Ask A Scientist Chemistry Archive

name         Mrs. A.
status       educator
grade        6-8
location     CA

Question -   How can I measure different levels of carbonation 
in different sodas.   This is a students' science fair question. 
He wants to know why if he puts a balloon over the top of a two 
liter bottle and shakes it up why the balloon will rise.
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Hello,
Basically the experiment your student proposes will work somewhat 
for measuring carbonation, although there are still a number of 
variables introduced by using a balloon.

By shaking the bottle, the soda (supersaturated with carbon dioxide) 
will release CO2 until it comes into equilibrium with the 
air.  However, the balloon itself applies some pressure (e.g. it 
blows out air when you release it), and manufacturing 
inconsistencies will cause some balloons to expand more than others 
under the same pressure.  Something like a flattened plastic bag 
could also capture the gas and would exert less variable back 
pressure on the bottle.

Another problem is that in opening the bottle you likely lose some 
gas (opening even a chilled bottle will make a hiss, indicating 
escaping gas when you open the cap).  Ideally you could capture that 
gas, perhaps enclosing the entire botlle in the bag, or sealing a 
bag around the neck before opening.  Less than ideal, you can 
perhaps chill (but not freeze) the bottles to the same temperature, 
and open them carefully but quickly to minimize the initial loss 
before capturing the gas.   It will be hard to determine how much 
you have lost however in that initial opening, but if quickly and 
consistently done, you can minimize the variability introduced in that step.

Don Yee
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