Dancing raisins

We had a bottle of sparkling water and some raisins so we tried a quick and simple experiment.  What happens when you put a few raisins into a beaker of carbonated water?

Floating raisins

Sparkling water, or any fizzy drink, has carbon dioxide dissolved in the water under pressure, and this comes out of solution and forms bubbles which float to the surface when you open the bottle.  Raisins are more dense than water, and sink to the bottom.  However, when bubbles of carbon dioxide attach to their surface, this makes them buoyant enough to float to the surface.  At the surface, the bubbles pop, and the raisin sinks again.  This process repeats, and different raisins become sufficiently buoyant to float.

My almost 4 year old didn't know whether the raisins would sink or float, so we started out by testing this with a small handful - they sink.  He was pretty impressed to see one float to the surface, then sink, and numerous others followed, bobbing up and down!  We watched the raisins for a while before adding more to see if the same thing happened (it did).  We talked a bit about what was in the bubbles, and why they floated because carbon dioxide gas is less dense than water as we watched the raisins dancing up and down.  

Bubbles of carbon dioxide

He asked if he could eat some raisins, and then gave up watching, although he did have a look later and saw that they were still dancing, although more slowly (because there is less carbon dioxide dissolved in the water as much of it has already bubbled away).