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Name: Steve
Status: student
Grade: 9-12
Country: United Kingdom
Date: Winter 2009-2010


Question:
What happens to the solubility of carbon dioxide in water when the water is frozen. I did this to a bottle of soda pop and when I opened it, there was a lot of fizzing which means the CO2 is not soluble.



Replies:
Steve,

If we look at the solubility of CO2 in water as a function of temperature, we find that solubility actually increases. Here is the first graph I could find showing that:

http://www.rocketscientistsjournal.com/2006/10/_res/CO2-06.jpg

When water turns to ice, this is no longer an issue of solubility but a measure of how much CO2 can be physically trapped in the ice. Unfortunately, the amount of CO2 trapped in the ice will depend very strongly on the conditions of freezing. Are the ice crystals formed very slowly so that the CO2 is pushed out of the system? Did ice form on top of the liquid first trapping most of the dissolved CO2? Was the system agitated? etc. It would be very hard to predict physical trapping.

Moreover, fizzing is not a good measure of how much CO2 was trapped. Fizzing can result from agitation, small crystals becoming nucleation points for bubble formation, etc.

I do not think we can answer this question because the term solubility does not apply and the "measure" of fizzing is not a very good indicator of solubility.

Greg (Roberto Gregorius)
Canisius College


You have several processes going on here. First, as the temperature DECREASES the solubility of CO2 in water INCREASES. However, the solubility of CO2 in ice essentially ZERO. So the same amount of CO2 is "forced" to dissolve in a decreasing amount of water, which increases the pressure, because it is confined to the space available (Henry's Law). When you release the pressure, the CO2 / Soda will come to a new set of conditions, namely atmospheric pressure. That is the fizzing you observe. Additionally, as the CO2 evaporates rapidly heat is lost and you may observe soda freezing, which feeds back into the cycle. Evaporation costs heat energy. What you ask does not have a simple answer-- many "things" are going on.

Vince Calder



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