# Fizz Keeper Partial Pressures

1. Jul 13, 2014

### ContangoJoe

There is a device called a Fizz Keeper that attaches to carbonated beverage bottles and is supposed to keep them from going flat after they are opened by pressurizing them with air. Dalton's law and Henry's law have been used to debunk the device as ineffective.

The argument against the device's effectiveness goes something like this:

Henry's law states that the concentration of a gas dissolved in a liquid is proportional to the gas's partial pressure. Since the concentration of carbon dioxide in the atmosphere is low, the partial pressure of carbon dioxide contributed by the pressurized air is insignificant. Therefore, about the same amount of carbon dioxide must still come out of solution to reach an equilibrium partial pressure in a resealed bottle.

But there's something I don't understand.

According to Wikipedia, the carbon dioxide in the headspace of an unopened soft drink bottle exerts a pressure of about 2 atmospheres. When the bottle is opened, that carbon dioxide escapes and is replaced with air. If the bottle is closed again, carbon dioxide will come out of solution until its partial pressure again reaches 2 atmospheres (actually a bit less because the concentration of carbon dioxide in solution has decreased some, but let's call it 2 atmospheres for simplicity). But now we also have air in the headspace at 1 atmosphere, so why wouldn't the total pressure be 3 atmospheres (the sum of the partial pressures per Dalton's law) at equilibrium?

The total pressure doesn't really go up 50% after a bottle opened and resealed, does it? Can anyone alleviate my confusion?

2. Jul 13, 2014

### Staff: Mentor

Right, but that is mainly an effect of your approximation - the partial pressure of CO2 in the liquid will drop significantly the first time you open the bottle.
Another effect: depending on the way the bottle is filled, it might have O2 and N2 partial pressures below the atmospheric pressure, so some gas can get dissolved there.

3. Jul 13, 2014

### ContangoJoe

I have another question that's a little deeper I think.

It's always seemed strange to me that molecules in a situation like this apparently make a distinction between themselves and molecules of a different compound. It seems to me that molecules would bump into molecules of another kind in basically the same way they would bump into molecules of their same kind.

How is it that the carbon dioxide molecules in this situation can just ignore the pressure of the atmospheric molecules in the headspace?

4. Jul 14, 2014

### Staff: Mentor

They do, but if there is no CO2 in the gas, no CO2 molecules can dissolve. To reach equilibrium, both fluxes have to be the same, and that needs a certain density of CO2 in the gas. More other atoms just make the motion more jagged (shorter mean free path), but the collision rate of CO2 molecules with the water surface stays the same.

5. Jul 16, 2014

### Staff: Mentor

At the pressures you are considering, the mixture of CO2 and air is still essentially in the ideal gas region where the various species act essentially independently.

Chet