Re: Carbon dioxide in soda

There are actually two separate things going on here, but I think one of them is more important than the other in this case.

If you expose a bottle of soda to the atmosphere, it will eventually go "flat". This is because there is an "equilibrium" established, which means that the amount of CO2 in the soda is "balanced" against the amount in the atmosphere, and there isn't very much CO2 at all in the atmosphere. It doesn't go flat in the unopened bottle because the first amount that escapes makes for a larger amount in the closed "atmosphere" at the top of the bottle, which has no place to go.

But if you just let the open bottle sit there, it will take a long time to go flat. This is because the CO2 must get together and form a bubble to come out of solution (this is called "nucleation"), and forming bubbles does not happen easily when liquids are still. If you disturb the liquid by pouring it into a glass, especially over ice, you provide all sorts of motion and surfaces that can help the bubbles get started. Putting in salt accomplishes the same thing -- you are giving the liquid a little nudge to help the bubbles start forming.

There is also another effect. For CO2 (or any gas) to dissolve in water, the water molecules have to sort of grab on to it (that is called "hydration") If you put more gas in, eventually you use up all the ability of the water to hydrate gas molecules, and that is the limit of how much you can dissolve. When you put salt into water, it breaks up into charged particles called "ions". These ions are strongly hydrated, so they tie up a lot of water molecules. That means there is less water free to hydrate the dissolved gas, so less gas can remain dissolved in the salt solution. This effect is called "salting out", and it is important in many processes.

But the salting-out effect is not that large unless you dump a LOT of salt into the water, so the main effect you are seeing is because the salt is providing sites for bubbles to form and allowing the gas to come out of solution faster.

Allan Harvey, Physical and Chemical Properties Division, NIST

aharvey@boulder.nist.gov

"I don't speak for the government, and they don't speak for me."