Got 5 minutes? Then you’ve got time to make dancing raisins.
What you need:
- 2 clear glasses
- clear carbonated soda (like Sprite or 7-up)
- a handful of raisins
Fill one of the glasses with water, the other with the clear soda.
Now drop a few raisins into each glass.
At first the raisins sink to the bottom of both glasses. But watch as the raisins in the soda quickly take on a life of their own.
What makes these raisins dance?
The trick is in the mix of density and buoyancy. Raisins are denser than water, so at first they sink.
The bubbles in the soda are made of carbon dioxide gas (CO2), which soda makers pump into the water to make it fizzy. The air inside the bubbles is less dense than the water, so the bubbles rise to the top of the glass. As they do, they get caught in the grooves of the raisins, lifting them up through the water.
You can think of the bubbles like little life jackets for the raisins. Or you can think of Charlie and his grandpa rising up through Willie Wonka’s factory after drinking fizzy lifting drinks.
When the bubbles attach to the raisins, it increases their buoyancy and decreases their average density, and up they go.
When the raisin gets to the top of the glass, the bubbles come in contact with the air outside and pop. Basically, the raisin loses its little life jackets—the extra buoyancy it got from the bubbles. This causes the raisin’s average density to increase and its buoyancy to decrease, so it falls back down into the soda water.
As it falls, more bubbles rush in to fill the raisin’s little grooves, and lift it back up to the top of the glass. Pop. Drop. Repeat.
And then it stops.
Why do the raisins stop dancing?
Two things happen: one has to do with the soda, the other with the raisins themselves.
1. The soda runs out of gas. The bubbles come from the carbon dioxide gas pumped into the water, and the CO2 in the soda is limited, so eventually it just runs out of gas.
Ever taken a sip of Coke that’s gone flat—like when an open can’s been sitting on the counter all day, or from a 2-liter bottle that’s been open in the fridge after a few days? It tastes awful. That’s because the carbonation caused by the bubbles has gone. And when the bubbles are gone, there are no little life jackets for the raisins.
2. The raisins get heavier. Dry raisins are not completely solid, they have empty spaces inside of them. But as a raisin sits in the soda, it stars to absorb some of the water, which fills its empty spaces.
So a fresh raisin that enters the soda is denser than water, that’s why it sinks to the bottom. But when a couple dozen little CO2 bubbles hook on, it’s enough to pick the raisin up off the bottom of the glass. But as a raisin absorbs extra water it gets heavier, which means it needs even more bubbles to lift it up through the water.
The combination of less bubbles and heavier raisins means no more dancing.
It’s kind of like when mom and dad escape their kids for the evening and head to a party. At first the bubbly gets them on the dance floor. But their bodies get heavier when filled with food, then the bubbly starts to run dry. So the dancing stops, and everyone drops to the floor.