Try This: Swimming Raisins

You probably have friends who have trained their pets to do tricks. Some may have dogs that can roll over, catch a ball in their mouth, or play dead. Others might have cats that can meow on command, or birds that can speak several sentences. Some people even claim that they can train their fish to swim upside down, but most people don't consider that “swimming,” if you know what I mean.

Yes, all of these are worthy accomplishments, but just wait until you tell your friends that you own trained raisins! They will watch in amazement as your “pets” perform feats of skill even their pets wouldn't dream of attempting.

Question: Can raisins be trained to swim and dive?

Materials

  • 1 (12-ounce) can of clear soda (lemon-lime, ginger ale, club soda, or similar)

  • Tall, clear drinking glass

  • Several raisins

Procedure

1. Open the can of soda and pour it all into the glass.

2. One at a time, drop the raisins into the glass and watch to see what happens.

The Science Behind the Magic

This really isn't magic at all. Carbonated sodas contain a gas called carbon dioxide (the carbon is where the word “carbonated” comes from). When you pour the soda into the glass, the bubbles you see are bubbles of carbon dioxide being released from the soda and drifting up to the surface.

When you drop the raisins into the glass, at first they sink to the bottom because they are denser than the soda. This is the same thing that would happen if you dropped a rock into the glass. However, while the raisin rests on the bottom of the glass, the carbon dioxide bubbles gather on its skin. When enough of the bubbles (which are less dense than the soda, and therefore tend to float to the surface of the drink) gather on the skin of the raisin, it begins to float upward.

At the surface of the drink, the carbon dioxide bubbles burst, and the raisin sinks back down to the bottom of the glass. This process repeats as long as there is carbon dioxide gas remaining in the drink. You should be able to watch your “trained” raisins dive and swim in the soda for some time.

Follow-Up

This experiment should work with other objects. Can you think of some you might test out? You might try small pieces of uncooked pasta, small berries, and even very small rocks. Can you think of any others to try? Challenge yourself to find out which characteristics of the objects you test lead to the same results as that of the raisins.

What principle does this experiment demonstrate in the real world? How can people, particularly small children, take advantage of this principle when they go swimming?

The principle in this experiment is buoyancy. It is what determines whether or not something floats. Small children learning to swim often wear little tubes around their arms or swim rings around their waists to stay afloat. Those air-filled rings serve the same purpose as the carbon dioxide bubbles in the Swimming Raisins experiment. The difference is that the rings don't burst at the surface of the water. As a result, the children can stay afloat.

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