Why Is Milk White?: & 200 Other Curious Chemistry Questions (2013)

PROJECT: DANCING RAISINS

Materials

Sometimes an extremely simple demonstration can stimulate quite a bit of thought.

I had my assistant drop some dried blueberries, dried cherries, and raisins into a glass of flavored sparkling water. They all quickly sank to the bottom, surrounded by a cloud of bubbles. After a short while, the raisins began to float up. When they reached the top, they sank back down to the bottom again, to start the process all over. They looked like wrinkled little submarines, bobbing up and down.

But the dried cherries and dried blueberries just sat there at the bottom.

The questions began very soon. What makes them go up? What makes them fall down again? Why do only the raisins seem to be active?

The carbonic acid in the soda water is slowly converting to water and carbon dioxide gas, because it is no longer under high pressure in the can. The easiest way for the gas to leave the water is at the water’s surface. Making a new bubble from scratch takes more energy than simply enlarging an existing bubble.

At the top of the glass is a large surface open to the air. Most of the carbon dioxide gas escapes at that large surface. But as the dried fruit was dropped into the soda, bubbles of air were trapped in the wrinkled surface of the fruit. These bubbles of air act as extra surfaces, and the carbon dioxide gas escapes into them, making the bubbles bigger. When the bubbles are big enough, they rise to the surface. If they are stuck to a raisin, they can lift the raisin to the surface with them.

Once the raisin is at the surface, the bubbles continue to grow, and they either detach from the raisin or they pop (since they are now at the surface). In either case, the raisin no longer has enough gas to keep it afloat, and it falls back down. Usually the bubble that popped or detached left a tiny remnant of itself stuck to the raisin, and that tiny bubble starts to grow again.

But why did the dried cherries stay at the bottom?

The cherries are much like the raisins, but they are larger. This makes something interesting happen. Imagine a box just big enough to hold a marble. It has one marble in it, and it has six sides that can collect bubbles like the raisin. Now imagine that we double the width, height, and depth of the box, so that eight marbles can fit into it. The box now has eight times the weight but only four times the surface area to collect bubbles. Now there aren’t enough bubbles to lift the weight. The dried cherries are just too big for the bubbles on the surface to lift.

But the small dried blueberries also stayed at the bottom. Why is that?

We looked closely at the blueberries. The bubbles there seemed to leave the surface of the blueberry before they got big enough to float it. It looked as though the surface of the berry was not as sticky to the bubbles as the surfaces of the other fruits. The blueberries also seemed to be a little denser than the other fruits, so there was more weight for each bubble to lift. Something about the surface of the blueberries seemed to be the most important thing, though. They just didn’t have as many small trapped bubbles as the other fruits. It was like they were smoother or covered in a soap-like substance that allowed the water to wet them better.

All of this from a simple glass of soda and some dried fruit. Who would have thought?