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If you carbonate water, and then expose the resulting soda water to the atmosphere and wait until it goes completely flat, will the (former) soda water end up more acidic in final equilibrium than water that was never carbonated and is also exposed to the atmosphere?

(Assume that both the water that was and was not carbonated are exposed to the exact same atmosphere, so that (a) they end up at the same temperature, and (b) the tiny addition to the level of atmospheric CO2 released during the carbonation process itself doesn't have any differential impact between the two waters.)

I understand that the soda water will be significantly more acidic than the flat water while it's still bubbly, because of the carbonic acid formed by the water and the injected pressurized CO2. I also believe that both waters will end up slightly acidic (although not as acidic as the soda water while it's still bubbly) because of the (modest) partial pressure of atmospheric CO2 in contact with both waters. My question is whether the high concentration of carbonic acid introduced into the soda water during the carbonation process permanently changes the equilibrium acidity of the water.

I'm also curious whether the answer would be different depending on whether you start with pure distilled water vs. "realistic" tap water, where the presence of impurities might somehow change the story.

My strong physical intuition is that the flattened (former) soda water will eventually "forget" about the carbonation and both waters will end up at the same equilibrium acidity, but some chemist friends disagree with me.

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    $\begingroup$ You're right. Henry's law states that at equilibrium the amount of gas dissolved in solution depends on the partial pressure of that gas which is in contact with that solution. So it doesn't matter if you have no carbonation or excess carbonation, when the solutions reach equilibrium then they will have the same amount of carbonation. $\endgroup$
    – MaxW
    Dec 29 '20 at 7:16
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Both water samples ( assuming pure water ) would theoretically end up slightly acidic with $\mathrm{pH} \approx 5.6$, but the carbonated one after long time. Tap water would have its baseline $\mathrm{pH}$ determined by its $\ce{CO2(aq)/HCO3-(aq)}$ $\mathrm{pH}$ buffer.

This is valid for an ideal case, as side processes - especially for tap water - will affect the result. As examples come to mind microbial/algae consumption of carbon dioxide, forming compounds affecting $\mathrm{pH}$, or precipitation of carbonates.

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This is an interesting question and environmentally related significant topic.

First, a pertinent related observation on dissolved oxygen content, to quote a question and its answer, which I believe to be factually accurate:

How does salinity affect Dissolved Oxygen solubility?

Dissolved oxygen decreases exponentially as salt levels increase. That is why, at the same pressure and temperature, saltwater holds about 20% less dissolved oxygen than freshwater.

So, likely adding say $\ce{NaCl}$ to carbonated water reduces dissolved carbon dioxide levels.

Per another source discussing the environmental problem of ocean acidification:

As atmospheric CO2 levels increase due to anthropogenic causes, dissolved CO2 also increases, which in turn decreases the pH of water. When water becomes saturated with CO2, it not only reduces the ocean's pH, but depletes the calcium carbonate sources as well ³⁵

As such, I would expect the impact of adding salt to carbonated distilled water would result in some reduction of dissolved $\ce{CO2}$ leading to an elevation of pH.

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