MadSci Network: Chemistry

Re: Experiment to test Carbonation

Date: Mon Dec 6 16:02:10 2004
Posted By: John Christie, Faculty, Dept. of Chemistry,
Area of science: Chemistry
ID: 1102004638.Ch

I think you have picked out an interesting thing to test, and something that should be possible. 
There are a few difficulties, though.

The experiment is all about carbon dioxide. Carbon dioxide dissolves moderately well in water. It 
dissolves much better if the water is cold, and much better if the carbon dioxide is under pressure.

To make a carbonated drink, carbon dioxide at high pressure is pumped through very cold water, 
and the solution is stored in a bottle under pressure. When you take the cap off the bottle, the 
pressure is released, and less carbon dioxide can dissolve in the water. So the gas comes out of 
solution -- that is what makes the fizz.

Soft drink makers can get their drinks more or less fizzy, depending on how large a pressure they 
use, and how cold the water they use.

Have you ever noticed that bottles for fizzy drinks are made from PET, but bottles for non-fizzy 
drinks are usually polyethylene? (Glass bottles can be used for either type of drink, of course). That 
is because carbon dioxide can leak through most types of plastic quite quickly, including 
polyetheylene. PET is special because it is the only suitable and readily available plastic that can 
stop carbon dioxide from escaping. It is important for the "shelf life" of a carbonated drink that the 
compressed carbon dioxide not be allowed to escape.

OK -- here are the basics for you experiment: get equal samples of the drinks you want to 
compare, at the same temperature, and poured from the bottle in the same way (that bit might be 
very hard to arrange). De-fizz them very rapidly, and measure the amount of gas given off.


1. How to de-fizz?  If you add any finely divided solid, like flour or caster sugar, the carbonation 
will escape very rapidly. Problem is that these fine solids will also cause frothing and foaming, and 
you will make a huge mess! If you have access to a chemical lab. "boiling chips" would be good for 
the purpose. Very clean fine sand should work fairly well, but it might be a bit slow. Broken 
crockery, smashed to sand-sized pieces would be ideal.

2. How to measure the amount of gas given off? Do not use a latex balloon, because the pressure 
build-up as you inflate it will (a) cause too much carbon dioxide to stay dissolved in the water, and 
(b) cause too much carbon dioxide to escape through the balloon skin. Probably best would be a 
large polyethylene bag, squashed empty and sealed over the mouth of the glass you are using. You 
would have to arrange some way of dropping the de-fizzing material into the glass AFTER you 
have sealed on the collection bag.

You will find that there are still a lot of issues with designing the experiment that you will need to 
think about and work out, but that should give you a good start.

Best wishes for your project, and have fun!

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