What is needed

• Plastic cup filled with root beer or other dark carbonated beverage.
• Spoon or other utensil

What to Do

• Take the spoon, and tap gently in the side of the cup, near the place on the side of the cup that the root beer comes up to.
• What do you notice?

What is Happening

When we do this, we see a lot of drops of root beer that float on top of the liquid in the cup. If we tap just right, we can get dozens of drops, some of which stay floating for several seconds.

So what is going on? To be honest, we don't really know. Here are some ideas that we have:

• Carbonation seems to make the effect stronger. Perhaps some of the carbonation comes out of the drops, which makes their surface tension a bit different than that of the rest of the liquid.
• The drops could also be cooler, from evaporation. We have found, by playing around in the sink, that drops of warm water can be made to float on cool water. So maybe this works the other way too. Again, the idea would be that the drops are cooler, and so their surface tension is just a bit different, and so they don't mix with the other liquid right away.
• Maybe a layer of carbon dioxide gas from the fluid forms, and makes a barrier to keep the two fluids from mixing right away. This might be why hot water drops float on cool water too. You have noticed that hot water from the tap is cloudy; this is because there are a lot of air bubbles in it. When you heat water, some air which is dissolved in it comes out. (Gases are less soluble in warm liquids than cold, which is why you need to chill a soda to keep it from fizzing.)

The real truth of the matter is, though, that we don't really know! We welcome your ideas.

Other Things to Try

Since this is one that we don't understand, there is room for experimentation. I think that this would make a cool science project. There are a lot of things you could test:

• Does carbonation really matter?
• How about sugar content? Do regular and diet drinks work the same?
• How about temperature? Does this work the same with hot and cold drinks?

It's wide open, really. Share anything you find with us, and we will note your results on this page

Follow-up 1. What is it that is observed to floart to the top of the liquid in the cup? 2. What might be happening to the carbonation when you tap on the side of the glass? 3. How could the temperature of the drops change? Why would that have anything to do with floating? 4. Try floating warm colored water on colorless cool water and report your findings. What materials did you use? 5. Gas may form some sort of barrier in this experiment. Where does the gas come from, and what does it do? 6. Explain why hot tap water is cloudy.   Obtaining credit for your efforts Print a copy of the blank laboratory write-up paper. Read and complete each section, incomplete write-ups receive no credit. Have your parent check your work and sign the paper.

The contents of the page was modified from that created for the Little Shop of Physics, Colorado State University