# Pressing Scientific Issue: Do Pump-Caps Preserve Soda Fizz?, Part II **URL:** \ **Category:** Factual Questions\ **Created:** [April 13, 2003, 3:28am UTC](https://boards.straightdope.com/t/pressing-scientific-issue-do-pump-caps-preserve-soda-fizz-part-ii/168380 "2003-04-13T03:28:22Z")\ **Posts on this page:** 1\ **Showing post:** 3
**Author:** ![Squink](https://avatars.discourse-cdn.com/v4/letter/s/b5e925/32.png) [@Squink](https://boards.straightdope.com/u/Squink)\ **Post date:** [April 13, 2003, 5:24am UTC](https://boards.straightdope.com/t/pressing-scientific-issue-do-pump-caps-preserve-soda-fizz-part-ii/168380/3 "2003-04-13T05:24:36Z")
(4) Why CO[sub]2[/sub] ? CO[sub]2[/sub] is a good choice for making fizzy drinks, because not only does it dissolve in a liquid, like other gases, but also [reacts with water to form carbonic acid](http://scifun.chem.wisc.edu/chemweek/CO2/CO2.html), and its ions. Thus the equilibrium concentration of CO[sub]2[/sub] in the air ,vs solution, lies towards more total carbonates in solution than with a ideal, nonreactive gas. If you charged a bottle of Soda with helium, rather than CO[sub]2[/sub], most of the gas would come out in one great bump immediately upon opening. However, the kinetics of CO[sub]2[/sub] \<-\>HCO[sub]3[/sub][sup]-[/sup] \<-\>CO[sub]3[/sub][sup]2-[/sup] are slow (~seconds), so not only do you end up with a lot more potential gas dissolved when you use CO[sub]2[/sub], but it comes out more slowly. The CO[sub]2[/sub] \<-\>HCO[sub]3[/sub][sup]-[/sup] \<-\>CO[sub]3[/sub][sup]2-[/sup] equilibrium depends on temperature, pH, and yes, **[pressure](http://cdiac.esd.ornl.gov/oceans/co2rprt.html#preseff)**. Data on the pressure effect is hard to come by, however before the last thread on this subject crashed-and-burned, I did find a site that gave a few figures. Can’t locate it now, but it’s a small, nonzero, effect. With plastic bottles at least, there may be a simple explanation for why some the pump seems to work for some people: The soda pop is supersaturated with respect to CO[sub]2[/sub] (all those carbonates…). The bottle is flexible. When you close the bottle you tend to squeeze it. This results in a low initial air-pressure inside the bottle, and walls that are easy for increased pressure to deflect. Since the pop is supersaturated, carbon dioxide gas _will_ come out of solution, until the point where the walls of the container supply enough rigidity for pressure to build and stop the process. Using a pump lets you use air, rather than fizz to make the container rigid. If that’s not too clear, think about what would happen if you quickly pulled a vacuum on a soda bottle, and then sealed it. You’d end up with a normally inflated bottle with a pressurized atmosphere of nearly pure CO[sub]2[/sub] above the pop. - Flat soda. --- _[View the full topic](https://boards.straightdope.com/t/pressing-scientific-issue-do-pump-caps-preserve-soda-fizz-part-ii/168380)._