some of the bubbles may burst, and without them, the raisin again sinks to the bottom of the cup. A life jacket works in the same way to help a person who might otherwise sink in the ocean float. The overall density of the life jacket-person tandem is lower than the density of the ocean water.

Discussion Questions

• How did the bubbles help the raisin and grape float?
• What gas is in the bubbles that form on the grape and raisin?
• In what ways is the bubble-covered raisin like a person in a life jacket? How can a life jacket be so small compared to our bodies and still keep us afloat?
• The grape and the raisin have very different volumes. Why do you think the air bubbles allowed both to float?

• Transcript

(electricity crackling)

CAROLINE: Watch this. Mary Elizabeth G. of Tallahassee, Alabama calls this cool Phenom "dancing raisins." All you need to make it is a tall, clear glass, some club soda, and a raisin cut in half.

RACHEL: Pour some club soda into a glass.

CAROLINE: Oh, wow, it's really fizzy.

RACHEL: I know, it's cool.

CAROLINE: Then, drop in a half a raisin.

RACHEL: Wait for about 20 to 30 seconds and watch what happens to the raisin. Oh, there it goes— look at that one.

CAROLINE: It went right up to the top and then came down.

RACHEL: Yeah, it goes up and down.

CAROLINE: It's dancing. Oh, wow. The reason why the raisin floats to the top is because the bubbles stick to the side of the raisin and make the raisin more buoyant.

"Buoyant" means that something floats more easily. The bubbles make the raisin float the way a lifejacket makes a person float.

RACHEL: We're going to see if we can get something heavier than half of a raisin to float.

CAROLINE: I think the weight might have something to do with it. I'm not sure, though.

CAROLINE: All right.

RACHEL: So, I'll take one... Here we go.

CAROLINE: I don't think it will work. Wow! Oh, my gosh, it works!

RACHEL: It's kind of like a buoy.

CAROLINE: That's so weird! It was buoyant.

RACHEL: It was awesome. It was so fast.

CAROLINE: I know, we just dropped it and it was, like, whoosh.

RACHEL: It's faster than the raisin going up.

CAROLINE: It's big and it doesn't have any creases.

RACHEL: Try this at home. Think of a question you'd like to answer, like: Can something heavier than a grape float, using bubbles? Will a walnut or a superball float? Make a prediction, then test it out.

Standards

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