Originally posted by: DrPizza
Bigsmooth, I was the one that pointed out that pressure does have something to do with it... and I still think it does.
From Wiki: Water will absorb its own volume of carbon dioxide, and more than this under pressure.
From observation: take a clear bottle of soda, so you can see inside.
Open the cap. Notice bubbles coming from the soda? It's because it's no longer in equilibrium. It's rapidly losing carbon dioxide because the pressure is lower.
Also, I've never seen vapor pressure refered to for a dissolved gas.
I agree that pressurization is very important. However, I think you might be looking at the problem in a different light than I am. I mentioned vapor pressure because that's a relatively simple concept that the layman can understand, while the gibberish I'll type out below is not.
Basically, I used 'vapor pressure' as a dummy variable to replace the complexities of the reaction equilibrium that one needs to consider when looking at liquid absorption phenomena.
Since the can is a closed system (when one neglects transport of CO2 through the metal, which is a good assumption over small time periods), the net amount of CO2 in the can must not change with temperature. Thus, the only thing that changes with respect to carbonation is the amount of CO2 in the liquid phase relative to the amount in the gaseous phase.
In reality, liquid absorption of a gas is a chemical process. In the case of CO2 absorption, carbonic acid (HCO2) is formed, and the reaction is CO2(g)+H2O(l) <=>HCO2(l) (unbalanced because I'm really that lazy
). This is a reversible reaction in which the equilibrium is drastically affected by the pressure, as you stated. The kinetics of the reaction will depend on the partial pressure of CO2 in the gaseous state and the concentration of HCO2 in the liquid state, as well as some rate constant that is independent of pressure but exponentially dependent on temperature. So, as you say, the pressure will definitely impact the level of carbonation, as high pressure will force the reaction to favor the formation of HCO2.
However, in the case of a closed container, the pressure is a result of the temperature. The pressure within the can will decrease very, very slightly as temperature decreases to a point, then it will begin to increase very rapidly as the water freezes and the liquid/solid complex expands. This is why there is an optimal temperature at which you should drink your soda. I figured this out accidentally when I was a little kid, experimenting with how long I should leave my Dr. Pepper in the fridge before drinking it (yes, I've always been a huge dork). When the liquid starts to freeze, the moles of liquid present decrease such that it can no longer have the same concentration of HCO2. This in turn creates an increase in gas pressure and CO2 concentration, with a corresponding decrease in carbonation levels.
What does all this mean?
1. Temperature cycling should have almost zero effect on the carbonation of the soda, This is true because the only effect one needs to consider here is mass transfer of CO2 through the can. While the temperature will effect the diffusivity of CO2 in the steel or aluminum, this effect will be very, very small - far too small to be important except over months. Storing the soda at high temperatures will also always result in a faster loss of CO2 and decrease in carbonation.
2. The temperature at which your soda is opened will have a very large impact on the carbonation. Low temperature will increase the amount of carbonation to a point, as well as decreasing the rate at which the carbonation is lost (since the HCO2 --> CO2 reaction is slower at low temperatures).
/more than anyone should ever have thought about carbonation
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