# Dalton's Law Of Partial Pressures

My dad put one of those fizz savers on the top of a two-liter of soda. But doesn’t Dalton’s Law Of Partial Pressures show that fizz savers wouldn’t work? The fizz savers pump in more air, but not an appreciable amount of carbon dioxide, so the carbon dioxide in the soda would still keep coming out of solution, wouldn’t it?

Not sure exaclty what you’re talking about, but if I understand my fizz savers correctly, what’s happening is that you’re increasing the pressure, and thus forcing more gas to stay in solution. As soon as the pressure returns to normal, more gas will come out of solution.

Dalton’s law of partial pressure
For ideal gases, “each gas in a mixture acts as if the other gas is not present and that the pressures that come from each gas can simply be added.”

However, the equilibrium between gas phase CO[sub]2[/sub] and an acidic solution of CO[sub]2[/sub]<=>HCO[sub]3[/sub] is hardly ideal. The added pressure will increase the amount of fizz that stays in the soda, but not as much as if you had pumped CO[sub]2[/sub] into the headspace.

By increasing the air pressure in the bottle you’re rasing the surface tension of the liquid, there by trapping the CO2 in the liquid.

(I think)

Whether or not the concentration of CO[sub]2[/sub] in the liquid is constant after the cap is put on depends upon the CO[sub]2[/sub] in the air above the liquid and is independent of the presence of other gasses isn’t it? Isn’t it a statistical thing? That is, CO[sub]2[/sub] molecules are bouncing around in the liquid and in the air above the liquid. If the concentration of the gas in the air is low and that in the liquid high then the probablility is that more of it will leave the liquid to the air than will leave the air and go into the liquid. This process will continue until the air is “saturated” with CO[sub]2[/sub] and equal numbers of molecules are going both directions.

At least that’s the way evaporation works. Aren’t the molecules of a dissolved gas just a part of the liquid so they act just like the molecules of the liquid? If so, then the cap doesn’t do any good.

This has been discussed several times before with, to my mind, no real conclusion. If there are any young dopers, or dopers with kids, looking for a school science project then I humbly suggest this as an excellent topic for investigation. It would be of great service both to the fight against ignorance and to the soda-guzzling masses at large.

If the thing does work it might be because the added air pressure forces the cap against the sides of the bottle and makes a tight seal that won’t allow any gas to escape. In that way, a little CO[sub]2[/sub] would come out of solution but the small air volume would soon become saturated and the loss of the gas out of the liquid would stop. Without a tight seal, the CO[sub]2[/sub] would continue to escape to the outside and would thus be lost.

That method of making a tight seal, i.e. letting the confined pressure make the seal better, is how cup seals and O-rings work and could be the case here.

Assuming ideal gases, the OP’s thought process is essentially correct. However, if you double or triple the pressure in the container, you are also doubling or tripling the initial pre-equilibrium partial pressure of the carbon dioxide in the air space, which would tend to keep more carbon dioxide in solution than without the “fizz saver.”

Sorry, picunurse, but you are completely off the mark. Increasing the pressure over a liquid does not affect the surface tension of the liquid, nor does the surface tension of a liquid have anything whatsover to do with keeping a gas dissolved in solution.

How does adding air pressure change the CO[sub]2[/sub] partial pressure? And what is “initial pre-equilibrium partial pressure of the carbon dioxide?”

Which would have more fizz in the soda:

1. a bottle with 1 atmosphere of air in the headspace (including atmospheric CO2)
2. a bottle with 10 atmospheres of N2 in the headspace.

A lot of people try to squeeze a 2-liter bottle to remove air before sealing. But the bottle tries to return to shape, pulling more gas out of solution until the pressure with the outside equalizes. So never do that.

Using those valves, though will help like this: Normally, all the co2 in the neck will pour out with the liquid. With a valve, none of the free gas escapes, only the dissolved gas. While the latter may be replaced with air coming in, it’s still a savings.

Not quite. Air or other gas pressures have little effect on surface tension. OTOH a tiny tiny bit of detergent decreases surface tension dramatically.
The solution of CO2 in water solutions is entirely depencent on pressure!

I probably should know the answer to this, or at least know how to find the answer, but I don’t. Sure wish somebody smart would chime in on this one.

My guess is #2. Reason: vapor pressure of the CO2 would have to exceed 10 atm to come out of solution.

We went through this about 2 years ago but I’m having problems finding the thread. In my experiments, the pressure of the CO2 was insufficient to return the bottle back to it’s original shape.

[QUOTE=flex727]
I probably should know the answer to this, or at least know how to find the answer, but I don’t. Sure wish somebody smart would chime in on this one.

I don’t think so. CO[sub]2[/sub] molecules are escaping from the liquid surface into the headspace in the bottle all of the time. If there are CO[sub]2[/sub] molecules in that space some of them will reenter the liquid. As the molecules leave the liquid the number of them increases in the headspace and so the number reentering the liquid also increases. Eventually an equilibrium is reached where the number escaping into the headspace equals the number reentering the liquid.

I think this process goes on independently of other gasses in the headspace unless the gas in the headspace is so dense that there isn’t room for the CO[sub]2[/sub] molecules to enter without gravitational or electrical interactions. And barring chemical interactions with other gasses that would bind the CO[sub]2[/sub] molecules so that they wouldn’t be free to reenter the liquid.

Atmospheric air contains carbon dioxide. If the air contains 330 parts per million by volume (ppmv) of CO[sub]2[/sub], air at 1 atmosphere (atm) of pressure contains 0.00033 atm of CO[sub]2[/sub]. If I double the air pressure to 2 atm, there is now 0.00066 atm of CO[sub]2[/sub] in the air space above the liquid.

The carbon dixode is going to come out of solution to an equilibrium partial pressure dependent on the temperature of the solution, in accordance with Henry’s Law. The more CO[sub]2[/sub] that is intially in the air space, the less that will come out of solution.

Yes, a fizz saver would work better on the surface of a neutron star!

My friend has blown up two bottles of Diet Rite in his beer fridge using these Fizz Saver contraptions.

Maybe it kept the soda REAAAAALY fizzy.

Here’s the thread:
Pressing Scientific Issue: Do Pump-Caps Preserve Soda Fizz?, Part II