Blueberries and bubbles: An accidental science experiment.

Summer is seltzer water time in the Coryell Castle. We slice some fruit and toss it in to the seltzer with a little ice and sip away. Yesterday, I decided to crush a few blueberries and mix them into my seltzer. Believe me, it tasted better than it looked.

As I stood by the kitchen counter, sipping and chatting with the kids, Max’s keen, observing eyes led him to yelp, “Mom! The blueberries are floating up and down in your drink!”

“That’s impossible, Max. What do you mean?”

Lo and behold, Max was right. The blueberries would descend to the bottom of the glass and then one would float back to the top, as if to check out the scene, before descending again. The blueberries took turns doing this as the munchkins took turns bellowing and exclaiming. Even little Milla joined in the hubbub.

Why were the blueberries rising and descending in the seltzer water? Had we discovered the secret pathway to another dimension? Or could we find a scientific explanation for this phenomenon?

Max remembered reading somewhere that the bubbles and carbon dioxide played a role in moving mothballs in seltzer water. Since he couldn’t remember where he read it, we turned to our friend Google for a little help. And we found that you can make raisins or mothballs “dance” in water with a little help from vinegar and baking soda. Or you can just pour yourself a glass of champagne, toss in a few raisins, and sip away in the name of science.

Bizarre Labs explains why the blueberries and raisins dance in bubbly environments:

The irregular surfaces on the mothballs or raisins hold some carbon dioxide bubbles. When enough bubbles accumulate to lift the weight of the mothball (or raisin), it rises to the surface. There, some of the bubbles of air escape into the atmosphere, and the mothball/raisin, which is denser than the water or soda, sinks to the bottom to start the cycle over again. The effect will last longer if the container is sealed, as less carbon dioxide will be able to escape.

There are quite a few other small objects that will work. As I am writing this, in fact, I just received a note from someone about an oscillating couscous grain in seltzer water! (Thanks, by the way.) The key is that they are able to trap air bubbles on their surface, that they are light enough to be buoyed to the surface by the bubbles, and that they won’t dissolve in the liquid.

Does this mean that the blueberries would not have “danced” if they had not been crushed a little before being placed in the water?

The Dancing Raisins Experiment fleshes out the phenom a little more:

Raisins are denser than the liquid in the soda, so initially they sink to the bottom of the glass. The carbonated soft drink releases carbon dioxide bubbles. When these bubbles stick to the rough surface of a raisin, the raisin is lifted because of the increase in buoyancy. When the raisin reaches the surface, the bubbles pop, and the carbon dioxide gas escapes into the air. This causes the raisin to lose buoyancy and sink. This rising and sinking of the raisins continues until most of the carbon dioxide has escaped, and the soda goes flat. Furthermore, with time the raisin gets soggy and becomes too heavy to rise to the surface.

You might want to try other objects to see if they exhibit this behavior. Any object whose density is just slightly greater than water’s and has a rough surface to which the gas bubbles can attach should be able to dance in the carbonated water.

I think we’ll add the following to our test to get a good sense of how this works:

  • Uncooked pasta
  • Raisin
  • Raw baby carrots
  • Raw broccoli heads
  • Cherry
  • Crushed cherry
  • Rice cracker
  • Whole wheat bread crumb

Max can add this experiment to his Science Notebook by filling out this little table that I put together which examines the objects, their behavior in seltzer water, and what might account for this behavior. It has ten columns for 10 different items.

Seltzer Water Experiment, a printable PDF table

I left the items blank so you can add what you have on hand. Oh, I just LOVE it when the days lessons just fall into my glass…..

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