What is needed
- Plastic cup filled with root beer or other dark carbonated
- 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
- 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
- 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
- How about temperature? Does this work the same with hot and
It's wide open, really. Share anything you find with us, and
we will note your results on this page
1. What is it that
is observed to floart to the top of the liquid in the cup?
might be happening to the carbonation when you tap on the side
of the glass?
could the temperature of the drops change? Why would that have
anything to do with floating?
floating warm colored water on colorless cool water and report
your findings. What materials did you use?
may form some sort of barrier in this experiment. Where does
the gas come from, and what does it do?
why hot tap water is cloudy.
Obtaining credit for your
- Print a copy of the blank laboratory write-up
- Read and complete each section, incomplete
write-ups receive no credit.
- Have your parent check your work and sign
The contents of the page was modified from
that created for the
Little Shop of Physics, Colorado State University